GREENSTONE SLATE...There is concern with the durability of roofing slate from some quarries. Slate can be tested to recognized standards if the source of a slate at a quarry provides

GREENSTONE SLATE Guide to Roof Installations

© 2002 Greenstone Slate Company, Inc.© 2001 National Roofing Contractors Association

We wish to thank the NRCA for allowing us to reprint portions of their Fifth Edition Roofing and Waterproofing Manual

General InformationSlate Roof Systems.............................................................................................................................................................. 1

Types of Slate Roof Systems .............................................................................................................................................. 1

Active Slate Quarries............................................................................................................................................................ 1

Slate Tools .......................................................................................................................................................................... 1

ComponentsSlate .................................................................................................................................................................................... 3

Material Description ............................................................................................................................................................ 3

Colors .................................................................................................................................................................................. 3

Weight ................................................................................................................................................................................ 4

Quantities per Square. ........................................................................................................................................................ 5

Standards ............................................................................................................................................................................ 5

Underlayment ...................................................................................................................................................................... 6

Asphalt-saturated Felt .......................................................................................................................................................... 6

Polymer-modified Bitumen Sheet Underlayments ................................................................................................................ 7

Ice Dam Protection Membranes .......................................................................................................................................... 7

Asphalt Roof Cements ........................................................................................................................................................ 8

Design ConsiderationsRoof Decks .......................................................................................................................................................................... 9

Wood Plank and Board Decking .......................................................................................................................................... 9

Structural Wood Panel Decking............................................................................................................................................ 9

Preservative-treated and Fire-retardant-treated Wood.......................................................................................................... 10

Other Deck Types ................................................................................................................................................................ 10

Underlayment and Roof Slope ............................................................................................................................................ 10

Underlayment Configurations .............................................................................................................................................. 11

Roof Slope .......................................................................................................................................................................... 11

Ice Dam Protection Membranes .......................................................................................................................................... 11

Fasteners ............................................................................................................................................................................ 14

Exposure and Appearance .................................................................................................................................................. 15

Starter Course .................................................................................................................................................................... 16

Eave Cant ............................................................................................................................................................................ 16

Flashings.............................................................................................................................................................................. 16

Perimeter Edge Metal .......................................................................................................................................................... 16

Penetration Flashings .......................................................................................................................................................... 16

Valley Construction .............................................................................................................................................................. 17

Open Valleys........................................................................................................................................................................ 18

Enhanced Open Valleys ...................................................................................................................................................... 20

Slate Roofing ManualTable of Contents

Closed Valleys...................................................................................................................................................................... 20

Rounded Valleys (Closed) .................................................................................................................................................... 20

Vertical Surfaces .................................................................................................................................................................. 20

Apron Flashings .................................................................................................................................................................. 21

Step Flashings .................................................................................................................................................................... 21

Cricket or Backer Flashings.................................................................................................................................................. 22

Counterflashings .................................................................................................................................................................. 24

Ridges ................................................................................................................................................................................ 25

Slate Ridge .......................................................................................................................................................................... 25

Slate Saddle Ridge ............................................................................................................................................................ 25

Strip Saddle Ridge .............................................................................................................................................................. 25

Combing Slate Ridge .......................................................................................................................................................... 26

Metal Ridge.......................................................................................................................................................................... 26

Hips .................................................................................................................................................................................... 26

Slate Hip .............................................................................................................................................................................. 27

Saddle Hip .......................................................................................................................................................................... 27

Mitered Hip .......................................................................................................................................................................... 27

Fantail Hip............................................................................................................................................................................ 27

Metal Hip ............................................................................................................................................................................ 27

GREENSTONE SLATE 5

Slate Roof SystemsSlate roof systems may be classified into three general categories:

• Standard

• Graduated

• Textural

Standard Slate Roof SystemsStandard slate roof systems are those roof systemscomposed of standard commercial slate that isapproximately 1/4 inch to 3/8 inch (6 mm to 9 mm)thick. The most common have uniform lengths andwidths, with square tails or butts. Standardcommercial slate may be used to form a variety ofdesigns on a roof, and are suitable for many buildingswhere a long service-life steep-slope roof covering isdesired.

If desired, butts or corners of each slate may betrimmed to a diamond, hexagonal, or other pattern forspecific portions or for all the roof system. Variety inthe pattern of standard slate roof systems issometimes attained by using random widths. Astaggered butt line can be achieved by using longerlength slates randomly.

Graduated Slate Roof SystemsGraduated slate roof systems feature variations inthickness, size and exposure. In graduated slate roofsystems, the thickest, widest and longest slates areplaced along eaves. As slate courses progressupslope to ridges, slates of gradually diminishing size and/or thickness are used, creating a “graduated”effect. This effect is intended to make the eave-to-ridge distance look longer and give the roof a more massive appearance. Slate for roofs of this typecan be obtained in any combination of thicknessesfrom 3⁄16 inch (4 mm) to 11⁄2 inches (38 mm).Thicker slates are sometimes specified. Graduatedslate roof systems may be installed in a wide variety of patterns.

Textural Slate Roof SystemsBecause slate shingles come in a wide variety of sizes, thicknesses, textures and colors, a wide rangeof appearance can be created. When a range ofthicknesses are mixed throughout a roof system, the roof system is some-times referred to as atextured or textual roof system. Textural slate roofsystems are composed of slate that has a roughertexture than standard slate. This slate type also hasuneven tails or butts.

Active Slate QuarriesActive roofing slate quarries in North America exist inNew York, Pennsylvania, Vermont, Virginia, and theCanadian provinces of Quebec and Newfoundland.The main area of slate production in Pennsylvania isthe Lehigh district. The active Vermont quarry districtlies in Bennington and Rutland counties and extendsinto Washington County in New York. Virginia slateoperations are now conducted only in BuckinghamCounty at Arvonia.

Imported slate is available from Spain, Wales, China,Brazil and South Africa.

There is concern with the durability of roofing slatefrom some quarries. Slate can be tested to recognizedstandards if the source of a slate at a quarry providesa consistent quality. Tests include those in ASTM C406, “Standard Specification for Roofing Slate,”especially the modulus of rupture test. British Stan-dard 680 has an acid resistance test for carbonateimpurities especially calcite. A new European Standardfor petrographic examinations can describe themineralogy and fabric of the stone and identify mostproblematic features of building stones.

Slate ToolsSpecialized tools for slate roofing include the slatehammer, stake, ripper, roof scaffold bracket, and roofridge or hook ladder. A slate hammer has many uses.The hammer head is used for driving nails, the claw forpulling nails, the point for punching slate and the knifeedge on the shaft for cutting slate.

www.greenstoneslate.com • 800.619.4333

GENERAL INFORMATION

A slate stake is used with a slate hammer to cut slatein the field. It is a T-shaped tool that can be stuck intoa roof deck or scaffold plank and provides a solidsupport at the underside of slate along the line wherea cut or hole punch is to be made. It is also used as astraight edge to mark slate before cutting or trimmingslate at valleys and other flashing areas.

A slate ripper is used to take out broken slate bycutting or pulling the concealed nails. The upperportion of a ripper is designed to hook around nailsand loosen or cut them by force. The handle isdesigned to be struck with a hammer to assist incutting or loosening a slate nail after the ripper hasbeen positioned around or against the nail.

A roof scaffold bracket is used to secure toe boards.Roof scaffold brackets are adjustable and hold woodplank toe boards parallel to the ground. A toe board isa wood plank that provides a platform for a workerand holds material and tools.

A roof ridge or hook ladder is lightweight and has ahook that goes over a roof ridge. It enables a workerto climb on a slate roof system without putting fullweight on individual pieces of slate. Weight istransferred along the length of a ladder.

GREENSTONE SLATE 6

The Greenstone Slate Guide to Roof Installations • General Information


GREENSTONE SLATE 7

SlateMaterial DescriptionSlate rock originates from sedimentary particles of clayand silt. Silt and other particles were originally washeddown into streams, which transported the particlesinto ancient seas. As quantities of these particles weredeposited over time, successive parallel layers of thematerial accumulated. These accumulations ofsediment, built-up over thousands of years to varyingthicknesses, and are called beds.

The pressure of successive overlying layerscompressed and consolidated the lowerunderlying sedimentary layers, whichformed shale. Years of these geologicalforces caused great pressure on theunderlying shale layers and, aspressures rose, heat and force loadscombined to chemically change originalclay materials into harder materials suchas chlorite, mica and quartz. Many yearsof these combined geological forcestransformed relatively weak shale rockinto hard slate.

The original layering of materials givesslate natural cleavage planes. Thesenatural cleavage planes are what allowsslate to be easily split into relatively thinlayers. Slate also possesses a naturalgrain that usually occurs at right anglesto cleavage. Roofing slates are com-monly split so that the length of the slateruns in the direction of the natural grain.See Figure 1.

The surface texture of slate, after it issplit for commercial use, depends on thequality and characteristics of the rockfrom which it was quarried. Many slatessplit to a smooth, practically even,uniform surface, and others split to asurface that is rough and uneven.

As a result, a wide range of surface textures areavailable.

Some slate contains narrow “ribbons of rock” that aredifferent in chemical composition and color from theprincipal slate. Slate that has been trimmed so that the ribbons are eliminated is known as clear slate.Slate that contains acceptable ribbons is sold asribbon stock.

ColorsThe color of slate is determined by its chemical andmineral composition. Because these factors differ invarious regions it is possible to obtain roofing slate in a


Figure 1: Example of quarried slates to roofing slates

COMPONENTS

QUARRIED BLOCKS OF SLATE ROCK ARE CROSSCUT INTO SECTIONS, THEN BLOCKS ARE SPLIT(SCULPTED) ALONG THE GRAIN. BLOCKS ARE THEN SPLIT ALONG THE CLEAVAGE PLANE INTO THIN -NER BLOCKS OFTEN REFERRED TO AS "EIGHTS." THE EIGHTS ARE SPLIT ALONG CLEAVAGE PLANE TOYIELD 8 INDIVIDUAL ROUGH SLATES. THESE ROUGH SLATES ARE THEN TRIMMED AND THE NAIL HOLESPUNCHED TO PRODUCE THE FINISHED ROOFING SLATE.

2. CROSSCUT BLOCKS

CLEAVAGE PLANE

1. QUARRIED BLOCK OF SLATE ROCK GRAIN PLANE

3. SCULPTED BLOCKS

4. EIGHTS

5. ROUGH ROOFING SLATES

GRAIN PLANE

GREENSTONE SLATE 8

The Greenstone Slate Guide to Roof Installations • Components

variety of colors and shades. Various shades of thesame slate may be used to provide interesting colormottling or shaded patterns when applied across aroof, or when interspersed randomly. The diversity ofcolor makes slate a sought-after material for thecreation of certain architectural or aestheticconditions. When multiple colors are used, a roofsystem is sometimes called polychromatic.

Exposure to weather causes all slate to change slightlyin color and composition. The degree of color changevaries. Slate that exhibits minimal color change isclassified as non-weathering. Slate that exhibits amore obvious color change is known as weathering.Weathering slate offers the designer another variationin roof aesthetics. Weathering and semi-weatheringgray and green slates contain some slates that canweather to buff or tan as the mineral particles in theslate oxidize, providing a varied color patternthroughout a roof system.

When texture and color are essential considerations in roof design, architects and owners should considerthe source of the slate, its potential for color changeand the effect of weathering on the slate to be used.Slate producers know from experience the probabledegree of color change for various slate materialsobtained from their quarries. Producers may beconsulted on selection of particular slate for a specific project.

For the purpose of classifying the basic natural colorsof roofing slate currently available in large quantities forgeneral use, the following color nomenclature may beused:

• black

• green

• gray

• gray green

• mottled purple

• mottled green

• purple

• red

In addition, slate distributors use the following namesand nomenclature for certain roofing slate:

• blue-black

• blue-gray

• mottled gray-black

• mottled gray-green

• mottled purple-green

• mottled green-purple

• purple variegated

• sea green

When selecting a slate color from a distributor, thecolor should be preceded by the word “unfading” or“weathering” to designate the color stability or changethat may be expected for a particular type of slate. Inaddition to the colors previously mentioned, slatedistributors use different combinations of names toidentify the colors of a particular slate.

WeightSlate of commercial standard thickness weighsapproximately 850 pounds per square (41 kg/m2).A square of slate on a roof system set at the standard3 inch (75mm) headlap will vary in weight from 650pounds to 8,000 pounds (295 kg to 3,629 kg)depending on the thickness of each slate.

Slate varies significantly in weight because of thenumerous thicknesses, and types available from thedifferent quarries. Table 1 shows the approximateweights of roofing slates of different thicknesses. Theactual weight of slate products may be from 10 per-cent above to 15 percent below the weights shown.

A quarry or supplier should be consulted for specificslate weight(s), length, width dimensions andthickness. Slate is also available with sawn orchamfered edge, depending on the slate producer.


Approximate Weight of Roofing Slate at Various Thicknesses(Based upon 3 inch [75 mm] exposure)

Slate thickness Weight3⁄16” to 1⁄4” (4 mm to 6 mm) 700 lbs/sq to 1,000 lbs/sq (3,400 kg/m2 to 4,900 kg/m2)

3⁄8” (9 mm) 1,500 lbs/sq (7,300 kg/m2)1⁄2” (13 mm) 2,000 lbs/sq (9,800 kg/m2)3⁄4” (19 mm) 3,000 lbs/sq (14,600 kg/m2)

1” (25 mm) 4,000 lbs/sq (19,500 kg/m2)

11⁄4” (32 mm) 5,000 lbs/sq (24,400 kg/m2)

11⁄2” (38 mm) 6,000 lbs/sq (29,300 kg/m2)

13⁄4” (44 mm) 7,000 lbs/sq (34,200 kg/m2)

2” (50 mm) 8,000 lbs/sq (39,000 kg/m2)

Table 1: Weights of Slate Weight at Various Thicknesses

GREENSTONE SLATE 9

Components • The Greenstone Slate Guide to Roof Installations

Quantities Per SquareIn the United States, slate is sold by the square. Asquare of roofing slate is defined by the U.S. Depart-ment of Commerce, National Bureau of Standards inSimplified Practice Recommendation No. 14 as:

“… a sufficient number of slate shingles of any size tocover 100 square feet of plain roofing surface, whenlaid with the approved or customary standard headlapof 3 inches (75 mm).” Slates for surfacing flat roofs areusually laid tile-fashion, without lap, in which case asquare of slate would cover an area greater than 100square feet.”

The quantity of slate per square varies from 89 piecesof 26 by 14 inch (660 by 360 mm) slate, to 686 piecesof 10 inch by 6 inch (250 mm by 150 mm) slate.These quantities include an allowance for the 3 inch(75 mm) headlap.

It should also be noted that for roofs of lower slope,where a 4 inch (100 mm) headlap is used, anadditional quantity of slate is required to cover 1square of roof. For very steep roofs or verticalsiding/cladding applications, where a 2 inch (50 mm)overlap is sufficient, fewer slates will be needed.

Table 2 shows length and width dimensions for stan-dard slate, the minimum number of slates required persquare and respective exposures for the slates listed.

StandardsTo satisfy the need for slate classification, the NationalSlate Association established various standards forcommercial slate during the 1800’s. The primarystandard was based on visual grading of slate bycolor, surface texture, straightness, thickness andweight. Even though grading was visual and variedfrom region to region, the standard helped ensure thatslate products (that met the standard) would containcertain physical characteristics essential for qualityslate roofing. For many years, roofing slates werespecified according to the National Slate Association’sstandard.

Since 1957, the American Society for Testing andMaterials (ASTM) has established a consensusmaterial standard for slate used for roofing shingles.This material standard is designated as ASTM C 406,“Standard Specification for Roofing Slate.” ASTM C406 addresses material characteristics, physical


SCHEDULE FOR STANDARD 3/16" (5 mm) THICK SLATE

SIZE OF SLATE(L x W)

SLATES PER

SQUARE

EXPOSURE WITH 3" (76 mm) Lap

LENGTH

(L)

WIDTH(W)

SIZE OF SLATE(L x W)

SLATES PER

SQUARE

EXPOSURE WITH 3" (76 mm) Lap

26 in. x 14 in. 89 111/2 in.

24 in. x 16 in.

(660 mm x 360 mm)

(610 mm x 410 mm)24 in. x 14 in. (610 mm x 360 mm)24 in. x 13 in. (610 mm x 330 mm)24 in. x 12 in. (610 mm x 310 mm)24 in. x 11 in. (610 mm x 280 mm)

22 in. x 14 in. (560 mm x 360 mm)22 in. x 13 in. (560 mm x 330 mm)22 in. x 12 in. (560 mm x 310 mm)22 in. x 11 in. (560 mm x 280 mm)22 in. x 10 in. (560 mm x 250 mm)

20 in. x 14 in. (510 mm x 360 mm)20 in. x 13 in. (510 mm x 330 mm)20 in. x 12 in. (510 mm x 310 mm)20 in. x 11 in. (510 mm x 280 mm)20 in. x 10 in. (510 mm x 250 mm)20 in. x 9 in. (510 mm x 230 mm)

18 in. x 14 in. (460 mm x 360 mm)18 in. x 13 in. (460 mm x 330 mm)18 in. x 12 in. (460 mm x 310 mm)18 in. x 11 in. (460 mm x 280 mm)18 in. x 10 in. (460 mm x 250 mm)18 in. x 9 in. (460 mm x 230 mm)

8698

106114138

108117126138152

121132141154170189

137148160175192213

(290 mm)

101/2 in. (270 mm)101/2 in. (270 mm)101/2 in. (270 mm)101/2 in. (270 mm)101/2 in. (270 mm)

91/2 in. (240 mm)91/2 in. (240 mm)91/2 in. (240 mm)91/2 in. (240 mm)91/2 in. (240 mm)

81/2 in. (220 mm)81/2 in. (220 mm)81/2 in. (220 mm)81/2 in. (220 mm)81/2 in. (220 mm)81/2 in. (220 mm)

71/2 in. (190 mm)71/2 in. (190 mm)71/2 in. (190 mm)71/2 in. (190 mm)71/2 in. (190 mm)71/2 in. (190 mm)

16 in. x 14 in. (410 mm x 360 mm) 160 61/2 in. (170 mm)16 in. x 12 in. (410 mm x 310 mm) 184 61/2 in. (170 mm)16 in. x 11 in. (410 mm x 280 mm) 201 61/2 in. (170 mm)16 in. x 10 in. (410 mm x 250 mm) 222 61/2 in. (170 mm)16 in. x 9 in. (410 mm x 230 mm) 246 61/2 in. (170 mm)16 in. x 8 in. (410 mm x 200 mm) 277 61/2 in. (170 mm)

14 in. x 12 in. (360 mm x 310 mm) 218 51/2 in. (140 mm)14 in. x 11 in. (360 mm x 280 mm) 238 51/2 in. (140 mm)14 in. x 10 in. (360 mm x 250 mm) 261 51/2 in. (140 mm)14 in. x 9 in. (360 mm x 230 mm) 291 51/2 in. (140 mm)14 in. x 8 in. (360 mm x 200 mm) 327 51/2 in. (140 mm)14 in. x 7 in. (360 mm x 180 mm) 374 51/2 in. (140 mm)

12 in. x 10 in. (300 mm x 250 mm) 320 41/2 in. (110 mm)12 in. x 9 in. (300 mm x 230 mm) 355 41/2 in. (110 mm)12 in. x 8 in. (300 mm x 200 mm) 400 41/2 in. (110 mm)12 in. x 7 in. (300 mm x 180 mm) 457 41/2 in. (110 mm)12 in. x 6 in. (300 mm x 150 mm) 533 41/2 in. (110 mm)

11 in. x 8 in. (280 mm x 200 mm) 450 4 (100 mm)11 in. x 7 in. (280 mm x 180 mm) 515 4 (100 mm)

10 in. x 8 in. (250 mm x 200 mm) 515 31/2 in. (90 mm)10 in. x 7 in. (250 mm x 180 mm) 588 31/2 in. (90 mm)10 in. x 6 in. (250 mm x 150 mm) 686 31/2 in. (90 mm)

Table 2: Schedule for standard slate

GREENSTONE SLATE 10


properties and sampling procedures appropriate tothe selection of slate for use as roofing shingles.

ASTM C 406 lists three grades of roofing slatedepending on geographic location and environmentalexposure: Grade S1 slates are to have specificproperties that are said to provide for an expectedservice life of more than 75 years; Grade S2 slates areto have an expected service life of 40 to 75 years; andGrade S3 slates are to have an expected service life of20 to 40 years. Actual service lives for quality S1 slateroofs range from 75 to more than 300 years.

There are three ASTM standard test methods thatapply to roof slate and provide the basis of ASTM C406. These are:

• ASTM C 120, “Standard Test Methods of FlexureTesting of Slate (Modulus of Rupture, Modulus ofElasticity)”

• ASTM C 121, “Standard Test Method for WaterAbsorption of Slate”

• ASTM C 217, “Standard Test Method for WeatherResistance of Slate”

These test method standards provide the basis for thegrade classifications in ASTM C 406 which establishesthe physical requirements shown in Table 3.

The grade of slate used should be commensurate withthe expected design life of the building. NRCArecommends designers specify slate that meets therequirements of ASTM C 406, Grade S1, for allgeographic regions and environmental exposureconditions in North America.

UnderlaymentNRCA recommends the use of underlayments withslate roof systems. Underlayment is applied over aroof deck before the application of roofing slate.Underlayment performs two primary functions: Itprovides temporary weather protection until a roofcovering is installed, and it provides a secondaryweatherproofing barrier should moisture migrate belowthe slate roof covering.

In addition, underlayments are generally necessary forthe following reasons:

• to comply with local building codes

• to help prevent dust, dirt and insects from enteringbuildings

There are different underlayment configurations thatcan be used for slate roof systems. Generally, theseconfigurations can be categorized as follows:

• single layer of underlayment

• single layer of self-adhered membrane

• double layer underlayment system

A single layer of underlayment is the most commonunderlayment configuration for slate roof systems.

Asphalt-saturated FeltAsphalt-saturated felt underlayments are the mostcommon underlayments used in steep-slope roofsystems and typically use organic reinforcing mats.

The weatherproofing material used to manufactureroof underlayment felt is asphalt flux. Asphalt flux isobtained from the fractional distillation of petroleumthat occurs toward the end of the petroleum refiningprocess. Asphalt flux is sometimes further refined byair blowing to produce roofing-grade asphalt at therefinery or at the product manufacturer’s facility.

Depending on the type of underlayment, asphalt maybe used in two processes: first as a saturant andsecond as a coating.

Saturant-grade asphaltAsphalt used in the saturation process is a “soft,” lessviscous asphalt than a coating asphalt and is used toimpregnate organic reinforcing mats. Saturant-gradeasphalt has a lower melting point than coating-gradeasphalt. Common underlayments such as No. 15 andNo. 30 are asphalt-saturated felts.

Coating-grade asphaltIf the felt underlayment is a coated felt, a coating-grade asphalt is applied to the felt after the saturationprocess. Coating-grade asphalt is more viscous thansaturant-grade asphalt. Mineral additives, or “fillers,”are added to the coating-grade asphalt to stabilize thebitumen and reduce its natural flow characteristics,and increase fire resistance and weatherability, makingit more suitable as a coating material.


Physical requirements for roofing slateClassification Modulus of Rupture Absorption, Depth of Softening,

Across the Grain, maximum maximumminimum (ASTM C 121) (ASTM C 217)

(ASTM C 120)Grade S1 9,000 psi (62 kPa) 0.25% 0.002 in. (0.05 mm)Grade S2 9,000 psi (62 kPa) 0.36% 0.008 in. (0.20 mm)Grade S3 9,000 psi (62 kPa) 0.45% 0.014 in. (0.36 mm)

Table 3: Physical requirements for roofing slate

GREENSTONE SLATE 11

Components • The Greenstone Slate Guide to Roof Installations

Underlayments are reinforced with mats that aredesigned to “carry” or support the asphalt. Reinforcingmats of different thicknesses are used to produceunderlayments of different weights. These reinforce-ments, sometimes referred to as carriers, are mostcommonly made out of organic fibers. However, someunderlayments are produced that use reinforcing matsmade of glass fibers.

Organic MatOver the years, organic mat has been produced fromvarious combinations of cotton rag, wood fiber andother cellulose fibers. Currently, wood and othercellulose fibers are the types of reinforcements mostwidely used in organic mats. Organic mats are thensaturated with a soft, saturant grade asphalt intendedto fill voids between fibers.

Glass Fiber MatsGlass fiber mats are composed of inorganic, glassfibers. The fibers may be continuous or random andare bonded together with plastic binders or resin.Additionally, these glass fiber mats may be furtherreinforced with chopped glass fiber strands or withcontinuous random or parallel glass fiber strands. Themats are then coated with asphalt.

The following American Society for Testing andMaterials (ASTM) standards apply to asphalt saturatedorganic felt underlayments for slates:

• ASTM D 224, “Standard Specification for Smooth-Surfaced Asphalt Roll Roofing (Organic Felt).” Thisstandard addresses material characteristics andphysical properties and provides four classifica-tions: Type I – minimum net mass per unit area ofroofing 39.8 lb/100 ft2 (1943 g/m2); Type II – mini-mum net mass per unit area of roofing 54.6 lb/100ft2 (2666 g/m2); Type III – minimum net mass perunit area of roofing 51.1 lb/100 ft2 (2495 g/m2);and Type IV – minimum net mass per unit area ofroofing 39.8 lb/100 ft2 (1943 g/m2).

• ASTM D 226, “Standard Specification for Asphalt-Saturated Organic Felt Used in Roofing andWaterproofing. This standard addresses materialcharacteristics and physical properties and pro-vides two classifications: Type I, commonly calledNo. 15 asphalt felt, and Type II, commonly calledNo. 30 asphalt felt.

NRCA recommends that asphalt organic felt underlay-ments meet or exceed the minimum physical propertyvalues listed in ASTM D 224 or D 226, Type II. ASTMD 226 covers asphalt-saturated organic felts with orwithout perforations. When used as underlayment,only non perforated asphalt felts should be used.

Polymer-modified Bitumen SheetUnderlaymentsPolymer-modified bitumen sheet membrane productsare now being used as underlayments in some steep-slope roof systems. Some of these materials aremarketed for use specifically as underlayments for steep-slope roof systems. Others are heavy, non-porous sheets commonly used in low-slope roof systems.

Bitumen used in this type of underlayment is asphaltthat has been modified with polymers. The commonpolymers currently used are atactic polypropylene(APP) and styrene butadiene styrene (SBS). Thesepolymers alter the basic physical characteristics of theasphalt and provide enhanced weathering, aging andsealing characteristics. Polymer-modified bitumenbase sheets are generally reinforced with either a glassfiber or polyester mat. Polymer-modified bitumenproducts specifically marketed for use as underlay-ments range in weight from 35 mils to 90 mils (0.7 mmto 2.3 mm) thick. In some cases, self-adheringpolymer-modified bitumen membranes most oftenused as ice dam protection membranes, are installedover roof decks as underlayments. Self-adheredpolymer-modified underlayments range from 20 mils to60 mils (0.5 mm to 1.5 mm) thick. However NRCArecommends the use of a minimum thickness of 40mils (1.0 mm).

Some of these materials are more resistant towrinkling and distortion upon exposure or after instal-lation and exhibit better watershedding properties thantraditional asphalt-saturated organic felts. As a result,more of these types of underlayments are being usedfor projects where higher degrees of underlaymentperformance are desired.

Ice Dam Protection MembraneAn ice dam protection membrane is a specialized typeof underlayment. This underlayment provides addition-al protection from moisture intrusion along eaveswhere ice dams can occur during winter.

An ice dam protection membrane may consist of:

• a single layer of self-adhering polymer bitumenmembrane

• two plies of No. 30 asphalt-saturated, non perfo-rated felt where the first ply sheet is fastened to adeck and the second ply sheet is adhered to thefirst with roof cement or cold adhesive

• a combination of one heavyweight coated basesheet and one ply of No. 30 asphalt-saturated nonperforated felt where the heavyweight, coatedbase sheet is fastened to a deck and the ply sheetis set in roof cement or cold adhesive.


GREENSTONE SLATE 12


The most common product used for ice damprotection membranes is a self-adhering, polymer-modified bitumen membrane. The bitumen used in this type of membrane is an asphalt that has beenmodified with polymers, typically SBS. These polymers alter the basic physical character-istics of asphalt and provide enhanced weathering andsealing characteristics.

Self-adhering polymer-modified bitumen membranesare generally reinforced with either glass fiber or a thinlayer of polyethylene on the top side. Thesemembranes should not be exposed for extendedperiods of time before the application of the slate.Some polymer-modified bitumen membrane productsincorporate granule surfacings to provide more slip-resistant surfaces for workers.

The product is manufactured with an adhesive on theback side of the membrane to create a self-adheringmaterial. A release paper covers the adhesive layer toprevent the membrane from sticking to itself after it iswound into a roll.

The following ASTM standard is applies to modified bitumen ice dam protection membranes:

• ASTM D 1970, “Standard Specifications for Self-Adhering Polymer Modified Bituminous Sheetmaterials used as Steep Roofing Underlayment forIce Dam Protection.” This standard addressesthickness, tear resistance, adhesion properties,low-temperature flexibility and thermal stability aswell as other physical properties.

NRCA recommends that self-adhering polymer-modified bitumen sheet membranes used as ice damprotection membranes in slate roof systems be aminimum of 40 mils (1.1 mm) thick and comply withASTM D 1970.

Asphalt Roof CementsRoof cements are commonly used in the application of slate roof systems. The base material used in themanufacture of roofing cement is an air-blown asphalt.The asphalt is thinned, or “cut-back,” with a petroleum-based solvent to create a softened,workable mixture. Some roofing cements containmineral fibers as stabilizers. Some manufacturers are now offering modified bitumen roofing cements.

There are two common types of asphalt roofingcement: “flashing cement” and “lap cement.” Flashingcements are commonly used on vertical surfaces andare of a trowelable consistency. Lap cements are used more specifically for bonding asphaltic materialstogether, and their consistency is characterized aseither trowelable or brushable.

Asphalt roof cements are also available in differentgrades. The two most common grades are referred toas winter or summer. The primary difference between

the winter and summer grades is their softening pointtemperature; winter grades have a lower softeningpoint temperature than summer grades.

Common uses for asphalt roof cement in slate roofsystems are:

• to bond two layers of felt underlayment together toform an ice dam protection membrane along theeaves in lieu of a self-adhering modified bitumenmembrane

• as a bedding cement for the purpose of sealingthe base or flange of a metal accessory to a roof

• to provide a temporary seal around roof penetrations or at walls before installing flashingcomponents

• as a bedding cement to secure and seal hip/ridge units

The following ASTM standards apply to asphalt roofcement used as a utility cement or flashing cement:

• ASTM D 2822, “Standard Specification for Asphalt Roof Cement.” This standard addressescomposition, pliability, high-heat behavior andadhesion properties, as well other physical requirements. Type I is a cement composed of alow-softening point asphalt, and Type II is com-posed of a high-softening point asphalt. These classifications are further categorized by the intended application: Class I for dry surfacesor Class II for damp, wet surfaces.

• ASTM D 4586, “Standard Specification for AsphaltRoof Cement, Asbestos Free.” This standardaddresses composition, pliability and high-heatbehavior, as well other physical requirements. TheType I and Type II classifications are the same asthose in ASTM D 2822; however, this standarddoes not differentiate between wet or dry surface usage.

The following ASTM standard applies to asphalt roofcement used as a lap cement:

• ASTM D 3019, “Standard Specification for LapCement Used with Asphalt Roll Roofing, NonFibered, Asbestos Fibered, and Non AsbestosFibered.” Three types of lap cement are described in this standard: Type I, brushing consistency with no stabilizers: Type II, heavy brushing or light troweling consistency with aquantity of shortfibered asbestos stabilizers: Type III, heavy brushing or light troweling consistency with non-asbestos stabilizers.


GREENSTONE SLATE 13

Roof DecksA roof deck provides the structural substrate overwhich a roof system is applied. It also must be able toprovide adequate withdrawal resistance for fastenersused to attach the roof system. Because of the typicallongevity of slate roof systems, a roof deck material ofcomparable long-term expected service life isimportant. Slate roof systems require a structurecapable of supporting heavier loads than most othersteep-slope roof systems. The thickness andstructural characteristics of wood sheathing/deckingshould be increased with thicker layers or heavierslates.

Slate may be applied over the following substrates:

• wood plank

• wood board

• structural wood panels

For new construction, NRCA recommends designersspecify roof deck slopes intended for the applicationof steepslope materials at 4:12 (18 degrees) orgreater. Slate should be applied only over continuousor closely spaced wood plank or board decking andnot over a spaced, or “skipped,” application of woodplank or board decking.

The proper thickness and species of wood deckingrequired for a specific roof is determined by designloads, including uplift, anticipated for the roofassembly and the distance between the supportingmembers. End joints of each piece of decking shouldbe staggered. The end joints, except for matchedends (e.g., tongue-and-groove) should also becentered over supporting members.

Wood Plank and Board DeckingSlate may be applied over wood plank or boarddecking that is not warped, cupped or bowed. Theterms “plank” and “board” are generally differentiatedby thickness. The Western Wood ProductsAssociation (WWPA) defines the term “plank” as “Apiece of lumber, from 2 but not including 5 inches

thick, generally used with wide face horizontal.” Theterm “board” is defined by the WWPA as “… a termgenerally applied to lumber when the nominal size is 1inch thick and 2 or more inches wide.” Wood boardsused for decks should be a minimum of 6 inches (150mm) nominal width.

A wood plank or board roof deck is composed ofsolid-sawn dimensional lumber. It is normallysupported by wood beams — often glue-laminatedtimber or glulams — and/or solid lumber joists, purlinsor rafters. Wood planks or boards may be single ordouble tongue-and-groove, straight-edged, ship-lapped, or grooved for splines or longitudinal edges.

Structural Wood Panel DeckingStructural wood panel decking consists of eitherplywood or oriented strand board (OSB).

Plywood Roof DecksA plywood roof deck is composed of panels made ofthin wood layers called veneers that are peeled fromlogs. The veneers are laid at right angles to each otherthen glued together under heat and pressure. Thiscross-lamination adds strength and stability to all-veneer panels. Panels consist of a number ofcross-laminated layers that vary in number accordingto a panel’s thickness.

Oriented Strand Board (OSB) Roof DecksAn OSB roof deck is composed of panels made fromlayers of compressed, glued wood strands. Thesestrand layers are oriented at right angles to oneanother before being glued and formed into panels.

NRCA is concerned with potential fastener-holdingproblems and dimensional stability because of theeffects of moisture where OSB and other non-veneerproducts are used as roof decking.

All plywood and wood-based panel roof deckssuitable for application should be made from plywoodor wood-based panels rated for structural use as roofsheathing. Most building codes require a label onplywood or wood-based panels ensuring that theplywood or wood-based panel complies with the


DESIGN CONSIDERATIONS

GREENSTONE SLATE 14

The Greenstone Slate Guide to Roof Installations • Design Considerations

criteria set forth in U.S. Product Standards (PS) PS 1-95, “Construction and Industrial Plywood”, for allveneer plywood or PS 2-92, “Performance Standardfor Wood-Based Structural-Use Panels;” | or APA—The Engineered Wood Association (APA) standardPRP-108, “Performance Standards and Policies forStructural-Use Panels,” for structural-use panels (e.g.,oriented strand board, all-veneer plywood).

Performance standards PS 1-95 and PS 2-92, whichwere initiated by APA — The Engineered WoodAssociation, formerly the American PlywoodAssociation, have been developed under the“Procedures for the Development of Voluntary ProductStandards” of the U.S. Department of Commerce.Performance standard PRP-108 was developed byAPA — The Engineered Wood Association.

NRCA recommends that plywood or wood-basedpanels intended for use as roof sheathing meet orexceed the requirements set forth by PS 1-95, PS 2-92 or PRP-108. Some roofing materials manufacturerswill allow application of roof products over other woodsubstrates.

When installing slate roof systems, if plywood is usedas the roof deck material, 5⁄8 inch (15 mm) CDXnominal thickness is the minimum recommended by NRCA.

All wood panel roof decks should consist of panelsrated for structural use as roof sheathing. For particularly demanding applications, such as prefabricated panelized roof deck systems wherecross-panel strength and stiffness or shear propertiesare critical, designers are recommended to use panelsmeeting the standard’s requirements for “Structural IRated Sheathing.”

Wood sheathing panels should be spaced approxi-mately 1⁄8 inch (3 mm) to allow for expansion. Endjoints of wood sheathing panels that do not occur oversupporting members should be blocked to provideadequate support for the sheathing panel ends.

NRCA does not recommend the direct attachment ofslate to gypsum; concrete plank; cementitious woodfiber; or similar, non-wood materials.

Preservative-treated and Fire-retardant-treated WoodCaution should be exercised when roof decks areconstructed of wood that has been treated with an oil-borne preservative. Many roofing materials man-ufacturers recommend that wood roof decks beconstructed with wood that has been treated with anon-oil preservative pressure treatment, or with non-treated air- or kiln-dried lumber. For additionalinformation regarding preservative wood treatment,consult the American Wood Preservers Association(AWPA).

Because of the deterioration of some fire-retardant-treated wood panels caused by chemical reaction,special care should be given to investigate the use offire-retardant-treated wood panel decks in the designof a steep-slope roof system.

Other Deck TypesNRCA recognizes that other types of structuraldecking are becoming more common in steep-sloperoof construction, specifically in the commercialsector; however, NRCA recommends that slates beattached only to structural wood panel decks or towood plank or board decks or batten systems unlessa wire tie attachment system is used. NRCA does notrecommend the direct attachment of slate to gypsum;concrete plank; cementitious wood fiber; or similarnon-wood materials. If deck types or attachmentmethods other than these are used, NRCA suggeststhat a nailing substrate consisting of structural woodpanels, such as plywood, is installed over thestructural deck.

Examples of deck types over which NRCA suggests alayer of plywood are:

• steel roof decks

• poured and precast, structural concrete decks

For ventilation purposes, if necessary, and to allowacceptable clearance for proper fastener penetrationof a plywood nailing substrate, NRCA suggests theuse of wood battens or metal channels attached overa structural deck to raise and separate the plywoodpanels from the deck surface below. The design,placement, spacing, height and attachment of woodbattens or metal channels is the responsibility of thedesigner. A complete roof assembly, including thestructural deck, plywood nailing substrate and roofcovering should be designed to meet local buildingcode requirements. Key factors to consider during thedesign phase are fire resistance, structural loading and wind resistance requirements of the applicablebuilding code.

When installing slate roof systems, NRCA does notrecommend the installation of a nailing substrate overcementitious wood fiber deck panels, poured andprecast lightweight insulating concrete, or poured andprecast gypsum roof decks.

Underlayment and Roof SlopeThe possibility of water migration through a slate roofcovering should be carefully considered. The water-shedding capabilities of a primary roof covering areclosely related to the slope of a roof, dimension of the


GREENSTONE SLATE 15

Design Considerations • The Greenstone Slate Guide to Roof Installations

overlap and headlap, distance between side joints inneighboring courses location of fastener holes, surfaceconditions of the slate and severity of weatherconditions anticipated.

Slate roof coverings are designed for use as multilayered, watershedding roof systems. Slate roofcoverings rely on slope to shed water off a roofsystem’s surface.

Many slate roof systems have outlived the underlayment felts over which they were installed.Therefore, where the long-term watershedding char-acteristics of the underlayment are necessary toprovide the weatherproof integrity of finished roofsystems, designers should carefully consider the typeand quality of underlayments to be specified.

An underlayment should be comparable to the designservice life of a primary steep-slope roof covering andcomponents. Consideration should be given to goodlocal practices, which draw upon experience withvarious underlayment products and methods thathave been developed to address special conditions.

Underlayment ConfigurationsThere are different underlayment configurations thatcan be used for slate roof systems. Generally, theseconfigurations can be categorized as follows:

• single-layer underlayment

• single-layer of self-adhered underlayment

• double-layer underlayment

A “single-layer underlayment” is one layer of underlay-ment fastened to a deck before the application of theslate. In single-layer applications, all underlayment feltsor sheets should be lapped a minimum of 2 inches (50mm) over the preceding sheet. End laps should be aminimum of 4 inches (100 mm). The underlaymentshould be fastened approximately for the slope of theroof, sufficient to hold the felts in place until the instal-lation of primary roof covering materials.

A “single layer of self-adhered underlayment” is onelayer of self-adhering polymer-modified bitumenmembrane applied over the roof deck. Typically, thistype of membrane is designed for use as an ice damprotection membrane. Designers should note thatthese types of membranes, when installed over anentire roof area, tend to act as vapor retarders.Potential problems with ventilation, moisture controland vapor-retarder placement should be consideredduring the design phase.

A “double-layer underlayment system” is two layers of underlayment fastened to a roof deck beforeapplication of slate. When a double-layer underlay-

ment is required, felt should be applied horizontally ata 19 inch (480 mm) overlap and an 17 inch (430 mm)exposure. An underlayment should be fastened suffi-ciently to hold the felts in place until primary roofcovering material is applied.

When a double-layer underlayment configuration isused and where the underlayment layers are identicalmaterials, they are commonly installed in a shingledfashion with a 19 inch (480 mm) overlap and a 17 inch(430 mm) exposure. However, if one layer is used to“dry-in” the building temporarily or underlayments oftwo differing compositions are used, each layer maybe applied in a single layer configuration.

Roof SlopeThe following are recommendations for underlaymentand slate headlap with respect to roof slope:

• for roof slopes of 8:12 (34 degrees) and above, aminimum of one layer of No. 30 underlayment feltor one layer of polymer-modified bitumen under-layment is recommended. See Figures 2 and 3.

Where weather conditions are severe and hard wind-driven rains are common, NRCA recommendsspecifying a minimum of two layers of No. 30 asphalt-saturated felt or one layer of a polymer-modifiedbitumen underlayment with 40 mil (1.0 mm) minimumthickness for use as underlayment with a slate roof.

• for roof slopes of 4:12 (18 degrees) to 8:12 (34degrees), a minimum of two layers of No. 30underlayment or one layer of a polymer- modifiedbitumen underlayment are recommended for useas underlayment with standard sized slate as longas the slate is installed with a 3 inch (75 mm) minimum headlap. See Figures 4 and 5.

NRCA does not recommend the design of slateroofing on slopes less than 4:12 (18 degrees). Atslopes less than 4:12 (18 degrees), slate is adecorative roof covering only, and a weatherproofmembrane installed under the slate is necessary.

Ice Dam Protection MembranesRegardless of the type of underlayment required orroof slope, in locations where the average temperaturefor January is 30° F (-1° C) or less, NRCA suggestsinstallation of an ice dam protection membrane. SeeFigure 6.

An ice dam protection membrane generally is a self-adhering polymer-modified bitumen membrane. NRCA recommends that these types self-adheringmembranes be a minimum of 40 mils (1.0 mm) thickand comply with ASTM D 1970.


GREENSTONE SLATE 16



WOOD DECK

NOTE - ALL DIMENSIONS ARE APPROXIMATE

FASTEN SHEETS

36"(910 mm)

2"(50 mm)

UNDERLAYMENT

A PLY OF SELF-ADHERING UNDERLAYMENT MATERIAL INSTALLED UP TO A POINT 24" (610 mm) OR 36" (910 mm), AS REQUIRED, INSIDE THE EXTERIOR WALL LINE OF THE BUILDING TO PROVIDE AN ICE DAM PROTECTION MEMBRANE

METAL DRIP EDGEAPPLIED OVER UNDERLAYMENT ATRAKE

METAL DRIP EDGE ATDOWNSLOPE EDGEAPPLIED DIRECTLY ON DECK

Figure 3: Example of a single layer of underlayment with an ice dam protection membrane 2"

WOOD DECK


FASTEN UNDERLAYMENT ASNECESSARY TO HOLD IN PLACE

36"(910 mm)

2"(50 mm)

WHEN METAL DRIP EDGE FLASHING IS SPECIFIED IT IS APPLIED OVER UNDERLAYMENT AT RAKE. AT THE EAVE IT IS APPLIED DIRECTLY ON THE DECK OR OVER A NARROW WIDTH OF ASPHALT SATURATED FELT TO SERVE AS A SEPARATOR

Figure 2: Example of single-layer underlayment

GREENSTONE SLATE 17



UNDERLAYMENT LAPPED 19" (480 mm)

FASTEN SHEETS

WOOD DECK

METAL DRIP EDGE AT RAKE APPLIED OVER

UNDERLAYMENT

19"(480 mm)

19"(480 mm)

36"(910 mm)


METAL DRIP EDGE AT AT DOWNSLOPE EDGE

APPLIED DIRECTLY ON DECK

STARTER SHEET

2"(50 mm)

FIRST AND SUCCEEDING COURSES OF UNDERLAYMENT LAPPED 19" (480 mm)

PLY OF SELF-ADHERING UNDERLAYMENT MATERIAL ADHERED UP TO A POINT 24" (610 mm) (OR 36" [910 mm] AS REQUIRED) INSIDE THE EXTERIOR WALL LINE OF BUILDING TO PROVIDE AN ICE DAM PROTECTION MEMBRANE

FASTEN SHEETS

WOOD DECK

METAL DRIP EDGE AT RAKE APPLIED OVER

UNDERLAYMENT

19"(480 mm)

19"(480 mm)

36"(910 mm)


METAL DRIP EDGE AT DOWNSLOPE EDGEAPPLIED DIRECTLY

ON DECK

Figure 4: Example of a double-layer underlayment 19" (480 mm)

Figure 5: Example of a double layer of underlayment with an ice dam protection membrane 19" (480 mm)

GREENSTONE SLATE 18



Underlayment felts, as well as polymer-modifiedbitumen base sheets, cemented together with asphaltroof cement or a cold adhesive can also be used asice dam protection membranes.

An ice dam protection membrane should be appliedstarting at the eaves and extending upslope aminimum of 24 inches (610 mm) from the inside of theexterior wall line of a building. See Figure 7.

Ice dam protection membranes, by themselves,cannot be relied upon to keep leaks from occurring.Careful consideration of roof ventilation, insulation andproject-specific detailing for particular climacticconditions is vital. Also, self-adhering polymer-modified bitumen underlayment must not be leftexposed for long periods of time. Self-adheringpolymer-modified bitumen underlayments should becovered with primary roofing material as soon as

practical to prevent premature degradation of themodified bitumen material. See manufacturers’individual product requirements.

Designers should note that these types ofmembranes, when used as underlayments over anentire roof area, tend to act as vapor retarders.Potential problems with ventilation, moisture controland vapor-retarder placement should be consideredduring the design phase.

FastenersBecause of the long-term service life of slate, seriousconsideration should be given to the type of fastenersto be specified. Several fastener types apply to certainprojects, such as copper slating nails, stainless steel,bronze or cut-brass roofing nails. NRCA suggests theuse of copper slating nails for slate roofs. Unprotected

Figure 6: Approximate area of North America that has an average January temperature below 30° F(-1° C). Map courtesy of Coastal Weather Research Centery, University of Alabama.

black-iron and electroplated nails are not recom-mended. An additional benefit or consideration for theuse of copper is that copper slating nails can be easilycut with a slate ripper when repair work is needed.

Nail shank should not be of larger diameter than thefastener holes in the particular slate being used. Nailsrecommended for most standard-sized slate roofsystems are sharp-point, 3⁄8 inch (9 mm) large flathead, copper-wire slating nails with a smooth, round,barbed or otherwise deformed shank. Generally,thicker slates require the use of larger diameter, longernails. Consideration should also be given to the slopeof a roof, weight of slate, wind loads anticipated, androof sheathing specified all these items relate to slatefastener selection for a specific project.

Fasteners should be long enough to penetrate throughall layers of roofing material and achieve secureanchorage into a roof deck. Fasteners should extendthrough the underside of plywood and penetrate atleast 3⁄4 inch (19 mm) into wood board or plankdecks. The required length of slate fasteners variesaccording to the thickness and exposure of the slatebeing used. For most 3⁄16 inch (5 mm) to 1⁄4 inch (6mm) thick slate, laid with a 3 inch (75 mm) head lapover an underlayment, a 11⁄2 inches (38 mm) or 13⁄4inches (44 mm) nail length is adequate.

Standard-sized slate is fastened with two nails. Allroofing slate should have a minimum of two nails.However, slate that is subject to high-wind conditionsand/or 3⁄4 inch (19 mm) and thicker should befastened with four nails.

Holes are punched from 1⁄4 inch to 1⁄3 inch (6 mmto 8 mm) the length of the slate from the upper end,and 11⁄4 inches to 2 inches (32 mm to 50 mm) infrom the edges. Where four holes are used, it is typicalto punch two additional holes approximately 2 inches(50 mm) above the two regular holes.

When attaching slates, nails should not be driven tightagainst the slate as if to draw the slate tight to thedeck. Slating nails should be driven so that a nail’shead just touches the surface of the slate so the slatehangs on the nail. See Figure 8.

In high-wind areas, a 1 inch (25 mm) dab of flashing-grade roof cement, roofer’s cement or polyurethanesealant placed under the exposed part of the slatenear the leading edge can help secure it.

Other methods of attaching/affixing slate in certainsituations are with slating hooks and wire tie systems.

Exposure and AppearanceThe exposure of slate is the portion of the slate shinglethat is not covered by the course above and is,therefore, the length of the slate roofing unit exposedto the weather. The proper exposure for a particularlength of slate is obtained by deducting the 3 inch (76mm) headlap from the total length of the slate thendividing that number by two. For instance, the properexposure for a 24 inch (610 mm) slate is:

24 inches - 3 inches = 21 inches; 21 inches ÷ 2 = 10 1⁄2 inches

(600 mm - 75 mm = 525 mm ÷ 2 mm = 263 mm)

Table 4 shows the proper exposures for variouslengths of slate if all are to be set with a 3 inch (76mm) headlap.

Slate can be installed to graduate by thickness and/orsize. Thicker and/or longer slates are laid at eavesgraduating to the thinner or smallest size at ridges. Atypical graduation by thickness is 1⁄2 inch (13 mm) to3⁄8 inch (10 mm) to 1⁄4 inch (6 mm). A typical grad-uation by size is 20 inches (500 mm) to 18 inches (450mm) to 16 inches (400 mm) to 14 inches (350 mm).

When multiple colors are used, the percentage ofeach color to be used throughout the roof systemshould be specified. An example is 40 percentunfading green, 40 percent weathering green and 20

percent purple. With regard toweathering slates, some quarries canreasonably predict the percent andintensity of color change from the basecolor to weathered color.

GREENSTONE SLATE 19



24" MIN. (610 mm)

ROOF DECK

ICE DAMPROTECTIONMEMBRANE

UNDERLAYMENT

ROOF COVERING

SNOW

WATER

ICE DAM

Figure 7: Example of ice damming

DRIVEN STRAIGHT, NAIL IS NOT TIGHT AGAINST SLATE

SLATES

UNDERLAYMENT

PLYWOODROOFDECK

Figure 8: Example of proper nailing of slates

GREENSTONE SLATE 20


Starter CourseBefore the first course of slate is installed, a row ofstarter slates is applied along the eave of a roofsystem to serve as the starter course. The starter

course’s primary purpose is to shed water that maymigrate through the joints of the slates in the overlyingfirst course.

The lower edge of the starter course should extendbeyond the downslope perimeter (eave) approximately2 inches (50 mm) to assist in directing runoff awayfrom the fascia board and other underlying buildingcomponents. When gutters or eave troughs are used,the overhang may be reduced to approximately 1 inch(25 mm) or less. Slates should be installed to extendapproximately 1 inch to 2 inches (25 mm to 50 mm)beyond the rake edge.

Starter slates may be applied face down. This allowsthe smooth backs of the starter course and firstcourse of slate contact each other.

Eave CantAn eave cant is necessary to raise the butt edge of astarter and first course of slate the same way theheadlap or third layer of slate raises the butt edge ofall succeeding courses. The thickness of the eave cantshould be about the thickness of the eave slates. Atraditional eave cant is a wood lath 1⁄4 inch (6 mm)thick and 2 inches (50 mm) wide. It is nailed to a deck and covered with eave flashing metal and underlayments. Beveled boards and raised metal eave flashings are also used.

FlashingsBecause steep-slope roof systems are frequentlyinterrupted by the intersection of adjoining roofsections; adjacent walls; or penetrations, such aschimneys and plumbing soil-pipe stacks, all of whichcreate opportunities for leakage, special provisions forweather protection must be made at these locations.These components used to control water entry arecommonly called flashings. Careful attention to

flashing details is essential to successful long-termroof performance regardless of the type of roofconstruction.Flashings in this section are divided into the followingcategories:

• perimeter/edge metal

• penetrations

• valleys

• vertical surfaces

Flashing metals should be made from a material ofthick enough gauge to achieve at least the expecteddesign life of the steep-slope roof covering used with it.

Perimeter Edge MetalDepending on the severity of the climate, anticipatedrainfall and freeze-thaw cycling, the use of perimeteredge metal should be considered.

Where climate or roof edge construction dictates theneed for perimeter edge metal, the type and minimumthickness of the metal should be commensurate withthe anticipated service life for the slate roof system.NRCA suggests metal penetration flashings for slateroof systems be fabricated from one of the followingmetal types and minimum thicknesses.

• 24 gauge (0.025 inch [0.64 mm] thick) prefinishedgalvanized steel

• 24 gauge (0.024 inch [0.61 mm] thick) stainless steel

• 16 ounce (0.022 inch [0.56 mm] thick) copper

• 16 ounce (0.026 inch [0.66 mm] thick) lead coated copper

In some regions, particularly those with mild climates,other types of metal and/or metals of lesser thicknessthan are shown above may be used successfully.NRCA considers these applications to be areapractices. Refer to the Introduction for additionalinformation about area practices.

Penetration FlashingsThere are many other smaller penetrations that needto be flashed into slate roof systems, such asplumbing soil stacks, vents, exhaust fans, furnace ofwater heater flue pipes, electrical standpipes andothers. This is typically accomplished with the use ofsome type of flat flange that extends around thepenetration and is installed under the slate and under-layment on the upslope side of the flange and extendsdown on top of the slate at the downslope side of the flange. Attached and sealed to the flange is acylinder, rectangular box or neoprene gasket that is


Length of Slate Slate Exposure

24” (610 mm) 101⁄2” (265 mm)

22” (560 mm) 91⁄2” (240 mm)

20” (510 mm) 81⁄2” (215 mm)

18” (460 mm) 71⁄2” (190 mm)

16” (410 mm) 61⁄2” (165 mm)

14” (360 mm) 51⁄2” (140 mm)

12” (300 mm) 41⁄2” (115 mm)

10” (250 mm) 31⁄2” (90 mm)

Table 4: Slate exposures with a 3 inch (75 mm) headlap.

GREENSTONE SLATE 21


used to seal around the penetration. The flange canbe set into mastic for additional protection. Theseflashing components are often supplied by othertrades but may be installed by a roofing contractor.See Figure 9.

The type and minimum thickness of the metal used forpenetration flashings should be commensurate withthe anticipated service life for the slate roof system.NRCA suggests metal penetration flashings for slateroof systems be fabricated from one of the followingmetal types and minimum thicknesses.

• 24 gauge (0.024 inch [0.64 mm] thick) prefinished galvanized steel

• 0.032 inch (0.81 mm) thick aluminum

• 0.032 inch (0.81 mm) thick prefinished aluminum

• 24 gauge (0.024 inch [0.61 mm] thick) stainlesssteel


• 16 ounce (0.026 inch [0.66 mm] thick) lead-coated copper

• 4 pound (0.062 inch [1.57 mm] thick) lead.


Valley ConstructionA valley is created at the downslope intersection of

two sloping roof planes. Water runoff from the portionsof the roof areas sloping into the valley flows towardand along the valley. Because of the volume of waterand lower slope along a valley line, this area isespecially vulnerable to leakage. A clear, unobstructeddrainage way is desired in valleys, so the valley maycarry water away quickly and perform successfully forthe life of the roof system.

Where roofs of two equal slopes join to form a valley,the slope of the valley is less than that of the twoadjacent fields of the roof. For example, where twosloped roofs with slopes of 4:12 (18 degrees) intersectat a valley, the actual valley slope is only about 3:12(14 degrees).

With slate roof systems, there are three basic types of valleys:

• open valleys

• closed or mitered valleys

• rounded valleys

These three general types of valleys are constructedonly after the necessary layer(s) of underlayment andany valley lining material specified have been appliedto a deck.

Valley underlayment construction consists of a full-width, 36 inch (900 mm) sheet of No. 30 underlaymentfelt or a polymer-modified bitumen underlayment, basesheet or ice dam protection membrane. This valleyunderlayment is centered in the valley. Valley underlay-ment sheets should be secured with only enoughfasteners to hold them in place until the balance of


METAL FLASHING SLEEVEWITH RAIN COLLAR

STATIC ROOF VENT LOUVERED ROOF VENT TURBINE ROOF VENT

METAL FLASHING SLEEVE

RAIN COLLAR

Figure 9: Examples of common roof penetrations

GREENSTONE SLATE 22


valley materials are applied. The courses of underlay-ment from the fields of two adjoining roof areas areextended so that each course overlaps the valleyunderlayment by at least 12 inches (300 mm). Thevalley is then lined with the balance of the valleyflashing and slate. Another recognized installationmethod is weaving the intersecting underlaymentcourses through the valley extending the underlaymenta minimum of 18 inches (460 mm) beyond the centerline of the valley on each side.

To prevent leakage it is important with all types ofvalley construction to avoid placing fasteners near thecenter of a valley. Generally, slate fasteners should bekept back from the center of a valley by a minimum of8 inches (200 mm).

To avoid fastening too close to the center of valleymetal, slate may be secured by wire-tie attachment.Wire-tie methods of attachment may be used withclosed and open types of valleys. Using wider slatemay provide an alternative to wire-tie attachment ofslates, because all wider slates allow fasteners to beinstalled without penetrating valley flashing metal.

Open ValleysOpen valleys are typically lined with sheet metal. Ametal valley is constructed by installing lengths,typically 8 feet or 10 feet (2.4 m or 3 m) of corrosion-resistant metal through the valley.

The slate and, with some area practices, the underlay-ment is lapped onto the flange on either side of thevalley metal, leaving a clear space between the roofingmaterial to channel runoff water down the valley. SeeFigure 10.

The type and minimum thickness of the metal used inan open valley should be commensurate with theanticipated service life for the slate roof system. NRCAsuggests valley metal for slate roof systems befabricated from one of the following metal types andminimum thicknesses.

• 24 gauge (0.025 inch [0.64 mm] thick) prefinishedgalvanized steel

• 24 gauge (0.024 inch [0.61 mm] thick) stainlesssteel





NRCA also suggests that valley metal be formed witha “W”-shaped splash diverter or rib in the center. Acenter rib can be especially beneficial in valleys whereadjoining roof areas are of unequal slope because therib helps prevent “wash over” of runoff. A center ribshould not be less than 1 inch (25 mm) high. Foreasier installation and controlling thermal expansionand contraction, NRCA suggests valley metal piecesused with slate roofing be no longer than 10 feet (3 m).NRCA recognizes that “V-”shaped valley metalperforms satisfactorily in certain environments but notwhen a valley is formed by two different roof slopes.

NRCA recommends that valley metal used with slatebe a minimum of 18 inches (450 mm) wide. Thismeans flanges on either side of the metal valley centerline are approximately 8 inches (200 mm) wide. Havinga flange width of approximately 8 inches (200 mm)allows the slate to lap onto the flange at least 4 inches(100 mm).

Open valleys permit clear, unobstructed drainage andthey are advantageous in locations where fallout fromsurrounding foliage settles on the roof system andtends to accumulate in the valley. Valley metal shouldbe made from a material thick enough to achieve atleast the expected design life of the steep-slope roofcovering.


Figure 10: Example of open valley flashing

GREENSTONE SLATE 23




COPPER VALLEY FLASHING PIECE (EXTEND 2" (50 mm) UPSLOPE FROM SLATE TO BE OVERLAID, EXTEND DOWNSLOPE 1/2"(13 mm) SHORT OF OVERLYING SLATE)

Figure 11:Example of a closedmitered valley usinginterwoven pieces ofvalley.

Figure 12: Example of aclosed mitered valley

GREENSTONE SLATE 24


Enhanced Open ValleysIn some climates, particularly those in areas prone toaccumulations of snow and ice or with regular freezethaw cycling open valley construction can beenhanced by one of the following procedures:

• lining the valley with a self-adhering polymer-modified bitumen underlayment material beforeapplication of the metal valley

• stripping in flanges on either side of the metal valley with a 9 inch to 12 inch (230 mm to 300mm) strip of self-adhering polymer-modified bitumen underlayment material. The self-adheringmaterial is adhered onto the valley metal flangeand onto an underlying width of similar self-adhering membrane material

• attaching valley flashing metal with cleats ratherthan through-fastening

• tapering the valley so that it is wider at the lowpoint than it is at the high point

Tapering the valley has the following advantages:

• allows for increase in runoff water volume to bereceived at the downslope end

• allows any ice that may form within the valley tofree itself when melting and slide down and exitthe valley rather than lodging somewhere alongthe length of the valley

Closed ValleysIn a closed valley, slate on both sides are cut at an

angle parallel to the center line of thevalley and are butted together, forming amitered joint. In areas of the UnitedStates where heavy accumulations offoliage fallout are anticipated or if mosscan be expected to grow between theslate roofing joints, a closed valley canhamper runoff. Therefore, specifying aclosed valley should be carefullyconsidered to be sure it is appropriate forthe particular project. See Figure 11. Closed valleys for slatemay also be formed by installing the slatetight to the valley line and placingindividual pieces of metal flashing undereach course of slate along the valleycenterline. See Figure 12.

Rounded Valleys(Closed) To form a rounded valley, field slates aretaper-cut and fanned or swept throughthe valley and slates are used to create

the valley surface. Individual metalvalley flashings are installed with eachcourse of slate. Note: Rounded valleysare the least common of the three slatevalley types. Of the three common slate valley types, closed rounded valleys are the most labor-intensive andintricate to construct.

Vertical SurfacesThere are four types of metal flashingsthat are commonly used at locationswhere a roof intersects a vertical wall:

• apron flashing — a metal used at ahead-wall transition, such as thedownslope side of chimney


STEP FLASHING

COUNTERFLASHING

APRON FLASHING WITH HEMMED DRIP EDGE

UPSLOPE CRICKET FLASHING

SLATE ROOFING

Figure 13: Example of a basic sheet-metal flashing components used at chimneys

APRON FLASHING

NOTE - ALL WEIGHTS AND DIMENSIONS ARE APPROXIMATE

APRON FLASHING SIZED TO FIT CHIMNEY. PROVIDE 4" (100 mm) MIN. OVERLAP ONTO THE ADJACENT DOWNSLOPE SLATES

SOLDERED CORNERS

NOTE:UNDERLAYMENT NOT SHOWN FOR CLARITY

Figure 14: Example of an apron flashing at a masonry chimney

GREENSTONE SLATE 25


• step flashing — used at a side-wall transition,such as the side of a dormer

• cricket or backer flashing — used at the upslopeside of a roof penetration, such as a chimney.

• counterflashing — secured to a vertical wall andused to cover and protect the top edge of anapron, step, and cricket or backer flashing.

Figure 13 shows a chimney penetration and the use ofall four flashing types. Generally, before flashings areapplied, an asphalt-saturated feltunderlayment should be applied to aroof deck around roof penetrations.However, in moderate and severeclimates, an ice dam protectionmembrane can be installed aroundthe bases of chimneys or curbs. Ifappropriately specified andconstructed, an ice dam protectionmembrane can assist in keepingwater from migrating into a roofsystem at roof-to-wall intersectionsduring times of severe winter freeze-thaw cycling.

Apron FlashingsApron flashings provide a weather-proofing transition material where aroof area intersects a head wall.Common locations for apron flashingsare the front, downslope, side of adormer or chimney; curbed roof pene-trations; and clerestory transitions.See Figure 14.

The type and minimum thickness ofthe metal used for apron flashingshould be commensurate with theanticipated service life for the slateroof system. NRCA suggests metalapron flashings for slate roof systemsbe fabricated from one of the followingmetal types and minimumthicknesses.

• 24 gauge (0.025 inch [0.64 mm]thick) prefinished galvanized steel

• 24 gauge (0.24 inch [0.61 mm]thick) stainless steel

• 16 ounce (0.022 inch [0.56 mm]thick) copper

• 16 ounce (0.026 inch [0.66 mm]thick) lead-coated copper

• 4 pound (0.062 inch [1.57 mm]thick) lead.


Step FlashingsFor slate roof systems, when a roof area intersects aside wall, flashing is installed at the end of eachcourse of slate.


LAPPED OVER SIDING

ROOF COVERINGNOT SHOWN FOR CLARITY

THIS DETAIL IS TYPICALLY USED FOR WOOD, ALUMINUM AND VINYL SIDING

OPTIONALCORNER

VARIES DUETO WALLCOVERINGTHICKNESS

Figure 16: Example of soldered one-piece flashing at an eave-to-wall interface

UNDERLAYMENT EXTENDING UP SIDEWALL

STEP FLASHING EXTENDING 4" (100 mm) MINIMUM UP WALL

3" (75 mm) MIN. STEP FLASHING HEADLAP

SIDING/CLADDING - - MAINTAIN 1" (25 mm) ABOVE ROOF SURFACEWALL CLADDING/SIDING/

BUILDING PAPER SERVES AS COUNTERFLASHING AND SHOULD OVERLAP STEP FLASHING A MINIMUM OF 2" (50 mm)

SLATE

BUILDING PAPER

NOTE - DIMENSIONS ARE APPROXIMATE

Figure 15: Step flashing at the intersection of a sloping roof and vertical wall

GREENSTONE SLATE 26


For most climactic regions, NRCA suggests usingmetal step flashing that is equal to the length of theslate by 8 inches (200 mm) wide so a minimum stepflashing headlap is achieved and a 4 inch (100 mm)extension is obtained onto each underlying shingleand 4 inches (100 mm) up the vertical surface. SeeFigure 15.

The type and minimum thickness of the metal used forstep flashing should be commensurate with theanticipated service life for the slate roof system. NRCAsuggests metal step flashing for slate roof systems befabricated from one of the following metal types andminimum thicknesses.• 24 gauge (0.025 inch [0.64 mm] thick)

prefinished galvanized steel



• 16 ounce (0.026 inch [0.66 mm] thick)lead-coated copper



The length of step flashing is generally the length of slate.

Special attention needs to be paid at the first (bottom)step flashing where an eave intersects a continuousvertical surface to ensure water is diverted to theoutside of the wall covering. See Figures 16 and 17.

Cricket or Backer Flashings

When a roof area intersects the upslope side of achimney or curbed roof penetration, either a cricket orbacker flashing should be installed at this location. Acricket diverts water around the penetration, and abacker flashing simply provides a weatherproofingtransition material where the roof intersects the backside of the penetration.

NRCA recommends that designers specify crickets at the upslope sides of chimneys or curbed roof penetrations when:

• large volume of water, snow, ice or debris isexpected.

• the chimney or curb is more than 24 inches (610mm) wide.

• the roof slope is 6:12 (27 degrees) or greater.

• the average January temperature is 30° F (-1° C)or lower and significant accumulations of snowand ice are anticipated on the upslope side of thechimney or curb.

The type and minimum thickness of the metal used forcricket or backer flashings should be commensuratewith the anticipated service life for the slate roofsystem. NRCA suggests metal cricket and backer

flashings for slate roof systems befabricated from one of the following metaltypes and minimum thicknesses.

• 24 gauge (0.025 inch [0.64 mm]thick) prefinished galvanized steel

• 24 gauge (0.024 inch [0.61 mm]thick) stainless steel

• 16 ounce (0.022 inch [0.56 mm]thick) copper

• 16 ounce (0.026 inch [0.66 mm]thick) lead-coated copper

• 4 pound (0.062 inch [1.57 mm]thick) lead.

In some regions, particularly those with mild climates, other types of metal and/ormetals of lesser thickness than are shown above may be used successfully.NRCA considers these applications to be area practices. Refer to the Introductionfor additional information about areapractices.


BOTTOM STEP FLASHING LAPPED OVER SIDINGROOF COVERINGNOT SHOWN FORCLARITY

SIDINGTHICK-NESS

THIS DETAIL IS TYPICALLY USED FOR WOOD, ALUMINUM AND VINYL SIDING

Figure 17: Example of a step flashing at an eave-to-wall interface

GREENSTONE SLATE 27



CRICKET

MASONRY CHIMNEY

ROOF DECK

FABRICATED CRICKET / VALLEY INTERSECTION SOLDERED, OR SOFT METAL RIDGE COVER

Figure 18: Example of a wood cricket built on the upslope side of a chimney

Figure 20: Example of a wood cricket roofed-in with slates

Figure 19: Example of a cricket flashing for the upslope portion of a masonry chimney

GREENSTONE SLATE 28


In some instances, the roof deck at theupslope side of a chimney forms a cricket.When this condition occurs, it is generallytreated in the same way as a dormer wheresimilar valley and ridge detailing conditionsoccur, and slates are used to cover the mainsurface area of the cricket. Figures 18, 19and 20 illustrate examples of crickets usedbehind chimneys.

Generally, before any valley metal (whenused) is applied, an asphalt-saturated feltunderlayment should be applied to a roofdeck around a chimney.

However, in moderate and severe climates,before the underlayment is applied, an icedam protection membrane should beinstalled around the base of a chimney. Ifappropriately specified and constructed, anice dam protection membrane can assist inkeeping water from migrating into a roofsystem at the intersection with the chimneyduring times of severe winter freeze-thawcycling.

CounterflashingsApron, step, cricket and backer flashingsrequire some form of counterflashing tocover and protect the top edge of theseflashings from water intrusion. In manyinstances, the wall covering or claddingmaterial performs the counterflashingfunction. Where this does not occur, a metal counterflashing mounted to thevertical wall should be installed along the topedge of the flashing metal. See Figures 21,22 and 23.

The type and minimum thickness of themetal used for counterflashings should becommensurate with the anticipated servicelife for the slate roof system. NRCA suggestsmetal counter flashings for slate roofsystems be fabricated from one of thefollowing metal types and minimumthicknesses.

• 24 gauge (0.025 inch [0.64 mm] thick)prefinished galvanized steel

• 24 gauge (0.024 inch [0.61 mm] thick)stainless steel

• 16 ounce (0.022 inch [0.56 mm] thick)copper

• 16 ounce (0.026 inch [0.66 mm] thick)lead-coated copper


STEP FLASHING

SLATES

WALL

CONTINUOUS COUNTERFLASHING EMBEDDED IN MORTAR JOINT

UNDERLAYMENTTURNED UP WALL

STEP FLASHING

SEALANT TOOLED INTO PLACE

WALLCOUNTERFLASHING

SLATES


Figure 21: Example of a through-wall metal counterflashing embedded in masonry

APPROX.1" NOMINAL

(25 mm)

COUNTERFLASHING

STEP FLASHING

SLATES

SEALANT

WALL

INSERT COUNTERFLASHING WITH UNDERBROKEN TOP HEM FOR

FRICTION FIT.

SOFT METAL WEDGE


Figure 23: Example of a metal counterflashing inset in masonry mortar joint

Figure 22: Example of a surface-mounted metal counterflashing attached to masonry

GREENSTONE SLATE 29



Where the wall cladding counterflashesthe wall flashing metal (e.g., stepflashing), NRCA recommends that thecladding material and underlaymentextend past and cover the top edge ofthe flashing metal a minimum of 2inches (50 mm).

RidgesThere are a variety of standard methods with regional variations that can be usedto finish the ridge of a slate roof. Twocommon materials used to finish theridges on slate roofs are slate andmetal.

Slate RidgeCommon methods of finishing a ridgewith slate are:

• Slate saddle ridge

• Strip saddle ridge

• Combing slate ridge

• Metal ridge

In all slate ridge applications, the ridgeslate can be set in sealant or asphaltroof cement and a secondary method ofsealing the joint at the apex of the ridgeslate is needed.

Slate Saddle RidgeWith a slate saddle ridge, wood nailersare installed at the peak of the roofalong the ridge. The first nailer isinstalled at the thicknesses of the slateto accommodate the last field slate, andthe second nailer is installed at abouttwice the thickness of the slate to acc-ommodate the ridge slate. The fieldslates are installed up the roof to theridge nailers on both sides of the roof. A soft metal or membrane ridge flashingmay be installed over the ridge nailer,and, then, ridge slate is installed per-pendicular to the field slate on each sideof the ridge. As the ridge slate is runacross the ridge and nailed, they over-lap and cover the nails. See Figure 24.

Strip Saddle RidgeThe strip saddle ridge is a variation of the slate saddleridge. It is installed in the same manner as the slatesaddle ridge except the ridge slates do not overlap.Instead they butt against each other. See Figure 25.


COPPER RIDGE NAILS, OR AS SPECIFIED, (LONGER THAN FIELD NAILS)

COPPER RIDGE FLASHING

WOOD NAILER

INJECT AND TOOL IN POLYURETHANE SEALANT, VERTICAL GRADE ASPHALT ROOF CEMENT

COMBING SLATE

FIELD ROOFING SLATE

HEADLAP NOT LESS THAN 3" (75 mm)

VERTICAL GRADE ASPHALT ROOF CEMENT

UNDERLAYMENT WRAPPED OVER RIDGE (FOR NON-VENTED RIDGE DETAILS)

Figure 25: Example of a strip saddle ridge

WOOD NAILER (THICKNESS VARIES ACCORDING TO SLATE USED)


OPTIONAL:VERTICAL GRADE ASPHALT ROOF CEMENT, OR POLYURETHANE SEALANT TO BED ALONG RIDGE JOINTS



WOOD NAILER EACH SIDE

COPPER SLATING NAILS, OR AS SPECIFIED

FIELD ROOFING SLATE

COPPER RIDGE NAILS, OR AS SPECIFIED, (LONGER THAN FIELD NAILS) COVERED BY SUCCEEDING RIDGE SLATE

OPTIONAL:SOFT METAL RIDGE FLASHINGPLACED BETWEEN EACH RIDGE SLATE

RIDGE SLATE

Figure 24: Example of a slate saddle ridge

GREENSTONE SLATE 30


Combing Slate RidgeWith the combing slate ridge, a nailerabout the thickness of the slate isinstalled at the ridge to accommodatethe field slate that is run up the roof tothe ridge on both sides of the roof. Theridge slate is installed in the samemanner as the strip saddle ridge exceptthe ridge slate facing the prevailingweather side is extended beyond theridge line. See Figure 26.

Metal RidgeWith a metal ridge, the nailers and fieldslate are run up to the ridge in the samemanner as the slate saddle ridge. Themetal ridge is fabricated to extend downover the field slate on both sides of theridge. There are many variations of theprofile of metal ridges. See Figure 27.

The type and minimum thickness of themetal used for metal ridges should becommensurate with the anticipatedservice life for the slate roof system.NRCA suggests metal metal ridges forslate roof systems be fabricated fromone of the following metal types andminimum thicknesses.• 24 gauge (0.025 inch [0.64mm]

thick) prefinished galvanized steel

• 24 gauge (0.024 inch [0.61mm]thick) stainless steel

• 16 ounce (0.022 inch [0.56mm]thick) copper

• 16 ounce (0.026 inch [0.66mm]thick) lead-coated copper

In some regions, particularly those withmild climates, other types of metaland/or metals of lesser thickness thanare shown above may be used success-fully. NRCA considers these applicationsto be area practices. Refer to the Intro-duction for additional information aboutarea practices.

There are additional methods to seal theridge of the slate roof by using variousprofiles of tile. These tiles are typically setin a bed or mortar to seal and bond thetile ridge cap.

HipsThere are a variety of standard methodswith regional variations that can be used to


WOOD NAILER SECURED TO DECK (THICKNESS VARIES ACCORDING TO SLATE USED)


METAL RIDGE CAP

WOOD NAILERSECURED TO DECK EACH SIDE

LEAD HEAD RING SHANK NAILS OR COPPER SLATING NAILS (LONGER THAN FIELD NAILS)



FIELD ROOFING SLATE

OPTIONAL :RIDGE FELT OVERLAYMENT COVERING WOOD NAILER

EXAMPLES OF OPTIONAL METAL RIDGE CAPS AND ATTACHMENTS

Figure 27: Example of a metal saddle ridge

BED EDGES IN VERTICAL GRADE ASPHALT ROOF CEMENT OR SLATER'S CEMENT

INJECT AND TOOL IN POLYURETHANE SEALANT, OR VERTICAL GRADE ASPHALT ROOF CEMENT

COMBING SLATES EXTENDED LEEWARD

1/8" TO 1/4" (3 mm TO 6 mm)





FIELD ROOFING SLATE

WOOD NAILERSECURED TO DECK EACH SIDE

OPTIONAL:SOFT METAL RIDGE FLASHING PLACED BETWEEN EACH RIDGE SLATE

Figure 26: Example of a combing ridge

GREENSTONE SLATE 31


finish the hip of a slate roof system. Two commonmaterials used to finish the hips on slate roofs are slateand metal.

Slate HipCommon methods of finishing a hip with slate are:

• Saddle hip

• Mitered hip

• Fantail hip

Saddle HipWith a slate saddle hip,wood nailers areinstalled along the hip.Field slate is installed tothe hip nailers on bothsides of the hip. A softmetal or membrane hipflashing may be installedover the hip, and, then,the hip slate is installedparallel to the hip oneach side of the hip. Thehip slate is run up thehip and secured withnails. They overlap theproceeding hip slate andcover the nails. SeeFigure 28.

Mitered HipWith a mitered hip, fieldslate is installed to thehip line and fit together.Hip slate is the sameslate as the field slateand is in the same planeas the field slate. Widerslates are often used atthe hip. The joint at thehip is sealed with ametal or membrane.Installed course forcourse with the hip slateor with a sealant. SeeFigure 29.

Fantail HipA fantail hip is a variationof the mitered hip. Themitered corner at the buttis cut or rounded to directthe water away from thehip joint and reduce thebreakage of the cornerpoint. See Figure 30.

Metal HipWith a metal hip, nailers and field slate are run up tothe hip in the same manner as the saddle hip. Themetal hip is fabricated to extend down over the fieldslate on both sides of the hip. There are manyvariations of the profile of metal hips. See Figure 31.


*OPTIONAL: SOFT METAL HIP FLASHING BETWEEN EACH COURSE

FIELD SLATE


ROOF DECK

WOOD NAILER (OPTIONAL: COVER WITH FELT PRIOR TO INSTALLING HIP SLATES)HIP SLATES*

(ALIGN WITH SLATE COURSES)

OPTIONAL:VERTICAL GRADE ASPHALT CEMENT

UNDERLAYMENT

OPTIONAL :VERTICAL GRADE ASPHALT CEMENT, OR POLYURETHANE SEALANT ALONG CENTERLINE

FIELD SLATE


ROOF DECK

MITRED FIELD (HIP) SLATES

OPTIONAL:VERTICAL GRADE ASPHALT CEMENT, OR POLYURETHANE SEALANT ALONG CENTERLINE


SOFT METAL HIP FLASHING BETWEEN EACH COURSE

UNDERLAYMENTDOUBLE WRAP HIP WITH UNDERLAYMENT

Figure 29: Example of a mitered hip

Figure 28: Example of a saddle hip

GREENSTONE SLATE www.greenstoneslate.com • 800.619.433332


The type and minimum thickness of the metal used for metal hips should be commensurate with theanticipated service life for the slate roof system. NRCA suggests metal hips for slate roof systems befabricated from one of the following metal types andminimum thicknesses.

• 24 gauge (0.025 inch [0.64 mm] thick) prefinished galvanized steel




• 4 pounds (0.062 inch [1.51 mm] thick) lead



ROOF DECK


UNDERLAYMENT DOUBLE WRAP HIP WITH UNDERLAYMENT

OPTIONAL :VERTICAL GRADE ASPHALT CEMENT, OR POLYURETHANE SEALANT ALONG CENTERLINE

FIELD SLATE

MITRED FIELD (HIP) SLATES

SOFT METAL HIP FLASHING BETWEEN EACH COURSE

FIELD SLATE


ROOF DECK

WOOD NAILER (OPTIONAL: COVER WITH FELT PRIOR TO INSTALLING HIP METAL)

METAL HIP


UNDERLAYMENT

Figure 30: Example of a fantail hip

Figure 31: Example of a metal hip

GREENSTONE SLATE...There is concern with the durability of roofing slate from some quarries. Slate can be tested to recognized standards if the source of a slate at a quarry provides

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