ROOFs

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) BUILDING TECHNOLOGY II

The ~aterial of a roof may range from banana leaves, wheaten straw or seagrass to lamininated glass, aluminium sheeting and precast concrete. In many parts of the world ceramic tiles have been the predominant roofing material for centuries.

The construction of a roof is determined by its method of support and how the underneath space is bridged ~u1'd whetrier or not the roof is pitched. The pitch is the ang1e 'afwhich the roof rises from its lowest to highest point. Most domestic architecture, except in very dry regions, has roofs that are sloped, or pitched The durability of a roof is a matter of concern because the roof is often the least accessible part of a building for purposes of repair and renewal, \vhile its damage or destruction can have serious effects.

. Types of roof structures

Roofs can be broadly.classified in three ways .

i) according to the plane of the outer surface, whether horizontal or sloping

ii) According to the structural principles on which their design is based

iii) According to their span

FLAT AND PITCHED ROO . ....:.F-=5

A roof is called a flat roof when the outer surface is horizontal or is inclined a tan angle not .

exceeding 10 degrees and a pitched roof when the outer surface is sloping in one or more directions at an inclination more than this.

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Climate and materials affect the choice between flat and pit~hed roofs. In hot dry areas the flat roof is common because it is not exposed to heavy rainfall. In areas of heavy a steeply sloped roof quickly throws off rain, while areas of heavy snow fall a less steeply pitched roof (35-40 degrees) preserves a useful insulating blanket of snow during the cold season, but thaw water to run off freely.

Coverings for roofs consist if unit materials such as tiles and slates lay close to and over-lapping each other, and a membrane or sheet materials, such as asphalt, bituminous felt or metal sheeting, with sealed or specially formed water-tight joints. With the former open joints necessitate the use of a pitched roof so that the water may run off quickly without passing through the covering.

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BUILDING TECHNOLOGY ll

Membrane materials can be used on pitched or. flat roofs. Sheet metals must be laid at a .

slight slope or fall, and some require the provisi.on of steps or drips at int~rvals down a flat roo[ : .

ROOF TERMINOLOGY

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top of rafters cut and nailed

either side of ridge

continuous ridge board

The ridge is the highest central horizontal part of a pitched roof.

. BUILDING TECHNOLOGY 11

Eaves describe the lowest part of a slope from which rainwater drains to a gutter or to ground.

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A gable end is the triangular part of a wall that is built up to the underside of roof slopes. '

A valley is the intersection of two slopes at right angles.

A hipped end is formed by the intersection of two, ge~erally similar slopes, at right angles.

PITCHED ROOFS

Usual construction of this roof is triangular frames of sloping rafters tied, trussed, together with. horizontal ceiling joists, usually with a system of struts and ties.

Strength and stability of this form of roof depends on the depth of triangular frames at mid span.

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There is an inherent instability across the slopes ofthis roof, parallel to the ridge, to the extent that wind pressur~ may cause the frames to rack or fall over. To resist racking the frames are braced by ga.Ole end walls, hipped ends or by cross bracing of diagonal roof boarding or braces across slopes.

The smaller the unit of roof covering, such as tile or slate, the greater must be the pitch or slope of the roof to exclude rain that runs down in the joints between the tiles. Larger units such as profiled sheets can be laid at a lower pitch than that required for tiles.

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BUILDING TECHNOLOGY II

Couple Roof ' . . .

Simplest form of pitched roof structure consists of timber rafters pitched up from supporting walls to a central ridge. This form of pitched roof is termed as a couple roof as each pair of rafters acts like two arms pinned at the top and the mechanical term for such an arrangement is a couple

ends of rafters . shaped

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common rafters

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roof tends to spread and overturn walls

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When this form of roof is covered with slates or tiles and subject to wind pressure there is a positive tendency for the foot of the rafters to spread and overturn the walls on which they

bear. Spreading of rafters is only weakly resisted by the nailed connection of rafters to .

ridge boa_rd which does not act as an effective tie. Maximum span of this roof is limited to 3.5 m

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Closed Couple Roof

Pitched roofs to small buildings such as bungalows are ftamed with rafters pitched to a central ridge board with horizontal ceiling joists nailed to the side of the foot of each pair of rafters. Ceiling joists serve the dual purpose of ties to resist the natural tendency of rafters to spread and as support for ceiling finishes.

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rafters

ceiling joists nailed to rafters

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BUILDING TECHNOLOGY II

;..-..;.. ridge board ' '

wallplate

The advantage of the triangular space inside the roof above the ceiling joists is that it will to some extent provide insulation, provide ~convenient space for water storage cisterns.

Collar Roof

Another form of tied coupled roof is framed with collars joined across pairs of rafters. The purpose of this arrangement is to extend first floor rooms into the roof space and utilize unused roof space. A disadvantage of this arrangement is that the head of windows formed in a wall will be some distance below ceiling and give less penetration of light. To provide normal height windows a form of half dormer window is often used.

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BUILDING TECHNOLOGY II

Purlin Roof

collar dovetail halved to rafter

ridge board

rafters

wall plate

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To economize in the section of roof rafters it has been practice to provide intermediate support up the slope of roofs by the use of purlins. Purlins are horizontal timbers supported by end walls or struts to internal load-bearing walls to support rafters usually half way up slopes. By the use of a comparatively substantial timber pur lin an .appreciable saving in timber rafter size can be effected.

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ridge board ......,._

rafters on

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BUILDING TECHNOLOGY II

_..__ division wall

corbel to support

purl in purl in

ceiling joists

wallplate ~-- load bearing ~

part1t1on

To limit spread of fire between houses, no combustible material may be built into division or separating wall.

Trussed Rafter Roof

Each pair qf rafters and ceiling joist to a pitched roof was formed as a truss. Trussed rafters are fabricated from light section, stress graded timbers that are accurately cut in shape, assembled and joined with galvanized steel c_onnector plates .

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EAVES

BUILDING TECHNOLOGY II

tiling battens 19 x 38 mm for rafters at 400 mm

/centres and 25 x 38 mm for 540 and 600 mm

galvanised steel gang-nail connector plates

trussed rafters at 400, 450 or 600 mm centres

rafters on wallplate on cavity wall

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-Eaves is a general term used to describe the lowest courses of slates or tiles and the timber supporting them. The eaves of most pitched roofs are made to project some 150 to 300mm beyond the external face of walls .

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Flush eaves

175 x25 mm fascia

flush eaves

BUILDING TECHNOLOGY II

125 x 50 mm rafters

~---- 125 x 50 mm ceiling joists

100 x 75 mm waflpfate

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Projecting eaves are constructed as either open or closed eaves. Open eaves project with the ends of roof rafters exposed beyond the face of the wall below. They provide protection for the wall below and rainwater runs off the edge of slopes to the ground or paved surface below.

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brickwork beam filling

open eaves (no soffit board)

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rafters

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JOIStS

watlplate

cavity wall

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BUILDING TECHNOLOGY 11

DORMER WINDOWS

Dormer windows are framed in a slope of pitched roofs as a vertical window for daylight to rooms inside the roof space whereas roof lights are formed as a glazed opening in the slope of the roof .

projecting

recessed

partly projecting

FLAT ROOFS

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flat ---.... roof

trimming rafter

trimming rafter

r-- head trimmer

t---:H-f-- studs to dormer cheek

In arid, dry climates closer to the equator, a flat roofhas served as protection from mainly the overhead sunduring the day and a roof platform on which to enjoy cooler air at night.

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Flat roof coverings include;

metal sheets e.g. lead, copper and zinc asphalt and bitumen felt

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rolls staggered at drip

conical rolls at 750 mm centres

firring pieces

cop1ng

dpc

drip

copper flat roof

BUILDING TECHNOLOGY II ~

double lock cross welts staggered

apron carried through wall

as dpc

upstand

70 drip at 3 .. 0 m centres

joists

rolls

sheet lead felt

rough boarding insulation board

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rolls spaced at 600 to 800 mm centres

roof joists

lead flat roof

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rolls staggered at drip

conical rolls at 750 mm centres

copng

dpc

drip

firring pieces

copper flat roof

BUILDING TECHNOLOGY II -

double lock cross welts staggered

apron carried through wall

as dpc

upstand

70 drip at 3 .. 0 m centres

joists

rolls

sheet lead felt

rough boarding insulation board

rolls spaced at 600 to 800 mm centres

roof joists

lead flat roof -

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asphalt finished over lead strip nailed to roof

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........ ~: .. asphalt in two coats to a ,, .. =::, . finished thickness of 20 mm : #::.,#~ .

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rough boards firring piece insulation

sheathing felt

asphalt flat roof

Flat roof draina"e

BUILDING TECHNOLOGY II

asphalt dressed over haff-round

wood rofl .. ..

asphalt apron

cavity wall

Flat roofs should be constructed so that the surface has a slight slope or falls towards rainwater outlets

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lead lined gutter

50 x 50 mm gutter bearer lead lined gutter run out through wall and dressed into rain water head

Rainwater outlet . .

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BUILDING TECHNOLOGY II

water outlet in wall lead shute

~ cast iron rain water head

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... BUILDING TECHNOLOGY II

DAMP PROOFING DETAILS FOR PITCHED ROOFS AND FLAT ROOFS

SKIRTI NG TO B~JCK PARAPET

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150rirn minmm

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BUILDING TECHNOLOGY II

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BUILDING TECHNOLOGY !J

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BUILDING TECHNOLOGY 1/

------------ Coping ~~~~2::~----------- Damp-proof coo.rsc =====t-~-. ....._ __ ~ -------~ Slacc or sim Jar cavty clelk"'f

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Damp-proof cor..rsc to drain irtrennally ~ ~ xtcmat f)

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weather:H:oo; covering insulation

vapour control layer boarding

iOists ceifing optional

ROOF GARDENS

J A roof garden is any garden on the roof of a building. PURPOSES OF ROOF GARDEN

-{ Decorative benefit

May provide food

Temperature control

Hydrological benefits

Architectural enhancement

Habitats or corridors for wildlife~ and Recreationalopportunities.

ROOF GARDEN DETAIL

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BUILDING TECHNOLOGY II

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Reference Roof

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BUILDING. TECHNOLOGY II ~

Vegetotion - ----i

Grow&lg medtum

Fllw M&mbrane - --Dnam- u et --~

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Support Panel

Vapour Bamor ._____ S:trucwraJ Support --

R.ooftop Garden

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