Measurement of Roofs and Upper Floors

Contents

1. Generally

2. In situ concrete upper floors 2.1 Concrete 2.2 Reinforcement 2.3 Formwork 2.4 Steel beams

3. Timber upper floors

4. Timber and concrete flat roofs 4.1 Generally 4.2 Timber construction 4.3 Concrete construction

5. Flat roof coverings 5.1 Asphalt coverings 5.2 Built-up felt roofing 5.3 Lead

6. Pitched roofs 6.1 Mensuration 6.2 Slating and tiling 6.3 Profiled sheet roofing 6.4 Construction of timber-framed pitched roofs 6.5 Trusses 6.6 Eaves and rainwater goods 6.7 Openings for chimney stacks, etc.

© The College of Estate Management 2013

Paper 1409V4-0

Measurement of roofs and upper floors

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1 Generally

The measurement of upper floors and of roofs has been grouped together in this paper as flat roofs are measured in a similar way to upper floors. Pitched roofs are considered separately.

2 In situ concrete upper floors

The measurement of in situ concrete upper floor slabs is split into concrete, reinforcement and formwork.

2.1 Concrete

In situ concrete floors are quantified in cubic metres and, according to the RICS New Rules of Measurement: Detailed Measurement for Building Works (NRM2), are included with ‘horizontal work’. The thickness must be stated in one of two bands: less than or equal to 300mm, or greater than 300mm. Horizontal work of the same classification may be aggregated together without the need to identify that which is in floors and that which is in beds, though the option is available to do so, if the taker-off thinks that it will assist the estimator.

Other standard methods of measurement (SMMs) may classify concrete differently.

2.2 Reinforcement

Bar reinforcement is measured lineally and quantified for billing in tonnes with bars of different diameters kept separate. The tonnage is calculated using the lengths and known weight of the bars in kg/m.

In many cases, where reinforcement is to be billed in detail, the engineer will provide a reinforcement/bar bending schedule with the lengths of the bars in each reinforced concrete component identified, along with shape codes and bending dimensions in accordance with BS 8666:2005. Most shape codes have a mathematical formula which uses the bending dimensions and the diameter of the bar to simply calculate the length of the bar.

Where a schedule has not been provided, reinforcement quantities will need to be calculated from the drawn information taking account of any bends, cranks, anchors or hooks in the bar. Example calculations of the additional lengths of an anchor and a hook are shown in Figure 1.

FIGURE 1

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Formation of the anchor and the hook require 5 × and 9 × the diameter of the bar to be added to the overall length of the bar. Twisted or deformed bars are commonly used to increase the bond between the steel and the concrete and these do not generally require anchors or hooked ends.

Bars over 12m long should be given separately and the length should be stated.

Fabric reinforcement is measured superficially and its weight and fabric reference included in the description. Strips required in one width, e.g. tension strips over beams, are separately identified with the width stated.

Note that the volume of reinforcement in a structure is ignored for the purposes of calculating the volume of concrete.

2.3 Formwork

Formwork is measured on the actual surface of the concrete which has to be supported. Formwork may be made from a variety of materials including plywood and steel, but the nature of the formwork is at the contractor’s discretion unless a special finish on the concrete is required, in which case this is identified. Sometimes, for practical reasons (e.g. lack of access making it impossible to remove once the concrete has cured), the lining material is required to be left in position and in these cases it should be given as a separate item. The classification of formwork given in NRM2 should be noted and also the rules for formwork to soffits of horizontal work which require the thickness of the concrete being supported to be stated and propping heights over 3m to be given in stages of 1.5m.

The reinforced concrete slab is usually supported at intervals on either reinforced concrete or steel beams.

Separate formwork is measured to the attached beams or upstand beams stating the shape and, if not square or rectangular, the dimensions.

2.4 Steel beams

Steelwork supporting concrete floors is usually of simple design. The sections are designated by giving the universal beam’s nominal size in mm and the weight in kg/m (e.g. ‘UB 254 × 146 × 37’, where ‘254 × 146’ is the nominal depth and width of the beam in mm and ‘37’ is the weight in kg/m). There are usually several weights in each nominal size and the exact widths and depths of the beams vary according to the weight. This must be borne in mind when calculating the concrete in a beam casing and the formwork, although, as with reinforcement, no deduction is made from the volume of concrete for the space occupied by the steel beam. Steelwork is quantified in tonnes.

In Figure 2, there is a difference of 16mm between the depths of the top and bottom weights of the 457mm × 152mm universal beam.

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Building in ends of steel joists as the work proceeds is deemed included with the brickwork and blockwork.

The following examples show a typical measure for a simple, reinforced concrete floor. No screed or floor covering is taken and it is assumed that these are to be measured in the Finishings section. It is important to make sure that this is the case and dimensions should be annotated to record assumptions.

FIGURE 2

As defined in BS 4–1: 2005

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FIGURE 3

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FLOORS 1

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FLOORS 2

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3 Timber upper floors

Structural timbers are not normally exposed to view in the completed building and are therefore assumed to be sawn on all faces unless otherwise stated. They are also assumed to be nail-fixed unless otherwise required. Timber floor joists are measured as lineal items.

The measurement may be set out as shown below:

It is good practice to arrange billed timbers in order of thickness and, to facilitate this, the thickness has been given first in the descriptions.

Timbers required to exceed 6m long in one piece must be so described and the length stated. Usually this is not necessary and an intermediate joint may be taken, the type of joint depending on the function of the member. If the joint is bolted, the bolts must be enumerated, but the labours to fabricate the joint and any holes are deemed to be included.

Trimmer and trimming joists are usually 25mm thicker than the common joists. The joists are initially measured across the opening and are later adjusted for the opening and for the thicker trimming joists. The trimmer should be measured the full length, including the tenons, for which an addition of 150mm should be made. Ground-floor joists are not normally trimmed as the ends of the joists abutting the hearth are usually supported on a fender wall.

The order for taking off follows the construction: plate or hangers, joists, strutting (measured through the joists), boarding, adjustments for hearths and stairwells.

The method of supporting upper-floor partitions must be considered as it may be necessary to allow for double joists underneath them if they run parallel with the joists.

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FIGURE 4

Drawing No MBW 1/5/1a

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FIGURE 5

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FLOORS 3

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FLOORS 4

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4 Timber and concrete flat roofs

4.1 Generally

Flat roofs are constructed and measured in a similar way to upper floors. They do need a fall to be introduced in order to discharge rainwater into gullies or outlets. For timber roofs, the fall is usually created by timber pieces, called firrings, splay cut along their length, being placed on top of the joists before the boarding is fixed. For concrete roofs, the fall may be achieved by casting the concrete slab with a slopping top surface, although a tapering screed is more often used, as this is more easily laid to a fall.

4.2 Timber construction

The boarding is measured superficially and described as flat roof boarding to falls stating the surface treatment after laying. Firring pieces shall be given in metres and described as supports, stating the width and average depth. The plates, joists, strutting and hangers are all measured as before.

You should examine your SMM for the rules regarding other timber roof components such as gutter boards.

4.3 Concrete construction

Concrete roof slabs are measured in the same way as the upper floors described in Section 2.

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5 Flat roof coverings

5.1 Asphalt coverings

This may be either natural rock asphalt or artificially made from ground limestone and bitumen. The asphalt is often laid on an underlay, which should be included in the description. Special attention must also be drawn to plans of each level, plant and restrictions on siting of asphalt heating pots. The principal items are as follows:

FIGURE 6

m²20mm asphalt roofing, over 500mm wide; laid to 5° falls on felt underlay; in two coats; to timber boarding

m 20mm asphalt vertical fascia 150mm high; to brickwork; with solid water check roll dressed into joint at top and splayed bottom edge

m Point asphalt skirting in cement mortar

m Code … horizontal lead flashing 200mm gt lapped 100mm at joints and including welted edge

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5.2 Built-up felt roofing

This consists of alternate layers of felt and bitumen, normally either two or three layers of felt being used. The felt is marketed in rolls and is specified by the weight (usually of a 10m-long roll). The top layer may be mineral surfaced or the roof may be covered with stone chippings.

Felt roofs are measured superficially, stating the number of layers, base, underlay and method of fixing and any surface covering. Forming laps and welts is separately measured lineally, but any additional felt sheet needed to form the lap is deemed included in the area measured.

FIGURE 7

m²

Built-up bitumen felt roof covering over 500mm wide; in three layers of … kg felt; laid to 5° fall; staggered joints; first layer nailed to softwood boarding with galvanised clout nails and the upper layers continuously bonded with bitumen; treating surface with bitumen and covering with stone chippings

Extra over for:

m forming laps; 50mm

m forming welts; 75mm long; bitumen sealed

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5.3 Lead

Sheet lead for roofing is specified in accordance with BS EN 12588:2006 using colour codes (green, blue, red, black, white, orange) representing sheets of various thicknesses and weights. Previously, code numbers were used (3–8) representing the approximate weight of lead sheets in lbs/ft². These equate with the colour codes and it is still common to see lead specified with code numbers (e.g. ‘code 5’), although the weights will be given in kg/m². Sheets of each thickness are available in various lengths and widths depending on supplier and requirement.

Lead roofing is usually laid in single layers directly onto the base material. Horizontal joints are generally lapped and vertical joints are often rolled over a timber former. Lead roofing is measured superficially with the formation of all laps, welts, drips, seams and rolls measured extra over in metres.

In the final description, it should be noted that although wedging the flashing is deemed to be included, there was a specification requirement to wedge at 1,200mm centres which needed stating. Raking out the joints of the brickwork and pointing is deemed to be included.

Copper, zinc and aluminium roofs are measured using the same measurement rules.

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6 Pitched roofs

The measurement of pitched roofs may be divided into coverings, construction, eaves finishings, rainwater goods, and adjustments for chimney stacks and the like. The coverings are often taken off first so that the surveyor becomes familiar with the general shape and geometry of the roof before dealing with the more detailed measurement of the structural timbers.

6.1 Mensuration

The first measurement required is the slope length and this is calculated as follows according to the information given on the drawing:

1 If the span and rise are given, use Pythagoras’ theorem –

2 If the span and pitch are given, use trigonometry –

The slope length is then used to find the area, which for a gabled roof is twice the product of the length and slope:

S = Span 2

cos

Area = 2 × L × S

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If the ends of the roof are hipped, the area of covering is the same as that of a gabled roof having the same plan, dimensions and rise. This simplification method only applies if the ends have the same angle of slope as the sides.

This principle may be applied to more complex shapes as follows. In both cases, the area of the hipped roof is the same as the sum of the areas of the two gabled roofs, assuming the same plan dimensions and pitch. Beneath the covering, the structures of these roofs will be very different in arrangement as are the labours required to form ridges, hips and valleys.

Proof:

In s EBC and DBC EB = DC = ½ span EC = BD = slope length CEB = CDB = 90º the s are congruent

Similarly s EAC and DAC are congruent DBC + DAC = EBC + EAC = ABC

the area of the two triangles omitted from the gabled roof is thesame as the area of the hipped end if the slope angle is the same.

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The length of hips and valleys may be found graphically if the drawing is to a sufficiently large scale, or alternatively it may be calculated. It must be emphasised that these methods can be applied only if the roof slope is constant.

If the angle of slope of the hip is different from the main roof area, the areas for the deduction from the main roof and the addition for the hip will need to be calculated separately.

1 Graphical method

2 Calculated

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6.2 Slating and tiling

The main area is taken first and the description must include all the particulars given in Section 18 of NRM2. The underlay and battens are included in the description of the slating or tiling but any boarding is measured separately. This has the same area as the coverings and is therefore ‘anded on’ after the principal item. It should be noted that if the coverings are vertical they must be given separately.

The additional labours are then measured lineally in a consistent order which should be adhered to. It is convenient to measure first the boundary work items such as the abutments, eaves, ridges, verges, valleys and hips. This is also the order in which the items are listed in NRM2.

Undercloak courses at eaves or verges (formed of a second layer of cut tiles under the top layer), bedding and pointing in mortar, cutting and forming angles and intersections at abutments are deemed to be included with the coverings, but leadwork and associated items would be taken separately at this stage.

The number of metal soakers is obtained by dividing the length of the abutment by the gauge of the slating or tiling. Typically, the dimensions of the soakers are found as follows:

Flashings are measured linear stating the profile as required in accordance with clause 17.5 and the description should include wedging into grooves, etc., giving a dimension diagram if thought appropriate.

Valleys may be lined with lead or, in the case of tiled roofs, with valley tiles, for example:

The valley tiles bond in with the general tiling and the cutting and bonding is deemed to be included.

Hips may be covered with capping tiles, or mitred or finished with hip tiles which bond in with the general tiling in the same way as the valley tiles described above. Cutting tiles and felt to hips is included with the hip item.

Cutting to the top edge at the ridge is deemed to be included with the slating or tiling.

Length = Lap + Gauge + 25mm Width = 100 + 50 = 150mm

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6.3 Profiled sheet roofing

Profiled sheet roofing and cladding systems are available in numerous profiles and various sheet or panel sizes depending on the manufacturer. Sheets may be manufactured from metal sheet, fibre-cement, plastic, glass reinforced plastic (GRP) or other composite materials. They may be single skin or dual skinned with insulation filler and may be supplied unfinished, galvanised, anodised, painted or coated depending on the product and material. The sheeting is measured superficially and described in accordance with Section 21 of NRM2. Special fitments such as eaves filler pieces, barge boards and ridge cappings are measured lineally. Roof lights and the like are enumerated as extra over the roof covering in which they occur if they are a component part of the roofing system, otherwise they are measured separately. A typical description for traditional fibre-cement sheeting could be:

6.4 Construction of timbered-framed pitched roofs

The carpentry for the construction of timber-framed pitched roofs includes the wall plates, common rafters, hip and valley rafters, ridge, purlins, struts, hangers, ceiling joists and trusses and is measured in accordance with Section 16 of NRM2.

The dimensions of the wall plate should be checked to see if any lengths are over 4.2m and if so, an allowance should be made for joints. This is not a requirement of the method of measurement but is based on marketable lengths of timber. The only timbers required to be kept separate by NRM2 are those exceeding 6m in one continuous length (16.1.1.x.9).

It is often necessary to measure bedding to roof plates where the brickwork is deducted for the volume of the plate. The method of calculating the number of rafters is as follows:

m² Fibre-cement corrugated sheet roofing; 40° pitch; 150mm end laps and one and a half corrugation side laps; fixed to purlins at 900mm centres with 6mm dia galvanised hook bolts and polypropylene washers

Or, for modern profiled composite panels:

m²

Composite panel roof cladding; 30° pitch; outer panel 0.8mm gauge; HPS200 colour-coated; HCFC free insulated core; 0.4mm embossed lining panel with polyester paint finish; fixed to purlins in accordance with manufacturer’s recommendation

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An example for a gabled roof –

The extra rafter is added because the result of the calculation 12.390 ÷ 400 gives the number of spaces between rafters. One final rafter must, therefore, be added for the last in the sequence. This gives 32 pairs of rafters; therefore do not forget to ‘twice’ the quantity for both slopes.

An example for a hipped roof –

In this case it is better to divide the overall length of the roof by the rafter spacing and deduct one to obtain the number of pairs of common rafters.

It may be assumed that the jack rafters in the hips require the same total length of timber as the common rafters of a gabled roof on the same plan dimensions and slope, i.e. aa = bb + cc.

If there is a rafter at the centre of the hipped end this must be added as extra because this takes the place that would be occupied by the ridge in a gabled roof.

All structural timbers are measured lineally and all holes and labours (e.g. for forming notches, joints, mortices and tenons) are deemed to be included.

Wall

2/255

510

13.000

Inner face of wall to centre of end rafter 2/50 100 610

400 ) 12.390 31 + 1 = 32

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Metalwork such as straps, shoes, hangers and bolts are enumerated. For example:

6.5 Trusses

Because there is the opportunity to manufacture them off-site, and owing to their speed of erection, trussed rafters are now commonly used in roof construction, combining rafters and ceiling joists in a single component with intermediate truss members providing additional support and stiffening. In Section 16 of NRM2, roof trusses and the like are to be described and enumerated as engineered or prefabricated items. Unless a manufacturer’s standard size is specified using a catalogue reference or similar, a dimensioned diagram may be the best way to describe the item.

Proprietary trusses and trussed rafters are built up of light timber sections which are joined together by means of nail plates or plywood gussets. These fixings are deemed included in the truss.

6.6 Eaves and rainwater goods

Fascia and barge boards are measured lineally in accordance with Section 16.4. All ends and mitres are deemed to be included in the items to which they relate.

Gutters and rainwater pipes are measured on the centre line over all fittings which are enumerated. Any painting is normally ‘anded on’ after these items.

6.7 Openings for chimney stacks, etc.

No deduction is made for holes or voids in roof coverings not exceeding 1m². For holes and voids not exceeding 1m², the work to form the opening and any boundary work is deemed included with the covering, but it is measurable for those larger than this.

Separate flashings, soakers and the like which are not part of the roof covering are measured next and, lastly, the roof timbers are adjusted.

Nr 50 × 10mm mild steel strap 850mm girth; holed for 12mm dia. bolts

Nr 12mm dia. bolt 190mm long with head and nut; 2 nr 50 × 50 × 3mm plate washers; 2 nr 64mm bulldog plate timber connectors