Flat roof: membrane upstands, rooflight deductions and tapered insulation
A flat roof is 32.0 m × 20.0 m. The waterproofing is single-ply membrane, turned up 300 mm at the 104 m parapet perimeter and 150 mm at two rooflights, each 1.50 m × 1.50 m. The tapered insulation runs from 60 mm at the perimeter to 180 mm at the drains; the two rooflight openings are not insulated. Contract rules state: membrane measured net; openings over 0.50 m² deducted; upstands measured separately and added; laps and fixings included in the rate; insulation measured to net area with average thickness, with no deduction for openings under 1.00 m². The contractor has measured the membrane as 640 m² plus 31.2 m² of parapet upstand and has omitted rooflight upstands. The QS must check the membrane and insulation quantities.
1. Calculate the measured membrane area, including all upstands, and identify the contractor’s error.
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Start with the gross roof area: 32.0 × 20.0 = 640.00 m². Each rooflight is 1.50 × 1.50 = 2.25 m², so two rooflights total 4.50 m². Both exceed 0.50 m², so the membrane area is 640.00 − 4.50 = 635.50 m².
Parapet upstand: 104 m × 0.30 m = 31.20 m². Rooflight upstands: each rooflight perimeter is 4 × 1.50 = 6.00 m; two rooflights give 12.00 m; at 0.15 m high this is 1.80 m².
Total membrane = 635.50 + 31.20 + 1.80 = 668.50 m².
The contractor’s 640 + 31.2 = 671.2 m² is wrong for two reasons: it fails to deduct the 4.50 m² of rooflight openings and it omits the 1.80 m² of rooflight upstands. The net error is 671.2 − 668.5 = 2.7 m² over-measure. Laps are not added because they are included in the rate.
2. Calculate the insulation volume using the average-thickness rule and state whether the rooflights are deducted.
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The insulation area rule is different from the membrane rule: no deduction is made for openings under 1.00 m², but each rooflight is 2.25 m², which exceeds 1.00 m². The rooflights are therefore deducted from the insulated area: 640.00 − 4.50 = 635.50 m².
For a linear fall from 60 mm to 180 mm, the average thickness is (60 + 180) / 2 = 120 mm = 0.120 m. Volume = 635.50 × 0.120 = 76.26 m³.
If the contractor used the gross 640 m², the volume would be 640 × 0.120 = 76.80 m³, over-measuring by 0.54 m³. If the contractor used the minimum 60 mm, the volume would be 38.13 m³, which would seriously under-measure the tapered fill. The QS should check that the average is appropriate to the actual slope geometry and drain positions, because a roof falling to multiple drains can have a different average thickness.
3. The contractor argues that laps should be measured because the membrane is 1.5 m wide, and that rooflight upstands are included in the rooflight item. Advise the QS.
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Both arguments should be rejected on the stated rules. Laps are included in the membrane rate; the fact that the roll width creates many laps is a pricing consequence, not a measurement item. The QS should not add a percentage for laps unless the contract expressly requires it. If the contractor believes the lap frequency is abnormally high, that is a rate build-up issue, not a quantity issue.
Rooflight upstands are not included in the rooflight item unless the contract says so. The measurement rules require upstands to be measured separately and added. The contractor omitted them, so the QS should add 1.80 m². The QS should also check that the rooflight opening itself is deducted from the membrane and from the insulation, and that the upstand height is measured from the finished roof surface to the top of the upstand, not double-counted with the rooflight kerb.