Felt
Reinforced bitumen membrane (felt) rolls with laps
- where
- effective cover per roll (m²)
- roll width (m)
- side lap (mm)
- roll length (m)
- end lap (mm)
- area including waste (m²)
- area of this layer (m²)
- waste allowance (%)
- rolls needed
Explain this equation
Works out how many rolls of reinforced bitumen membrane (felt) one layer needs once the laps are taken off. Run it once per layer for a two- or three-layer system.
A roll covers less than its full size because each sheet overlaps its neighbour along the side and at the end. Taking the side lap off the width and the end lap off the length (both converted from millimetres) gives the area that each roll really covers. The roof area is increased by the waste allowance and divided by that cover, then rounded up.
Effective cover is a smaller rectangle inside the roll; quantity is area divided by it.
Sense check: a 1 × 7.5 m roll with 75 mm side and 150 mm end laps covers about 6.8 m². 20 m² plus 5% needs 4 rolls.
Watch out: cap sheets and underlayers often come in different roll sizes and use different laps. Work each layer separately.
Works out how many rolls of felt you need for one layer once side and end laps are taken off each roll.
Enter your values to see the result.
Good to know
Work out each layer (underlay, intermediate, cap sheet) separately, as roll sizes and laps differ. IKO’s BS 8217 system guidance asks for cap sheet laps of at least 75 mm at the sides and 100 mm at the ends; the Tecnatorch mineral cap sheet comes in 1 m × 7.5 m rolls with a 75 mm selvedge and 150 mm end laps. Stagger end laps between rows and between layers.
Common mistake
Dividing the area by the nominal roll area. With 75 mm side and 150 mm end laps a 7.5 m² roll only covers about 6.8 m².
Limits
Laps only; cutting around outlets, upstands and complex shapes may need a higher waste allowance. Use the laps on the data sheet of the membrane you use.
Sources
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow the roof covering manufacturer’s fixing specification, BS 5534, BS 8000-6, BS 6229, the Lead Sheet Training Academy guidance and Building Regulations. Wind uplift and structural design need a competent designer.