Quantities
DPC / membrane roll allowance
- where
- total DPC length (m)
- net run (m)
- number of laps
- specified lap length (mm)
- roll length (m)
Explain this equation
This works out the total length of DPC or membrane needed for a run, including the extra taken by laps, and how many rolls to buy.
r is the net length of the run in metres. n is the number of laps, and l is the lap length in mm. L is the total length of material needed and L_roll is the length of one roll.
Each lap uses an extra lap length of material, so the laps add n × l. The lap length is in mm, so it is divided by 1000 to put it in metres. Dividing the total by the roll length and rounding up, with the ⌈ ⌉ brackets, gives whole rolls.
A 30 m run with four 150 mm laps needs 30.6 m. With 20 m rolls that is 2 rolls.
Watch out: joining rolls creates extra laps, so count those. Corners, stop ends and cavity trays need separate details.
Estimate strip length including a known number of specified laps.
Enter your values to see the result.
Good to know
Enter laps from the planned layout. Roll seams may add laps. Corners, stop ends and cavity trays need separate details; this does not determine compliant lap length.
Sources
- Geometry / arithmetic
Estimates are not design approval or British Standards compliance checks. Structural arch, retaining-wall, wind, foundation, lintel and masonry-capacity design are not performed. Use the project engineer’s specification.