Membrane roll calculator

Roofing membrane — breather membrane, roofing felt, underlay — comes in fixed-size rolls, and working out how many to order means accounting for two things that quietly eat into a roll's headline coverage: overlap and wastage. Skip either one and the last row on the roof is the moment you find out.

Overlap eats into coverage

A roll's gross area is length × width, but that's not what actually ends up covering new roof. Every row needs a headlap where it overlaps the row above, and often a sidelap at the joins too — typically 10–15% of the roll, depending on the pitch and the manufacturer's fixing instructions. That overlap is doubled-up material, not new coverage, so it has to come off the roll's effective area before working out how many are needed.

Why the lap changes with pitch

The minimum headlap generally goes up as the pitch comes down, because a shallower roof gives wind-driven rain more of a run to find its way under a lap before gravity pulls it back out. A steep roof might only need a modest lap; a low-pitch roof close to a membrane's minimum working pitch can need considerably more. That's set by the manufacturer for the specific product, not a fixed industry number, so the overlap allowance here should always be checked against the roll's own data sheet rather than assumed.

A worked example

A 50m × 1m roll (50m² gross) with a 10% overlap allowance gives 45m² of effective coverage. For a 200m² roof with a 10% wastage allowance for cuts and offcuts, the target area comes to 220m². Divide 220 by 45 and you get 4.9 — round up, because a part-roll doesn't exist as an order, and that's 5 rolls.

What this isn't

This is a flat-area estimate for a straightforward roof. It doesn't add extra material for valleys, hips, chimneys or rooflights, and it doesn't replace the manufacturer's minimum lap for the specific pitch and exposure — laps typically need to increase on lower pitches. Check the data sheet before ordering, and round generously on a roof with a lot of detail.