Roofing Underlay Calculator (UK): How Many Rolls of Membrane Do I Need?
By the Professional Roofers team
Updated 2026 · Independent cost guide
Underlay is sold by the roll, but a roll never covers its full face area once you take out the head laps between courses and the end laps where rolls join. Enter your roof size and this works out how many rolls of breathable membrane you actually need, with the laps and a wastage margin built in so you do not come up a strip short near the ridge.
How many rolls of underlay do you need?
Estimate for ordering and quote-checking. Underlay is laid in horizontal courses from the eaves up, each course overlapping the one below, so the covered width of every course is less than the roll width. Always confirm the head lap for your pitch against the membrane manufacturer’s data and BS 5534.
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