About this calculator
This calculator works out how many rolls of breathable (vapour-permeable) roofing membrane a gable roof needs, and how much tape for the laps.
The membrane is laid in courses parallel to the eaves, starting at the eaves and working up, each course lapping over the one below by the head lap so that water runs off. A course runs the full roof length from gable edge to gable edge; where a roll runs out mid-course, the next roll laps the previous one by the end lap, over a rafter. At the ridge, either each slope's top course runs over the ridge onto the other slope, or a separate course is laid along the ridge.
The roll count does not divide the roof area by the roll area. The calculator lays the real courses one after another off real rolls — so a course that does not fit on what is left of a roll starts a new one, and short roll ends are counted as offcut. That is the number you will actually buy.
Counter-battens, which hold the membrane down and form the ventilation gap, are not included: use the roof battens calculator for them.
Formula
roof length = building length + 2 × gable overhang
slope length = given, or (span / 2 + eaves overhang) / cos(pitch)
height per slope = membrane past eaves + slope length + run over the ridge
courses per slope = ceil((height − head lap) / (roll width − head lap))
courses = 2 × courses per slope (+ 1 ridge course)
rolls = lay every course of "roof length" off rolls in order:
a roll end ≥ shortest usable piece continues;
a course split across rolls gets an end lap
tape (end laps) = end laps × roll width
tape (head laps) = 2 × (courses per slope − 1) × roof length (+ 2 × roof length for a ridge course)
tape rolls = ceil(tape / tape roll length)
Worked example
A 10 m building with 300 mm gable overhangs, 5 m slopes, 1.5 m × 50 m rolls, 150 mm head lap, 300 mm end lap, 150 mm over the ridge, end laps taped:
roof length = 10 000 + 2 × 300 = 10 600 mm
height per slope = 0 + 5000 + 150 = 5150 mm
courses per slope = ceil((5150 − 150) / 1350) = ceil(3.70) = 4 → 8 courses
roll 1: 4 courses × 10 600 = 42 400, 7600 left → starts course 5
roll 2: rest of course 5 = 3000 + 300 lap, then courses 6–8 → 14 900 left
rolls = 2, end laps = 1
tape = 1 × 1500 mm → 1 roll of 25 m
Area division would say 106 m² ÷ 75 m² = 1.4 → 2 rolls here as well, but it goes wrong as soon as the course length does not fit the roll well — for 6 m courses on 10 m rolls (keeping only ends of 5 m or more) the courses need 8 rolls, while 48 m ÷ 10 m suggests 5.
FAQ
How big should the laps be? Follow the membrane manufacturer's instructions; they often depend on the pitch. The defaults (150 mm head lap, 300 mm end lap, 150 mm over the ridge) are the figures one manufacturer gives for its membrane and are not a standard.
Should the laps be taped? Many manufacturers require taping, especially for airtightness or a low pitch, and some membranes have integrated tape along the edge. Choose which laps you tape; head laps along an integrated tape edge need no extra tape.
Lap over the ridge or a separate ridge course? Both are used. With a ventilated ridge the membrane is often cut back at the ridge after the roof is covered; the calculator still counts it as laid, because it protects the roof during construction.
Why is there offcut even though the courses are short? Because a roll end shorter than the "shortest usable piece" is not used to start a course. Lower that value if you are happy to join short pieces.
Are counter-battens included? No — they are counted in the roof battens calculator.
Assumptions and limits
No standard is cited. EN 13859-1 is the product standard for flexible roofing underlays; it says nothing about how much to buy. National laying rules (e.g. BS 5534 in the UK) give lap guidance, but every lap here is your input and the calculator was not checked against any one code.
Defaults to replace: 1500 mm × 50 m rolls, 150 mm head lap, 300 mm end lap, 150 mm over-ridge lap, 0 mm past the eaves, 1000 mm shortest usable piece, 25 m tape rolls.
Geometry: a symmetric gable roof with no hips, valleys, roof windows or chimneys; extra membrane and tape around penetrations is not counted. Every course runs the full roof length, including the gable overhangs. Courses are laid in order, slope 1 then slope 2 then the ridge course; the end lap is placed wherever the roll runs out, which on site you shift to the nearest rafter.