How this calculation works
Unfold equal upstands into the roof rectangle. Lay continuous strips in the chosen direction, count overlap-adjusted strips, then count complete strip cuts per roll rather than dividing total area alone.
Strips = 1 + ceil(max(0, covered width − roll width) / (roll width − side lap)); rolls = ceil(strips / floor(roll length / cut length))A worked example
A 10 × 5 m roof with 1 m wide rolls and 0.1 m side laps needs six 10 m strips. A 20 m roll provides two strips, so three rolls are required.
Good to know
Rectangular cutting model, not waterproofing design or warranty approval. No end-to-end splices, corner details, penetrations or offcut reuse. Verify the product’s actual lap and installation requirements.
Reference: OpenStax: rectangle area and volume calculations (opens in a new tab) ↗
The first strip covers more new width
The first strip covers its full roll width. Every additional strip adds only roll width minus side lap. Using effective width for the first strip as well would overstate the strip requirement in some exact-fit cases.
The last strip may extend beyond the required rectangle. Its offcut is not assumed reusable, making this a deliberately conservative full-width strip plan.
Roll length imposes a cutting constraint
Enough total membrane area does not guarantee enough usable cuts. A continuous strip must fit inside the roll length, and only a whole number of those cuts can come from each roll.
If the required strip is longer than a roll, the input is rejected. This model does not invent an end-lap specification or splice detail to bridge the shortage.
Unfolded upstands are a simplified rectangle
The same upstand allowance extends all four sides before strip counting. Real corners and roof details need more than this unfolded rectangle and may require extra material or a different application method.
Try both strip directions when practical. A direction with fewer rolls is still only a material-cutting comparison; the product system, drainage and installation requirements determine whether it is appropriate.
Use the result with the next measurement
The curtain fabric calculator with seams and pattern repeat also counts full-width cuts, but fabric repeats and hems differ from membrane overlaps and should keep their own allowances.
Rectangular cutting model, not waterproofing design or warranty approval. No end-to-end splices, corner details, penetrations or offcut reuse. Verify the product’s actual lap and installation requirements.
A few questions, answered
Can strips be joined end to end?
No, each modeled strip must fit continuously in a roll.
Are edge offcuts reused?
No, each strip consumes full roll width.
Does the model choose compliant lap widths?
No, lap is an entered product-specific assumption.