Units:
Mode:
Actual nominal width of the membrane.
Linear length of one roll.
Percentage of roll width overlapped at side seams.
Percentage of roll length overlapped at end joints.
Cuts, detailing, upstands, damage. Usually 3–10%.
Direct input if overall area is already known.
One side of the rectangle.
Other side of the rectangle.
Choose the direction rolls are laid.
Results
Rolls Required (final)
—
Rounded up to whole rolls.
Coverage Provided
—
After overlaps and waste.
| Item | Value |
|---|
Note: Manufacturer lap dimensions vary. Always confirm recommended laps, primers, and detailing for your specific membrane system.
Example data
Click a row to load its inputs.
| # | Units | Mode | Area / L×W | Width | Length | Side lap % | End lap % | Waste % |
|---|---|---|---|---|---|---|---|---|
| 1 | Metric | Area | 150 m² | 1.0 m | 10 m | 10 | 15 | 5 |
| 2 | Metric | Layout | 20 m × 8 m | 1.2 m | 15 m | 8 | 12 | 7 |
| 3 | Imperial | Area | 2000 ft² | 3 ft | 65 ft | 10 | 12 | 5 |
Formulae used
Effective coverage per roll
weff = w × (1 − sideLap/100)leff = l × (1 − endLap/100)Aroll = weff × leffAnet = Aroll × (1 − waste/100)
By Total Area mode
Narea = ceil( Aproject / Anet )
By Rectangular Layout mode
- Number of courses:
ncourses = ceil( D⊥ / weff ), whereD⊥is the dimension perpendicular to run orientation. - Rolls per course:
nper-course = ceil( Drun / leff ) - Raw rolls:
Ncourse-raw = ncourses × nper-course - With waste:
Ncourse = ceil( Ncourse-raw × (1 + waste/100) )
Final selection
N = max( Narea, Ncourse )
How to use
- Choose your unit system and calculation mode.
- Enter roll width, length, and lap percentages from the datasheet.
- In Area mode, type the total area. In Layout mode, enter project length and width and choose orientation.
- Set an appropriate waste percentage for detailing and offcuts.
- Press Calculate. Export the summary as CSV or PDF, or copy a shareable link.
Tip: Increase end laps for slopes or critical waterproofing zones.
FAQs
Follow the membrane manufacturer’s datasheet. Typical side laps are 75–100 mm and end laps 100–150 mm. Enter as percentages relative to the roll width/length.
Yes. Waste covers offcuts, damage, patches, and detailing. Complex roofs may need higher waste allowances.
Area mode is quick for budgeting. Layout mode respects course counts and roll length, which can increase the roll requirement.
Yes. Toggle to Imperial to work in feet and square feet.
The calculator encodes your inputs in the page URL. Share or bookmark it to reload the same configuration later.
Disclaimers
- This tool provides estimates only and does not replace project-specific detailing.
- Verify substrate prep, primers, and accessory quantities separately.
- Always comply with the membrane system supplier’s published guidance.
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