Damp Proof Membrane Calculator
Enter the covered area, roll dimensions, overlap allowance and 1–20 supplied area or overlap values. Compare whole rolls and coverage, then copy or download the complete report as TXT. Supply all values for the chosen model. Enter zero explicitly when appropriate. This estimates area quantities; it does not optimize cutting, specify manufacturer laps, or certify roof installation or structure.
A QUICK WALKTHROUGH
How to use this tool
- Enter the area to cover in square metres.
- Enter the membrane roll length and width in metres.
- Choose an overlap allowance; check the required lap with the membrane manufacturer.
- Planes: length, width (m), one pair per line, 1–100
- Extra penetration/upstand area (m²)
- Waste (%)
Overlap-adjusted coverage
The estimate divides the area by roll length × roll width × (1 − overlap allowance / 100), then rounds up to a whole roll. The percentage represents area lost to overlaps; cutting waste is not added separately.
Confirm the required lap
Overlap requirements vary by membrane product and installation. Use the manufacturer’s specified lap and account for complex layouts, penetrations, upstands, and cuts separately.
Planning estimate
This is a material estimate, not an installation specification or guarantee. Calculations run in your browser; inputs are not uploaded or stored. Whole-roll ceilings use exact decimal fractions; other numbers are approximate. The report includes original and parsed inputs, units, model, exact fractions, gross/effective areas, overlap loss, rolls, purchased coverage, remaining coverage and every comparison row. Invalid overlap rows are identified without clamping.
General membrane coverage
Interior effective area = (roll width − side lap) × (roll length − end lap). Required = (sum of plane areas + extra) × (1 + waste / 100). Rolls = ceil(required / effective). Supply all values for the chosen model. Enter zero explicitly when appropriate. This estimates area quantities; it does not optimize cutting, specify manufacturer laps, or certify roof installation or structure.
GOOD TO KNOW
Common questions
Does the allowance include cutting waste?
No. It models only the area allowance for membrane overlaps. Add separate rolls if your layout has substantial cuts or obstacles.
How do I choose the overlap percentage?
Use product guidance and your layout to estimate the area consumed by laps. The calculator does not recommend a percentage.
Does it cover walls or upstands?
Enter the total membrane area you need, including any upstands. This estimate does not calculate those dimensions for you.
Coverage model
Percentage area loss / Side and end laps. Interior effective area = (roll width − side lap) × (roll length − end lap). Required = (sum of plane areas + extra) × (1 + waste / 100). Rolls = ceil(required / effective).