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MEASURE ONCE. PLAN YOUR PURCHASE.

Roof Membrane Roll Lap calculator.

Count membrane courses and rolls for supplied roof-plane dimensions and specified side and end laps.

ROOFING

Your inputs

Use measurements from your plan and properties from your chosen product. Any prefilled values are illustrative.

Where to get this value

Measure this dimension in your plan or on site and use the displayed unit. For irregular shapes, divide the geometry into the supported sections.

Where to get this value

Measure this dimension in your plan or on site and use the displayed unit. For irregular shapes, divide the geometry into the supported sections.

Where to get this value

Use the actual usable quantity or dimensions of the product you intend to buy. Check the label and the unit shown here.

Where to get this value

Use the actual usable quantity or dimensions of the product you intend to buy. Check the label and the unit shown here.

Quantity and planning estimate
YOUR ESTIMATE
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A clear estimate starts here.

Enter your dimensions and product details to see the quantity and purchase breakdown.

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Formula and assumptions
BEHIND THE NUMBERS

How this calculation works.

Lap values and roof-plane dimensions must be supplied. Does not select waterproofing details, roofing suitability or safe access.

Courses = 1+ceil(max(width−roll width,0)/(roll width−side lap)); rolls per course use end-lap coverageCalculated with normalised units

A worked example

Illustrative inputs only. Product properties and design dimensions are not recommendations for your project.

Plane length along roll
12 m
Plane width across courses
3 m
Roll length
10 m
Roll width
1 m
Specified side lap
100 mm
Specified end lap
200 mm

Membrane rolls without cross-course offcut reuse: 8 rolls

Before you purchase

Use dimensions from your plan and properties from your chosen product. Any design spacing, section or fixing pattern must already be specified.

Lap values and roof-plane dimensions must be supplied. Does not select waterproofing details, roofing suitability or safe access.

Method: dimensional arithmetic and user-supplied specifications. Software checks: 2026-10-04. Calculation and review policy.

Check your purchase assumptions

Keep nominal roofing dimensions separate from effective coverageAccount for both side and end laps in roof rolls

Keep the project moving.

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