Roof underlay Calculator: measuring notes and method
Use surface area and the stated lap
A 1 m wide roll with 100 mm side lap has 0.9 m effective width. The overlapped strip does not cover additional roof area.
Plan the actual roll runs
This area model does not place strips on the roof. Short runs, end laps, valleys and penetrations consume additional material. Check a run layout before deciding whether a partly used roll can cover another section.
Formula
Effective roll area = (roll width − side overlap) × roll length. Rolls = area with waste ÷ effective coverage, rounded up.
Worked examples in metric and imperial units
Illustrative values match Reset example and stay fixed when you edit the calculator. Both columns describe the same calculation; converted inputs are rounded.
Metric worked example
- Total surface area
- 24 m²
- Roll width
- 1 m
- Roll length
- 20 m
- Side lap
- 100 mm
- Waste allowance
- 10 %
Rolls to buy: 2
Imperial worked example
- Total surface area
- 258.3339 ft²
- Roll width
- 3.2808 ft
- Roll length
- 65.6168 ft
- Side lap
- 3.937 in
- Waste allowance
- 10 %
Rolls to buy: 2
Follow the calculation
Lengths in this working are in metres.
- Effective width = 1 − 0.1 = 0.9 m.
- Effective area per roll = 0.9 × 20 = 18 m².
- Divide 26.4 m² including allowance by effective roll area and round up.
Reviewed 19 October 2026. How calculations are checked · Report a correction