Roof Underlayment Overlap Calculator
Estimate effective underlayment coverage, rolls and cost after accounting for side laps, end laps, waste and roof area.
Default project preview
7 rolls
Nominal roll area
432 ft²
Effective width
2.67 ft
Effective length
142 ft
Order area
2,640 ft²
Calculator
Build the material order
Your order will appear here
Enter the project details and calculate a rounded material estimate.
Input and output guide
What each value means
The labels and units below are part of this calculator’s visible methodology. Replace every default with measurements or product data from the actual project.
Roof area
Enter this project assumption in ft². The current imperial default is 2400 ft².
Roll width
Enter this project assumption in ft. The current imperial default is 3 ft.
Roll length
Enter this project assumption in ft. The current imperial default is 144 ft.
Side lap
Enter this project assumption in in. The current imperial default is 4 in.
End-lap allowance per roll
Enter this project assumption in ft. The current imperial default is 2 ft.
Waste allowance
Enter this project assumption in %. The current imperial default is 10 %. Minimum: 0. Maximum: 50.
Price per roll
Enter this project assumption in a unitless value. The current imperial default is 95. Minimum: 0.
How to read the outputs
Nominal roll area
before laps. Default preview: 432 ft².
Effective width
after side lap. Default preview: 2.67 ft.
Effective length
after end-lap allowance. Default preview: 142 ft.
Order area
including waste. Default preview: 2,640 ft².
Purchased nominal area
full rolls. Default preview: 3,024 ft².
Material cost
rolls only. Default preview: $665.00.
Methodology
Transparent calculation
BuildMeter converts every input to a consistent internal unit, applies the selected allowance, and rounds products up to whole purchasable units.
Effective coverage per roll = (roll width − side lap) × (roll length − end-lap allowance). Rolls = roof area with waste ÷ effective coverage, rounded up.
BuildMeter Guides
Understand the measurements and assumptions
Use these visible reference guides before changing project inputs or planning allowances.
FAQ
Roof Underlayment Overlap Calculator questions
Why is effective coverage lower than roll area?+
Adjacent courses and roll ends overlap, so part of each roll does not add new roof coverage.
Should valleys and eaves be included?+
Include all areas receiving this product, but calculate separate ice-and-water membrane zones independently.
Can one lap value fit every product?+
No. Follow the exact manufacturer instructions and applicable roof requirements.