How much roofing underlayment do I need?
Use roof area, overlap allowance, and roll coverage to estimate whole underlayment rolls.
Short answer
Multiply roof area by the overlap allowance, divide by the product's usable roll coverage, and round up to whole rolls.
For homeowners checking an underlayment order after roof area is known.
How to do it
Enter roof area
Use a measured roof area or a roof plan rather than a rough building footprint.
Account for overlap
Choose an allowance that reflects the product instructions and roof details.
Use roll coverage
Enter the coverage shown on the roll or product information.
Check the work plan
Review eaves, rakes, overlaps, weather, local requirements, and the manufacturer's instructions separately.
Formula
Roof area multiplied by one plus overlap allowance gives required coverage. Divide by roll coverage and round up.
Illustrative example
A 2,000 sq ft roof with a 10% overlap allowance needs 2,200 sq ft of coverage. At 1,000 sq ft per roll, the estimate is 3 rolls.
Common mistakes
- Using roof footprint instead of surface area
- Ignoring overlap
- Using roll coverage from another product
Before you buy
Confirm roll width, length, overlap, eave and rake details, product type, and whether the supplier sells compatible accessories separately.
What this guide does not cover
This is a coverage estimate, not an installation method. Roof geometry, local code, weather, and manufacturer instructions control the actual work.
Use the calculator
Roofing Underlayment Calculator is the primary tool for this guide. You can also browse the other roofing calculators.