Formula
Roof area × (1 + overlap allowance) = required coverage. Divide by roll coverage and round up to whole rolls.
Roofing Underlayment Calculator
Estimate required underlayment coverage, whole rolls, package overage, and optional cost.
How this roofing underlayment estimate works
Underlayment coverage is shaped by roll dimensions and the overlap called for by the product. Enter the label coverage so the roll count reflects what you can actually buy.
Worked 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.
Before you buy
Confirm roll width, length, overlap instructions, eave and rake details, and whether a self-adhered or synthetic product changes the installation sequence.
Assumptions
Defaults are starting points. Replace coverage, density, package yield, spacing, and allowance values with the product or plan you are actually using.
Limitations
This is a coverage estimate, not an installation method. Roof geometry, local code, weather exposure, and manufacturer instructions control the actual work.