Roof Underlayment Calculator
Rolls of roofing underlayment needed.
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ⓘ Live results
Results
- Rolls needed
- 7
- Effective area with overlap
- 2760 sq ft
Quick answer
Roll count is the roof area plus waste for overlaps, divided by the usable coverage of one roll.
rolls = ceil(roof area x (1+waste%) / usable roll coverage)
How to use the Roof Underlayment Calculator
- 1Get the roof area, ideally already adjusted for pitch.
- 2Check roll coverage — synthetic underlayment often covers 400 sq ft (10 squares) per roll.
- 3Add 10-15% for the required horizontal and vertical overlaps.
Worked example
Using the values the calculator loads with:
Inputs
- Roof area2400 sq ft
- Roll coverage400 sq ft
- Overlap/waste allowance15 %
Results
- Rolls needed7
- Effective area with overlap2760 sq ft
Frequently asked questions
How much overlap is needed?
Typically 4 inches horizontal and 6 inches vertical, more in ice-dam zones.
Synthetic or felt?
Synthetic underlayment is lighter, tears less and covers more area per roll than 15 lb felt.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.