Roof Underlayment Calculator
Estimate the felt or synthetic underlayment needed for a roof.
Last updated 2026-08-21
What Is a Roof Underlayment Calculator?
A roof underlayment calculator estimates how many rolls of felt or synthetic underlayment you need under your shingles. Underlayment is the water-resistant layer that sits between the roof deck and the shingles. Homeowners use it to budget the underlayment portion of a roofing job and to know what roll size to buy.
How It Works
Rolls = Roof area ÷ Roll coverage. #15 felt covers 432 sq ft; #30 felt covers 216 sq ft.
Example
For a 2,000 sq ft roof with #15 felt: 2000 ÷ 432 = 4.6 → 5 rolls.
How to Calculate
- Find your total roof surface area. For a 2,000 sq ft roof, that is the number you start with.
- Choose your roll size. Standard #15 felt covers about 432 sq ft per roll, while heavier #30 felt covers about 216 sq ft per roll.
- Divide the roof area by the roll coverage. For #15 felt on a 2,000 sq ft roof: 2,000 ÷ 432 = 4.6 rolls.
- Round up to whole rolls and add waste for laps and overlaps. 4.6 rolls rounds to 5 rolls.
- Add a little more for valleys and flashings, where underlayment layers stack and overlap.
Underlayment Coverage per Roll
| Underlayment | Coverage per Roll | Laps to Allow |
|---|---|---|
| #15 felt | 432 sq ft | 10 to 15% |
| #30 felt | 216 sq ft | 10 to 15% |
| Synthetic (10 ft wide) | 1,000 sq ft | 5 to 10% |
| Synthetic (standard roll) | 500 sq ft | 5 to 10% |
| Ice and water shield | 66.7 sq ft | per course |
Sources
Frequently Asked Questions
What is underlayment for?
Underlayment is the water-resistant layer between the roof deck and the shingles.
When should I use #30 instead of #15?
#30 is heavier and often used on low-slope roofs or where more protection is needed.
Do rolls cover exactly their label size?
There is overlap loss at laps. Add 10–15% waste for realistic roll counts.
Estimate the cost with the Roofing Cost Calculator or browse Roofing Calculators