Roofing Felt Calculator
Calculate how many rolls of 15-lb roofing felt you need for your roof area, plus total material cost.
How this calculator works
Standard 15-lb roofing felt is sold in rolls sized to cover 4 roofing squares — 400 square feet — with manufacturer-specified overlap already factored into that rating. This calculator divides your roof’s surface area by 400 sq ft to get the roll count, rounding up since felt is sold as whole rolls.
Formula: Rolls = Roof area (sq ft) ÷ 400 sq ft per roll.
Worked example
A 30 ft × 20 ft roof section (600 sq ft of surface area):
- Rolls: 600 ÷ 400 = 1.50 → rounds up to 2 rolls
- At $25/roll, that’s $50.00.
Coverage reference
| Roof surface area | Felt rolls needed (15-lb) |
|---|---|
| 15 ft × 10 ft | 1 roll |
| 30 ft × 20 ft | 2 rolls |
| 60 ft × 40 ft | 6 rolls |
How to measure your roof
Measure each roof plane’s length and width and add the areas together, using actual sloped surface area rather than the ground footprint. If you’re using heavier 30-lb felt instead of the 15-lb default this calculator assumes, double the resulting roll count since 30-lb rolls cover roughly half the area per roll.
Frequently asked questions
How much does a roll of 15-lb roofing felt cover?
A standard roll of 15-lb felt covers 4 roofing squares — 400 square feet — per roll, with the manufacturer's overlap already built into that rating. This calculator divides your roof area by 400 sq ft to get the roll count, rounding up since felt is sold in whole rolls.
How does 30-lb felt coverage compare to 15-lb?
A 30-lb felt roll is roughly twice as heavy per square foot and typically covers about half the area per roll — around 2 squares (200 sq ft) instead of 4. If you're using 30-lb felt, double the roll count this calculator gives you for 15-lb felt covering the same roof.
Is felt still used under shingles, or has synthetic underlayment replaced it?
Both are still widely used — 15-lb felt remains a common, budget-friendly underlayment choice that meets most shingle manufacturer warranty requirements, while synthetic underlayment has gained ground for its lighter weight, larger roll coverage, and better tear resistance. Check your shingle manufacturer's installation requirements, since some warranties specify which underlayment types qualify.
How much overlap does felt need between courses?
Typically a 2-inch horizontal overlap between courses and a 4-inch overlap at end laps, per most manufacturer and code requirements — already factored into the standard 400 sq ft per roll coverage rating used here. Steeper roofs or ice-prone climates may call for wider overlap or a doubled first course, which would reduce actual coverage per roll.
How much waste should I add for roofing felt?
10% is a standard allowance for valleys, ridges, and trimming around roof penetrations. Since felt rolls are relatively inexpensive per roll, it's common to round up an extra full roll on smaller or more complex roofs rather than risk running short mid-install.
Estimates only. Verify quantities with your supplier before purchasing.