Roofing Underlayment Calculator: Rolls of Felt or Synthetic
Pick your underlayment type and enter the roof area to get the number of rolls to order. Synthetic underlayment covers far more per roll than felt, so the type you choose changes the count dramatically — a full roof can take 3 rolls of synthetic or 13 rolls of #30 felt.
Calculator
Covering 24.6 squares with synthetic (~10 squares/roll) takes 3 rolls — confirm the roll coverage on your product label.
Underlayment is the water-resistant layer that goes down over the deck before the shingles. It protects the sheathing during the tear-off-to-dry-in window and adds a secondary barrier for the life of the roof. It comes in rolls, and the only thing that changes the roll count is how much each roll covers.
The three common choices carry very different coverage. Synthetic underlayment is light and wide, covering roughly 10 squares per roll. Traditional asphalt-saturated felt covers much less: #15 felt about 4 squares per roll and the heavier #30 felt about 2 squares per roll. Those are typical figures — the exact coverage is printed on the wrapper, and this tool lets you keep the type you actually bought. Sizing underlayment uses the same roof area you used for shingles, so run the bundle calculator alongside it.
Formula
Rolls are squares-with-waste divided by the coverage on the label, rounded up:
squares = roof_sqft ÷ 100 × (1 + waste/100) rolls = ⌈ squares ÷ coverage_per_roll ⌉
- coverage_per_roll — squares covered by one roll: synthetic ≈ 10, felt #15 ≈ 4, felt #30 ≈ 2.
- ⌈ ⌉ — round up, because you buy whole rolls.
Worked example
Cover a 2,236 sq ft roof (10% waste = 24.6 squares) with each type:
- Synthetic (10 sq/roll): 24.6 ÷ 10 = 2.46 → 3 rolls.
- Felt #15 (4 sq/roll): 24.6 ÷ 4 = 6.15 → 7 rolls.
- Felt #30 (2 sq/roll): 24.6 ÷ 2 = 12.3 → 13 rolls.
Same roof, very different roll counts — which is exactly why one wide roll of synthetic often wins on both handling and labor even when the material costs more per square.
Coverage & product notes
Reading and using underlayment coverage well:
- The label is the source of truth. Roll widths and lengths vary by brand; a "10-square" synthetic is typical but not universal. Enter the coverage your product states rather than assuming.
- Overlaps eat coverage. Manufacturers print coverage after a standard head-lap; steeper laps for low-slope sections use more material, which is part of why a small waste allowance is built in here.
- Ice-and-water is separate. Self-adhered membrane at eaves, valleys and penetrations is bought and measured on its own — this tool sizes the field underlayment only.
- Code and warranty matter. Some regions require two layers of felt on low slopes, and some shingle warranties specify an underlayment class. Confirm both before you settle on a type.
Reference table
Rolls to cover 24.6 squares (your roof plus 10% waste) by product:
| Type | Coverage | Rolls |
|---|---|---|
| Synthetic (~10 squares/roll) | 10 squares/roll | 3 |
| Felt #15 (~4 squares/roll) | 4 squares/roll | 7 |
| Felt #30 (~2 squares/roll) | 2 squares/roll | 13 |
Frequently asked questions
How many rolls of synthetic underlayment do I need?
Divide your squares (with waste) by the roll coverage, which is about 10 squares for most synthetic underlayment. A 2,236 sq ft roof at 10% waste is 24.6 squares, so 24.6 ÷ 10 = 2.46, rounded up to 3 rolls. Always confirm the coverage printed on your specific product.
What is the difference between #15 and #30 felt?
Weight and thickness. #30 felt is heavier and tougher than #15, so it resists tearing and holds up better if the deck is exposed for a while — but it covers less per roll (about 2 squares versus 4). That is why the same roof might take 7 rolls of #15 but 13 rolls of #30.
Is synthetic underlayment better than felt?
Synthetic is lighter, wider, more tear-resistant and covers far more per roll, which usually means faster installation and fewer rolls to handle. Felt is cheaper per square and is still accepted everywhere. The right choice depends on budget, local code and your shingle warranty — this tool works with whichever you pick.
Do I add waste to underlayment?
A little. Underlayment overlaps at every horizontal seam and end lap, and you trim it at rakes and around penetrations. A 10% allowance generally covers those laps and off-cuts. Steep head-laps on low-slope areas use more, so bump it up if your roof has long low-slope runs.
Does underlayment coverage include ice-and-water membrane?
No. This calculator sizes field underlayment. Self-adhered ice-and-water membrane at the eaves, in valleys and around penetrations is a separate product bought by the linear foot or by the roll for those specific areas.