Roofing Underlayment Calculator
Calculate how many rolls of synthetic roofing underlayment you need for your roof area, plus total material cost.
How this calculator works
Synthetic roofing underlayment is sold in rolls sized to cover 10 roofing squares — 1,000 square feet — with standard overlap already factored into that rating. This calculator divides your roof’s surface area by 1,000 sq ft to get the roll count, rounding up since rolls are sold as whole units.
Formula: Rolls = Roof area (sq ft) ÷ 1,000 sq ft per roll.
Worked example
A 30 ft × 20 ft roof section (600 sq ft of surface area):
- Rolls: 600 ÷ 1,000 = 0.60 → rounds up to 1 roll
- At $90/roll, that’s $90.00.
Coverage reference
| Roof surface area | Underlayment rolls needed |
|---|---|
| 15 ft × 10 ft | 1 roll |
| 30 ft × 20 ft | 1 roll |
| 60 ft × 40 ft | 3 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. Because a single roll covers such a large area (1,000 sq ft), smaller roofs will often round up to one roll with substantial leftover material — factor that surplus into your total project material cost rather than expecting to use the whole roll.
Frequently asked questions
How much does one roll of synthetic underlayment cover?
A standard synthetic underlayment roll covers 10 roofing squares — 1,000 square feet — per roll. This calculator divides your roof's surface area by that figure to get the roll count, rounding up since rolls are sold whole.
Is overlap already factored into the 1,000 sq ft coverage rate?
Yes — the rated coverage per roll already accounts for the manufacturer's specified overlap between courses, so you don't need to add extra beyond this calculator's result for standard lap. Steeper roofs or high-wind zones sometimes call for wider overlap than the base rating assumes, which would reduce actual coverage per roll.
Why choose synthetic underlayment over traditional felt?
Synthetic underlayment is lighter per roll, covers more area per roll (1,000 sq ft versus 400 sq ft for standard 15-lb felt), and holds up better to UV exposure and foot traffic if left exposed during construction delays. It typically costs more per roll but often works out cheaper per square of coverage given how much more area each roll covers.
How much waste should I add for underlayment?
10% is a reasonable default, covering roof penetrations, valleys, and the extra course needed at hips and ridges. Complex roofs with many planes and penetrations may need to round up an extra roll beyond the toggle's suggestion, since a partial roll can't be split precisely to the last square foot.
Do I need underlayment under metal roofing too, not just shingles?
Yes — metal roofing systems generally require an underlayment layer as well, often a high-temperature synthetic or self-adhering product rather than standard felt, since metal panels can transmit more heat to the underlayment than shingles do. Check your metal panel manufacturer's specification for the required underlayment type.
Estimates only. Verify quantities with your supplier before purchasing.