Inputs to result
Required coverage = sloped roof area × (1 + waste% ÷ 100); rolls = ceil(required coverage ÷ effective installed roll coverage).
Roofing
Roof Underlayment Calculator
Estimate synthetic or felt roofing underlayment roll counts from measured sloped roof area, lap allowance, and installed roll coverage.
Enter measured sloped roof area and manufacturer-stated effective coverage per roll after required laps.
Calculation and worked example reviewed 2026-11-12. See our calculation methodology.
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Roofing
US UNITSEnter your measurements
YOUR ESTIMATE
4 rolls to plan1,320 sq ft material400 sq ft effective coverage per roll.
Use installed coverage after laps, not gross roll area.
Formula and example
How this calculation works
This estimator converts your entered jobsite inputs (measured sloped roof area, extra coverage, effective roll coverage) into consistent US trade units, computes the exact geometric baseline first, and then applies any selected waste or order allowance so you can verify both numbers before purchasing materials.
How does the roof underlayment calculator formula work?
Required coverage = sloped roof area × (1 + waste% ÷ 100); rolls = ceil(required coverage ÷ effective installed roll coverage).
(Roof Underlayment Calculator)where:
- Measured sloped roof area – Entered in sq ft (default reference: 1200 sq ft);
- Extra coverage – Entered in % (default reference: 10 %);
- Effective roll coverage – Entered in sq ft (default reference: 400 sq ft);
Check the math
For 1,200 sq ft of roof, 10% extra coverage and a roll covering 400 sq ft after laps, order 4 rolls.
Quick lookup chart
Roof Underlayment Roll Reference by Roll Type & Sloped Area
Compares synthetic 10-square rolls (~900–950 sq ft net after laps) vs. #15/#30 felt rolls.
| Sloped Roof Area | Synthetic (900 sf net) | #15 Felt (400 sf net) | #30 Felt (200 sf net) |
|---|---|---|---|
| 1,200 sq ft (+10%) | 2 rolls | 4 rolls | 7 rolls |
| 1,800 sq ft (+10%) | 3 rolls | 5 rolls | 10 rolls |
| 2,400 sq ft (+10%) | 3 rolls | 7 rolls | 14 rolls |
| 3,200 sq ft (+10%) | 4 rolls | 9 rolls | 18 rolls |
Before you use this result
Use installed coverage after overlap, not gross roll area. Ice-and-water protection, valleys, eaves and local roof-system requirements may need separate materials.
- Enter net installed coverage after horizontal and vertical lap lines, not the gross unrolled square footage printed on the wrapper.
- Low-slope roofs (2:12 to under 4:12) typically require double-layer overlapped underlayment per building code and shingle warranty rules, doubling roll count.
- Measure eaves, valleys, and penetrations separately for self-adhering ice-and-water membrane.
Questions answered
Calculator FAQ
Can I use the roll's printed gross area?+
Only if it already accounts for the installation overlaps. Otherwise enter the smaller effective installed coverage.
Is ice-and-water shield included?+
No. Calculate that membrane separately for the required roof areas.