Free Tool
Roofing Underlayment Calculator
Enter your roof's squares to estimate how many rolls of underlayment you'll need.
A roofing square is 100 sq ft of actual (sloped) roof surface. Don’t know yours yet? The Roof Squares & Shingle Calculator works it out from your roof’s footprint and pitch — use its “Roofing squares” number, not the waste-adjusted one, since waste gets applied here instead.
Underlayment type
10-square roll, ~1,000 sq ft — the most common modern choice
Waste factor
A basic gable roof, few or no valleys
Enter your roof area in squares above to see your results.
This covers general field-area waste and overlap only — valleys, hips, and any low-slope sections that need extra underlayment coverage should be sized separately.
Felt vs. synthetic underlayment
Felt underlayment (15 lb or 30 lb, asphalt-saturated paper) is the traditional option and generally the cheapest per roll. Synthetic underlayment (woven polypropylene or polyethylene) is lighter, more tear-resistant, holds up longer if left exposed before the roof covering goes on, and covers far more area per roll — a synthetic roll typically covers 2.5× what a 15 lb felt roll does.
Why coverage-per-roll already includes overlap
Underlayment rows overlap — commonly 2 inches on horizontal laps and 4-6 inches where two rolls meet end to end. Manufacturers already account for this when they publish a roll’s coverage (a “4-square roll” of 15 lb felt covers about 400 sq ft after overlap, not 400 sq ft of raw material). That’s why this calculator doesn’t apply a separate overlap deduction on top of the roll coverage figures.
Valleys and hips need extra
Underlayment typically needs to extend at least 36 inches beyond the centerline on each side of a valley or hip — more material per square foot than a flat field area. The waste factor above covers general cuts and overlap variance across the roof, not a dedicated valley/hip allowance. If your roof has several, size those sections separately and add the result.
Worked example
A 15-square roof, simple waste factor (10%), synthetic underlayment:
- 1. Roof area: 15 × 100 = 1,500 sq ft
- 2. With 10% waste: 1,500 × 1.10 = 1,650 sq ft
- 3. Rolls needed: 1,650 / 1,000 = 1.65 → 2 rolls
Frequently asked questions
What's the difference between felt and synthetic underlayment?
Felt (15 lb or 30 lb, asphalt-saturated paper) is the traditional option and generally the cheapest. Synthetic underlayment (woven polypropylene or polyethylene) is lighter, tears less, handles UV exposure longer if left uncovered, and covers far more area per roll — a synthetic roll typically covers 2.5x what a 15 lb felt roll does.
Do I need to account for overlap separately?
No — the coverage-per-roll figures this calculator uses (400 sq ft for 15 lb felt, 200 for 30 lb, 1,000 for synthetic) are already net of standard overlap. That's how manufacturers publish them, so building in a second overlap deduction would double-count it.
Do valleys and hips need extra underlayment?
Yes — valleys and hips typically need underlayment extended at least 36 inches beyond the centerline on each side, which is more material per square foot than a flat field area. The waste factor here covers general cuts and overlap variance, not a dedicated valley/hip allowance — size those areas separately if your roof has several.
How many squares of roof do I have?
If you haven't measured yet, the Roof Squares & Shingle Calculator on this site computes it from your roof's footprint and pitch — its "squares to order" number is exactly the input this calculator wants.