Roof Underlayment Calculator
Choose your product or enter the roll size from the label. The calculator adds the slope, subtracts laps to get the real coverage per roll, applies two layers where low-slope shingle roofs require it, and rounds up to whole rolls.
Roof Underlayment Calculator
Find how many rolls of roofing underlayment you need, including laps, waste and the double coverage required on low-slope shingle roofs.
Your results appear here and update as you change the numbers.
How this calculator works
- Roof area = plan area × √(1 + (rise ÷ 12)²).
- Single layer (4/12 and steeper): effective width = roll width − head lap.
- From 2/12 up to 4/12 shingle roofs need two layers (IRC R905.1.1): each course laps the one below by 19 in., so a 36-in. roll covers 17 in.; effective width = roll width ÷ 2 − 1 in.
- Effective roll coverage = effective width × (roll length − end lap).
- Rolls = roof area × (1 + waste) ÷ effective coverage, rounded up.
Worked example
2,000 sq ft of plan area at 6/12 = 2,236 sq ft of roof. A 48 in. × 250 ft synthetic roll with a 4 in. head lap and 6 in. end lap covers 44 ÷ 12 × 249.5 = 915 sq ft. With 10% waste: 2,460 ÷ 915 = 2.7 → 3 rolls; at $120 a roll, $360 (example price).
When to get a professional check
Follow the underlayment and shingle manufacturers’ lap requirements — they can be stricter than code — and add ice barrier at eaves where your local code requires it.
Assumptions used — example values, not yet reviewed against current local prices
| Assumption | Value |
|---|---|
| Synthetic roll price (preset) | $120 /roll |
| #15 felt roll price (preset) | $30 /roll |
| #30 felt roll price (preset) | $30 /roll |
| Ice & water shield roll price (preset) | $90 /roll |
Sources and references
Ready for real prices? Learn how to find and vet a local roofer.
How to choose a contractorResults are planning estimates, not quotes. Prices vary by location, contractor and the condition of your roof. Verify sizes and code requirements locally before buying or building.
Frequently asked questions
How much does a roll of synthetic underlayment cover?
Typically about 10 squares gross (1,000 sq ft) for a 48 in. × 250 ft roll, less after laps.
Do I need two layers?
For asphalt shingles on slopes from 2/12 up to 4/12, yes (IRC R905.1.1).
Is ice and water shield required?
In areas where ice dams are likely, local code usually requires an ice barrier at the eaves.
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