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RoofingRoof Underlayment Calculator — How Many Rolls of Felt?
Enter the roof area (the sloped surface, not the footprint) to estimate rolls of 15-lb felt underlayment. Synthetic underlayment covers far more per roll — see the notes.
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How this calculator works
A roll of 15-lb felt covers about 400 sq ft (4 squares) after overlaps. We divide your roof area by 400 and round up. 30-lb felt covers ~200 sq ft; synthetic underlayment often covers ~1,000 sq ft per roll.
What changes how much you need
- 15-lb felt ≈ 400 sq ft/roll; 30-lb felt ≈ 200 sq ft/roll; synthetic ≈ 1,000 sq ft/roll.
- Codes often require an ice-and-water barrier at eaves and valleys — buy that separately.
- Overlap rows ~2–4 inches; the coverage figure already accounts for typical overlap.
- Don’t leave felt exposed for long — it degrades in sun and wind.
Frequently asked questions
How much does a roll of roofing felt cover?
A 15-lb felt roll covers about 400 sq ft (4 squares) after overlaps; 30-lb covers about 200 sq ft.
Felt or synthetic underlayment?
Synthetic is lighter, tougher, and covers far more per roll (~1,000 sq ft), but costs more. Felt is cheaper and fine for many jobs.
Do I need ice-and-water shield too?
Most codes require it along eaves and in valleys in cold climates. Estimate and buy it separately from field underlayment.