Roof Underlayment Rolls Calculator
Find how many rolls of roofing underlayment a roof needs from its area and the coverage of felt or synthetic rolls.
How it works
rolls = ceil( roof area × (1 + overlap%) ÷ coverage per roll )
Underlayment is sold by the roll, each covering a set area before overlaps: a synthetic roll is usually a full 10 squares (1000 sq ft), 15 lb felt about 4 squares, and 30 lb felt about 2. Rows overlap by a few inches, so the roof area is inflated by roughly 10% before dividing by roll coverage. Always use the true sloped area, not the flat footprint, or you will fall short on steep roofs.
Worked example
A 2000 sq ft roof with synthetic underlayment: adding 10% for overlap gives 2200 sq ft, and at 1000 sq ft per roll that is 3 rolls. In 15 lb felt at 400 sq ft, the same roof would take 6 rolls.
Frequently asked questions
Do I use the roof area or the floor area?
The sloped roof area, always. A 6/12 roof has about 12% more surface than its footprint, and a 12/12 roof about 41% more, so run your footprint through a roof-area or pitch calculator first if you only know the ground dimensions.
Synthetic or felt underlayment?
Synthetic is lighter, tougher, and covers far more per roll, so most new work uses it. Felt is cheaper per roll and still common for repairs. The coverage difference is exactly why the roll count swings so much between them.
How much overlap should I add?
Horizontal laps are typically 2 to 4 inches and end laps around 6, plus extra at valleys and hips. Ten percent covers a simple roof; add more for cut-up roofs with many valleys and penetrations.
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Estimates only — add for waste and confirm against local building codes and supplier specs.