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Roof underlayment calculator

Find how many rolls of underlayment your roof needs by area and type.

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Underlayment is the water-resistant layer under your shingles. Enter your roof area and underlayment type to see how many rolls to order.

Rolls needed
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Order a spare roll for overlaps, valleys, and waste.

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How much underlayment to buy

Underlayment coverage per roll varies by type: synthetic covers about 10 squares (1,000 sq ft) a roll, #15 felt about 4 squares, #30 felt and ice-and-water shield about 2 squares. Most roofs use synthetic across the field and ice-and-water at eaves and valleys. Always round up and add a spare for overlaps.

Cost ranges reflect 2026 U.S. installed prices (BLS producer/labor data, Remodeling Cost vs. Value, and manufacturer pricing). Your real price depends on your roof, region, and pro — see the full cost guide.

FAQ

Roof Underlayment Calculator questions

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Divide your roof area by the roll’s coverage. A 2,400 sq ft roof needs about 3 rolls of synthetic (1,000 sq ft each) — always round up for overlaps.
Usually. Synthetic is lighter, tougher, more water-resistant, and covers more per roll. Felt is cheaper up front but tears and wrinkles more easily.
At the eaves, valleys, and around penetrations — anywhere water can back up. In cold climates, code often requires it along the eaves to stop ice-dam leaks.
Field underlayment (synthetic or felt) covers the entire deck. Ice-and-water shield is added on top in the vulnerable spots, not usually across the whole roof.

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