Calculate how many rolls of underlayment your roof needs. Choose felt 15#, felt 30# or synthetic — each has a different roll coverage, and the coverage field stays editable to match your product.

How Much Underlayment Do You Need?

Underlayment goes over the roof deck and under the shingles. It is sold in rolls, and each roll covers a fixed number of square feet depending on the material: felt #15 covers about 432 sq ft (roughly 4 squares), felt #30 about 216 sq ft, and modern synthetic underlayment about 1,000 sq ft (10 squares) per roll.

Underlayment Formulas

Area with waste

Adjusted Area = Roof Area × (1 + Waste)

1,677 × 1.10 = 1,845 sq ft

Rolls needed

Rolls = Adjusted Area ÷ Coverage per Roll

1,845 ÷ 432 = 4.27 → 5 rolls (#15 felt)

Worked Example

A 1,677 sq ft roof with 10% waste needs 1,845 sq ft of underlayment. In #15 felt at 432 sq ft per roll that is 4.27 rolls, so buy 5 rolls. The same roof in synthetic underlayment at 1,000 sq ft per roll needs only 2 rolls — fewer seams and less labor.

Felt vs Synthetic Underlayment

Type	Roll Coverage	Best For	Notes
Felt #15	≈432 sq ft	Standard steep-slope roofs	Lowest cost; tears more easily underfoot
Felt #30	≈216 sq ft	Areas needing extra durability	Heavier; more rolls per square
Synthetic	≈1,000 sq ft	Most new installs	Tear-resistant, slip-resistant, fewer seams

Coverage figures are typical industry values — confirm against the product data sheet before ordering.

What You Need

Your roof surface area (from our Roof Area Calculator), the underlayment type you plan to install, and the roll coverage from the product packaging. Add extra waste for roofs with valleys and hips, where underlayment is cut and overlapped more often.

Assumptions & Limitations

One layer of underlayment is assumed over the whole roof. Steep-slope installations typically require a minimum 2/12 pitch; low-slope and flat roofs use membrane systems that this calculator does not cover. Valleys and penetrations may require additional material beyond the waste factor. Planning purposes only.

Frequently Asked Questions

How many squares does a roll of underlayment cover?

Felt #15 covers about 4 squares (432 sq ft), felt #30 about 2 squares, and synthetic underlayment about 10 squares (1,000 sq ft) per roll.

Do I need to double underlayment in valleys?

Valleys usually get a dedicated valley lining or a double layer of underlayment. Increase your waste factor to 15% or add the valley length separately when ordering.

Is underlayment required under shingles?

Yes — asphalt shingles on a steep-slope roof require an underlayment layer per IRC Chapter 9. It is not optional, and it is not included in the shingle bundle count.

Sources & Methodology

Quantity dataset p2-data-1 (manufacturer packaging specifications, reviewed quarterly).

Formulas, assumptions and rounding rules: Methodology · Last updated: September 2026 · Calculation Engine: p1-v1 · Published by the RoofCalcWorks Editorial Team.

Roofing Underlayment Calculator

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