Roof Underlayment Calculator

Roof underlayment is the water-resistant layer installed between your roof deck and shingles — and buying the right number of rolls before the job starts saves time and wasted material. Enter your roof length, roof width, and roof pitch, then specify your underlayment roll width, roll length, side overlap, end overlap, and waste factor to get the rolls needed. Secondary outputs include total roof area, coverage per roll, and area with waste.

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Typical overlap is 4-6 inches

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Account for cuts, mistakes, and irregularities

Results

Rolls Needed

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Total Roof Area

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Coverage per Roll

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Area with Waste

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Frequently Asked Questions

How do I calculate the roof area for underlayment?

Calculate the base area (length × width) and adjust for roof pitch. Higher pitch roofs have more actual surface area than their base footprint.

How much overlap should I use for roof underlayment?

Standard practice is 4-6 inches of side overlap and 6 inches of end overlap. Check your local building codes as requirements may vary.

What waste factor should I include when ordering underlayment?

Include 10-15% waste factor to account for cuts around roof features, mistakes, and material defects. Complex roofs may need higher waste factors.

Does roof pitch affect how much underlayment I need?

Yes, steeper roof pitches have more actual surface area than the base footprint. A 6/12 pitch roof has about 18% more area than its base dimensions.

What is the standard width of underlayment rolls?

Most synthetic underlayment comes in 3-foot or 4-foot wide rolls. Traditional felt underlayment is typically 3 feet wide.

How long are typical underlayment rolls?

Roll lengths vary by manufacturer and type. Synthetic underlayment typically comes in 250-foot rolls, while felt may be 144 or 216 feet.