Roofing Underlayment Calculator
Estimate roofing-underlayment rolls, effective coverage, total order area, lap allowance and material cost for a simple gable roof.
Default project preview
5 rolls
Roof surface area
1,421 ft²
Order area
1,563.1 ft²
Effective roll coverage
368 ft²
Purchased coverage
2,000 ft²
Calculator
Build the material order
Your order will appear here
Enter the project details and calculate a rounded material estimate.
Input and output guide
What each value means
The labels and units below are part of this calculator’s visible methodology. Replace every default with measurements or product data from the actual project.
Building length
Enter this project assumption in ft. The current imperial default is 40 ft.
Building width
Enter this project assumption in ft. The current imperial default is 30 ft.
Roof overhang
Enter this project assumption in ft. The current imperial default is 0.5 ft. Minimum: 0.
Roof pitch rise
Enter this project assumption in per 12. The current imperial default is 6 per 12. Minimum: 0.
Published roll coverage
Enter this project assumption in ft². The current imperial default is 400 ft².
Lap and detail loss
Enter this project assumption in %. The current imperial default is 8 %. Minimum: 0. Maximum: 30.
Waste allowance
Enter this project assumption in %. The current imperial default is 10 %. Minimum: 0. Maximum: 50.
Price per roll
Enter this project assumption in a unitless value. The current imperial default is 75. Minimum: 0.
How to read the outputs
Roof surface area
slope included. Default preview: 1,421 ft².
Order area
waste included. Default preview: 1,563.1 ft².
Effective roll coverage
after lap loss. Default preview: 368 ft².
Purchased coverage
published roll area. Default preview: 2,000 ft².
Slope factor
pitch multiplier. Default preview: 1.118.
Material cost
underlayment rolls. Default preview: $375.00.
Methodology
Transparent calculation
BuildMeter converts every input to a consistent internal unit, applies the selected allowance, and rounds products up to whole purchasable units.
Roof surface uses the pitch slope factor. Effective roll coverage = published coverage × (1 − lap loss). Rolls = round up (roof order area ÷ effective coverage).
Reviewed project guidance
How to use this estimate in a real project
Last reviewed: July 29, 2026
Use this calculator to estimate roof-underlayment rolls from roof surface area, effective roll coverage and overlap allowance. Effective coverage should reflect side laps, end laps and the exact product installation method.
Calculation steps
- 1Use the sloped roof surface area.
- 2Include valleys, hips and small waste areas.
- 3Enter effective—not nominal—coverage per roll.
- 4Round the result to full rolls.
Worked numerical example
A 2,400 ft² roof with 12% allowance requires 2,688 ft² of effective coverage. A roll with 1,000 ft² effective coverage requires 3 full rolls.
Purchasing and rounding
Underlayment is purchased by whole roll. Nominal roll area can overstate installed coverage when laps and fastening zones are deducted.
Interpret the result
Extra purchased coverage is useful for transitions, repairs and damaged sections. A calculator cannot determine whether synthetic, felt or self-adhered membrane is required.
Page-specific assumptions
- One underlayment product.
- Uniform lap pattern.
- Roof deck is ready for installation.
- Special eave or valley membranes are estimated separately.
Common mistakes
- Using building footprint instead of roof area.
- Entering nominal roll area.
- Ignoring valley or eave membrane requirements.
- Assuming all underlayments are interchangeable.
Primary and technical references
BuildMeter provides planning estimates, not engineering, architectural, electrical, mechanical, plumbing, permit or code approval. Review the methodology and verify final decisions with product documentation and qualified professionals.
BuildMeter Guides
Understand the measurements and assumptions
Use these visible reference guides before changing project inputs or planning allowances.
FAQ
Roofing Underlayment Calculator questions
Does roll coverage already include overlaps?+
Some manufacturers publish effective coverage and others publish gross roll area. Check the product data before adding a lap-loss factor.
Is ice-and-water membrane included?+
No. Eaves, valleys and penetrations may require separate membrane quantities.
Can the same underlayment be used under every roof covering?+
No. Compatibility and fastening requirements depend on the roof covering and assembly.