Does one roll always cover the same area?

No. Coverage depends on the selected product and required laps; use manufacturer guidance.

Does this replace roof installation instructions?

No. Follow product instructions and local requirements for installation and detailing.

Roofing calculator

Roofing underlayment calculator

Pitched roof area and whole underlayment-roll quantity using stated roll coverage.

Calculator result and formula

Underlayment rolls to order: 2 rolls

Formula: L × W × √(1 + (rise ÷ 12)²) × (1 + waste) ÷ net coverage

Formula ID: ROOF-UNDERLAY-001 · Version 1.0.0 · Published · Last reviewed Aug 26, 2026.

Inputs

Roof length: Horizontal roof length Default: 30.
Roof width: Horizontal roof width Default: 20.
Pitch rise: Rise for every 12 units of run Default: 6.
Roll coverage: Net coverage after manufacturer-required laps Default: 432.
Waste factor: Allowance for laps, cuts and roof detail Default: 10.

How this calculator works

This calculator converts a pitched roof area into whole underlayment rolls using the selected product's net coverage after required laps.

Measure horizontal roof dimensions and enter pitch rise per 12.
Use net product coverage after overlap requirements, not nominal roll area.
Confirm product instructions and special detailing at valleys, eaves and penetrations.

Worked example: Example: a 30 ft × 20 ft roof at 6:12 has about 671 ft² of area. With 10% allowance and 432 ft² net roll coverage, it rounds to 2 rolls.

Assumptions and limitations

Roof surface is calculated from a rectangular horizontal footprint and rise-per-12 pitch.
Coverage must be the selected product's net usable coverage.
Waste is applied before whole-unit purchase rounding.

Common mistakes to avoid

Confirm the measurement basis and units before using the result.
Keep waste or allowance visible and adjust it for the actual job conditions.
Confirm supplier packaging, delivery increments, specifications, and final site requirements before ordering.

Calculator disclaimer

This planning calculator does not determine structural design, code compliance, installation requirements, or guaranteed pricing. Confirm final requirements with the appropriate responsible professional, authority, manufacturer, and supplier.

Formula review and sources

Internal QA complete · independent external review pending · 1 source reference.

Procore: underlayment coverage guidance

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