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.
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 questions
- 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.
Reference notes
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