Roof Underlayment Roll Count
Field underlayment rolls over the whole deck.
Example
You enter
- Roof deck area (ft²) 2500
- Coverage per roll (ft², ~1000 synthetic, ~400 felt) 1000
- Lap + waste allowance (percent) 10
You get
- Underlayment rolls to order 3
No published worked example is on file for this calculation. It is checked against a hand-worked derivation of its method and an independent reference test.
Details, formula, and sources
Scoperolls = ceil(area x (1 + lap + waste) / roll coverage). A 2,500 sf deck with 10% laps and waste, on 10-square synthetic rolls (1,000 sf), takes 3 rolls; on 15-lb felt (4 squares, 400 sf) the same deck takes 7 - the roll coverage, set by the product, drives the count. Distinct from eave and valley ice-barrier coverage; the code and manufacturer set the underlayment and lap requirements.
Formula or tablerolls = ceil(roof_area_sf x (1 + lap_waste_pct/100) / roll_coverage_sf).
Source and editionRoofing underlayment roll-count identity by name (area with lap/waste over roll coverage); first-principles count arithmetic.
Free accessThe roll-count arithmetic is public quantity geometry; the underlayment type and lap requirements come from the code and manufacturer.
What governsEstimate. AHJ and licensed professional govern.
API field namesroof_area_sf, roll_coverage_sf, lap_waste_pct, rolls
- Roll coverage ~1,000 sf for synthetic (10 squares), ~400 sf for 15-lb felt (4 squares); the product governsunderlayment manufacturer
- Lap + waste ~10% for head and side laps plus offcuts; the code sets the lapcode / manufacturer