Roof Underlayment Roll Count
Field underlayment rolls over the whole deck: rolls = 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 the eave/valley ice-barrier-coverage; the code and manufacturer set the underlayment and lap requirements.
Formula and source
rolls = ceil(roof_area_sf x (1 + lap_waste_pct/100) / roll_coverage_sf).
Roofing underlayment roll-count identity by name (area with lap/waste over roll coverage); first-principles count arithmetic.
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