Roof Underlayment Calculator (Synthetic & Felt)

Roof area is either entered directly, or computed in footprint mode as (building length + 2 x overhang) x (building width + 2 x overhang) x pitch factor, where pitch factor = sqrt(rise^2 + 144) / 12 (pure geometry — the roof surface is the hypotenuse of a rise/run triangle), capped at a 20,000 sq ft resulting roof area. The overhang correction matters because a building's wall-to-wall footprint understates actual roof area — the roof extends past the wall line at every eave and rake. Asphalt shingles require a minimum 2/12 pitch per IRC R905.2.2; from 2/12 up to (but below) 4/12, code requires double-layer underlayment — the underlayment quantity below already doubles automatically when this applies, flagged in the warning above. 1 roofing square = 100 sq ft. 3 bundles covering 100 sq ft is standard for most 3-tab and architectural shingles, but NOT universal — some heavier designer/laminate products use 4 or 5 bundles per square instead (currently set to 3 here), and each option uses that product's own EXACT package coverage rather than assuming every product covers exactly 100 sq ft per square — e.g. Owens Corning Duration MAX's 4 bundles cover 98.4 sq ft, not 100, which this calculator accounts for directly rather than rounding to a nominal figure. A waste allowance (10% simple gable, 15% moderate hip/valley, 18% complex cut-up) is applied to both shingle and underlayment quantities before rounding up to whole bundles/rolls. Starter strip covers both eaves AND rakes (100 ft/bundle, a representative mid-range figure — real products range roughly 50-120 ft/bundle). Ridge/hip cap uses 25 ft/bundle (real products range roughly 20-33 ft/bundle). Drip edge sums eave + rake length in standard 10 ft pieces (rake length may be 0 for a true hip roof). Nails are computed from your entered shingles-per-square figure (default 80, matching most standard products; some products like GAF Royal Sovereign Metric use 64) x 4 (standard) or 6 (high-wind) nails per shingle, converted to pounds at 150 nails/lb (a conservative, fewer-per-lb figure so the computed weight rounds up rather than under-buying). Ice & water shield (optional) is modeled as a simplified 6 ft eave swath plus a 3 ft valley swath at 200 sq ft/roll — a planning simplification, not a substitute for your local code's actual required extent. This calculator computes material quantities only — it does not verify local code compliance, structural roof deck adequacy, or ventilation requirements.