Sheathing covers the whole roof
Sheathing (decking) goes over the rafters before any shingle, so its area is the full sloped roof: footprint × pitch multiplier. Sheets are 4×8 = 32 square feet, installed with 1/8-inch gaps and H-clips, and the calculator adds the standard 10% waste for rakes, hips and offcuts.
Thickness and spacing go together
Half-inch OSB or plywood spans rafters 24 inches on center; 5/8-inch is the upgrade for 24-inch spans with heavy shingles, and any rafter spacing over 24 inches needs thicker decking. Roofers sheathe from the bottom edge up, starting with a starter row cut to keep seams staggered.
Sources and standards
Where these figures come from
Every formula on this page is implemented from the published reference material below. Where a value depends on a regional code, the calculator lets you override it rather than assuming one jurisdiction.
- National Roofing Contractors Association — installation standards for steep- and low-slope roof systems.
- FEMA Building Science — wind and snow load guidance for residential roofs.
- American Society of Civil Engineers — ASCE 7, the source of the ground snow load maps used in roof design.
Figures, formulas and reference tables last verified: 19 September 2026.