Enter the size of the building and the roof pitch to see how many 4 × 8 sheets of plywood or OSB it takes to sheath the roof.
How to use the roof sheathing calculator
- Enter the length and width of the building at the walls.
- Enter the pitch as rise in 12 and the overhang at the eaves and rakes.
- Keep 10% waste for a plain gable roof. Use 15% for hips, valleys and dormers.
Formulas
Footprint = (length + 2 × overhang) × (width + 2 × overhang) Pitch multiplier = √(1 + (pitch ÷ 12)²) Roof area = footprint × pitch multiplier Sheets = roof area × (1 + waste %) ÷ 32, rounded up
A 4 × 8 sheet covers 32 ft². The pitch multiplier converts the flat footprint into the sloped surface: a 6 in 12 roof has 11.8% more surface than its footprint and a 12 in 12 roof has 41% more.
Worked example: a 40 × 28 ft house with a 6 in 12 roof
- Footprint with 12 in overhangs: 42 × 30 = 1,260 ft².
- Pitch multiplier: √(1 + 0.5²) = 1.118.
- Roof area: 1,260 × 1.118 = 1,409 ft².
- With 10% waste: 1,550 ft² ÷ 32 = 48.4, so buy 49 sheets.
Questions
What thickness of plywood is used for roof sheathing?
The most common are 7/16 in OSB and 15/32 in (nominal 1/2 in) plywood over rafters or trusses 24 in on center, with 5/8 in used for heavier roofing or higher snow loads. The span rating stamped on the panel and your local code decide.
How many sheets of plywood do I need for a 1,500 sq ft roof?
1,500 ÷ 32 = 46.9, so 47 sheets with no waste and 52 sheets with 10%.
Do I need gaps between sheets?
Panel makers call for a 1/8 in gap at edges and ends so the sheets can expand, and H-clips between rafters where the span rating requires edge support. The gaps are too small to change the sheet count.