Roof sheathing, also called roof decking, is the layer of plywood or oriented strand board (OSB) that is nailed to the rafters or trusses and carries the underlayment and roof covering. It is sold in 4 × 8 foot sheets, so the number of sheets depends on the roof’s surface area, which is bigger than the footprint of the house. This calculator turns the building length, the horizontal run, the pitch and the overhang into sheets.
How to use the roof sheathing calculator
- Enter the building length along the ridge.
- Enter the run: the horizontal distance from the outside of the wall to the ridge, measured in plan.
- Enter the pitch as rise per 12 inches of run (a 6/12 roof rises 6 inches for each foot of run).
- Add the eave overhang, select one or two planes, and keep or change the waste allowance.
Roof sheathing formula
Slope length = (run + overhang) × pitch factor
Deck area = building length × slope length × number of planes
Sheets = ⌈ deck area × (1 + waste) ÷ 32 ⌉
The pitch factor is the hypotenuse of a right triangle whose base is 12 inches and whose height is the rise. At 12/12 it is √2 ≈ 1.414, so the slope is 41% longer than its run.
Worked example: 30 ft long, 12 ft run, 6/12 pitch, 1 ft overhang
- Pitch factor: √(1 + 0.5²) = 1.1180.
- Slope length: (12 + 1) × 1.1180 = 14.53 ft.
- Deck area: 30 × 14.53 × 2 = 872 sq ft (8.7 roofing squares).
- With 10% waste: 959 sq ft ÷ 32 = 29.98 → 30 sheets.
Sheets for common gable roofs (6/12 pitch, 1 ft overhang, 10% waste)
| Roof (length × run) | Slope length (ft) | Deck area (sq ft) | Sheets |
|---|---|---|---|
| 20 × 8 ft | 10.06 | 402 | 14 |
| 24 × 10 ft | 12.30 | 590 | 21 |
| 30 × 12 ft | 14.53 | 872 | 30 |
| 40 × 14 ft | 16.77 | 1342 | 47 |
| 50 × 16 ft | 19.01 | 1901 | 66 |
Panel and nailing guidance
- Span rating. A stamp such as 24/16 means the panel can span 24 inches between rafters on a roof and 16 inches on a floor. Choose a rating that matches the rafter spacing.
- Edge gaps. Leave about 1/8 inch between sheets so that moisture swelling doesn’t buckle the roof.
- Panel clips. On 24 inch rafters, H-clips between rafters support the unsupported edges of thinner panels.
- Fasteners. Nail on a regular schedule; typically closer on the edges than in the field. Your local code sets the nail size and spacing.
- Stagger the joints. End joints should land over a rafter and be offset between courses.
Reducing waste
A simple gable roof with dimensions that are a multiple of 4 feet wastes little. Complex roofs with hips, valleys and dormers waste more because many sheets are cut at angles. Offcuts from the rake edge are often good for the next course. Cheaper sheets with rough edges cost less but give more rejects.
To get the rafter length itself, use the Rafter Length Calculator, and for pitch conversions the Roof Pitch Calculator. Shingles and other coverings are estimated in the Roofing Calculator, eave membranes in the Ice and Water Shield Calculator, and metal panels in the Metal Roofing Material Calculator.
Frequently asked questions
How many sheets of plywood do I need for a roof?
Find the roof area (length × slope length × planes), add 10% waste, and divide by 32 sq ft per 4×8 sheet. A 30 × 12 ft-run gable roof at 6/12 with a 1 ft overhang needs 30 sheets.
Why not just use the footprint of the building?
A sloped roof has more surface than its footprint. The slope length is the horizontal run times the pitch factor √(1 + (rise ÷ 12)²), which is 1.118 at 6/12 and 1.414 at 12/12.
What thickness of sheathing should I use?
It depends on the rafter or truss spacing and the loads. Roof panels carry a span rating such as 24/16, where the first number is the maximum rafter spacing in inches; code tables (IRC R503.2) give the thickness. Ask your building department or truss designer.
Do I need to leave gaps between sheets?
Yes. Panel manufacturers such as APA recommend a 1/8 inch gap at panel edges and ends to allow for expansion, and special clips or tongue-and-groove edges between rafters.
How should the sheets be laid?
Run the long dimension perpendicular to the rafters and stagger the end joints so they fall on rafters and do not line up across rows. Follow the nailing schedule in your code.