Roof sheathing calculator: how many sheets for a roof?
Enter the building footprint, the overhang and the roof pitch. The calculator converts the flat footprint into sloped roof area with the pitch factor and counts 4×8 (or metric) sheathing panels with waste.
Footprint with overhang: 1,260.0 sq ft × pitch factor 1.118 = 1,408.7 sq ft of roof deck
You need 49 sheets of 4×8 ft (48 × 96 in) (44.02 before 10% waste)
Worked examples
| Job | Area | Panel | Waste | Sheets |
|---|---|---|---|---|
| 40 × 28 ft ranch, 1 ft overhang, 6/12 | 1,408.7 sq ft | 4×8 ft (48 × 96 in) | 10% | 49 |
| 24 × 24 ft garage, 1 ft overhang, 4/12 | 712.6 sq ft | 4×8 ft (48 × 96 in) | 10% | 25 |
| 10 × 12 ft shed, 6 in overhang, 4/12 | 150.7 sq ft | 4×8 ft (48 × 96 in) | 10% | 6 |
| Hip roof, 50 × 30 ft, 1.5 ft overhang, 8/12 | 2,102.0 sq ft | 4×8 ft (48 × 96 in) | 15% | 76 |
| 10 × 8 m house, 0.5 m overhang, 7/12 (30°) | 114.6 m² | 2440 × 1220 mm | 10% | 43 |
From footprint to roof area: the pitch factor
A sloped roof has more surface than the ground it covers. Multiply the horizontal area under the roof — the footprint plus the overhang on every side — by the pitch factor, the length of the rafter for each unit of horizontal run: pitch factor = √(12² + rise²) ÷ 12. For a 6/12 roof that is √180 ÷ 12 = 1.118, so a 1,000 sq ft footprint needs 1,118 sq ft of sheathing.
The same factor works for gable, hip and shed roofs as long as every plane has the same pitch: each plane covers its share of the footprint and is stretched by the same factor. For a roof with mixed pitches, run the calculator once for each section and add the results. Dormers add their own small roofs; estimate them separately.
| Pitch | Angle | Pitch factor | Roof area per 1,000 sq ft footprint | 4×8 sheets (10% waste) |
|---|---|---|---|---|
| 2/12 | 9.5° | 1.014 | 1,014 sq ft | 35 |
| 3/12 | 14.0° | 1.031 | 1,031 sq ft | 36 |
| 4/12 | 18.4° | 1.054 | 1,054 sq ft | 37 |
| 5/12 | 22.6° | 1.083 | 1,083 sq ft | 38 |
| 6/12 | 26.6° | 1.118 | 1,118 sq ft | 39 |
| 7/12 | 30.3° | 1.158 | 1,158 sq ft | 40 |
| 8/12 | 33.7° | 1.202 | 1,202 sq ft | 42 |
| 9/12 | 36.9° | 1.250 | 1,250 sq ft | 43 |
| 10/12 | 39.8° | 1.302 | 1,302 sq ft | 45 |
| 12/12 | 45.0° | 1.414 | 1,414 sq ft | 49 |
Waste allowance for roofs
Use about 10% for a simple gable roof: the only waste is the last course at the ridge and the angled cuts at the rakes. Hip and valley roofs need 15% or more because every course ends in a diagonal cut, and the triangular offcuts can only be reused when they are big enough to span two rafters. Roofers also count in squares — 100 sq ft of roof surface. One 4×8 sheet covers 0.32 square, so a square takes 3.125 sheets before waste.
Roof sheathing thickness and installation
- 7/16-inch OSB with a 24/16 span rating and 15/32-inch plywood (32/16) are the most common panels for rafters or trusses 24 inches on center. Many builders step up to 1/2 or 5/8 inch for heavy tile, high snow loads or a flatter-feeling deck.
- Run the long edge across the rafters and stagger end joints by at least one rafter bay, just like a subfloor.
- Where the long edges fall between rafters, codes often require edge support — panel clips (H-clips), blocking or tongue-and-groove edges — especially for thinner panels at 24-inch spacing. Check your local code.
- Leave a 1/8-inch gap at all panel ends and edges (an H-clip sets it for you) and nail on the schedule printed on the panel or in your code, typically 6 inches at edges and 12 inches in the field.
Metric roofs are usually described in degrees: a 30° roof is very close to 7/12 (30.3°), 22.5° is about 5/12 (22.6°) and 45° is 12/12. Use the table above to match them.
How to use the calculator
- Measure the footprint. Measure the building length and width at the outside of the walls.
- Add the overhang. Enter the eave and rake overhang — the horizontal distance the roof extends past the wall on each side.
- Choose the pitch. Select the rise per 12 of run. The calculator converts the footprint to sloped roof area.
- Count sheets. Pick a panel size and a waste allowance (10% gable, 15% hip or valleys) and round up.
Other plywood calculators
- Subfloor calculatorFloor area → 4×8 or 2440 × 1220 subfloor panels
- Wall sheathing calculatorWalls, openings and gables → sheathing panels
- Kitchen cabinetsSheets per base and wall cabinet
- BookshelvesSheets by bookcase height and width
Cutting parts rather than laying whole sheets? Area estimates undercount furniture and cabinets — use the cut list optimizer, and see plywood sizes and thicknesses for real panel dimensions.
FAQ
How many sheets of OSB do I need for a roof?
Multiply the footprint including overhangs by the pitch factor to get roof area, add 10–15% waste and divide by 32 sq ft per 4×8 sheet. A 40 × 28 ft house with 1 ft overhangs and a 6/12 roof needs about 1,409 sq ft of sheathing, or 49 sheets with 10% waste.
How many sheets of plywood are in a roofing square?
A square is 100 sq ft of roof, and a 4×8 sheet is 32 sq ft, so a square takes 3.125 sheets — about 3.5 sheets with 10% waste.
Does roof pitch change how much sheathing I need?
Yes. A steeper roof has longer rafters, so it needs more sheathing for the same footprint: 5.4% more at 4/12, 11.8% more at 6/12, 20.2% more at 8/12 and 41.4% more at 12/12.
Should I use 7/16 or 1/2 inch roof sheathing?
Both are common on 24-inch rafters or trusses. 7/16 OSB rated 24/16 is the code minimum in many areas (often with H-clips); 1/2 inch or thicker is stiffer, holds roofing nails better and is preferred under tile or in snow country.