Estimating sheathing
Whether you are sheathing walls, decking a roof or laying a subfloor, the question is the same: how many sheets of plywood or OSB do I need? The calculator divides the area to cover, minus doors and windows, by the area of one sheet, adds a waste allowance and rounds up to whole sheets.
The formula
Sheets = ⌈(area − openings) × (1 + waste) ÷ sheet area⌉
| Sheet | Area |
|---|---|
| 4 × 8 ft | 32 sq ft |
| 4 × 9 ft | 36 sq ft |
| 4 × 10 ft | 40 sq ft |
Worked example
A small house has 1,200 sq ft of exterior walls with 80 sq ft of doors and windows.
- Net area = 1,200 − 80 = 1,120 sq ft
- With 10% waste: 1,232 sq ft
- 1,232 ÷ 32 = 38.5, so 39 sheets of 4 × 8
- At $30 a sheet, about $1,170
How to measure
- Walls: perimeter × wall height, per floor. Add gable triangles (base × height ÷ 2).
- Roof: measure each plane along the slope, not the floor plan, or multiply the plan area by the slope factor (about 1.12 for a 6/12 roof, 1.20 for 8/12).
- Floors: length × width of each room or of the whole platform.
- Subtract only large openings; small windows often end up as waste anyway because the sheet must still be cut around them.
How much waste to allow
| Job | Waste |
|---|---|
| Simple rectangular walls or floors | 5–10% |
| Walls with many openings | 10–15% |
| Hip or valley roofs, complex shapes | 15–20% |
Plywood or OSB?
Both are accepted by US building codes for wall and roof sheathing and subfloors when they carry the right span rating stamp.
- OSB is cheaper, very uniform and the most common choice for walls and roofs.
- Plywood tolerates getting wet better, swells less at the edges and holds screws slightly better, which is why some builders prefer it for subfloors and roofs in wet climates.
Typical thicknesses
- Walls: 7/16 in OSB or 15/32 in plywood
- Roofs: 7/16 to 5/8 in, depending on rafter spacing and snow load
- Subfloors: 23/32 in (nominal 3/4 in) tongue-and-groove, glued and screwed
Always check the span rating against the framing spacing (16 or 24 in on center) and follow your local building code.
Tips
- Stagger the joints between rows and leave a 1/8 in gap between sheets for expansion.
- Run the long side of the sheet across the framing for strength.
- Buy one or two extra sheets: a damaged or miscut sheet is common, and returns are easier than a second trip mid-job.