How Many Sheets of Plywood or OSB Do You Need? Floors, Roofs, and Sheds
October 21, 2026
Plywood and oriented strand board (OSB) are sold in 4 × 8 foot sheets that cover 32 square feet. To estimate how many you need, divide the area to be covered by 32 and round up, then add an allowance for waste. A shed with an 8 × 10 foot floor has 80 square feet, which is 2.5 sheets, so you buy 3. With 10% waste, 80 × 1.1 ÷ 32 = 2.75, which also rounds to 3.
In practice, how the sheets fit matters as much as the area. A good layout plan reduces waste and ensures that joints fall on framing.
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Subflooring runs perpendicular to the joists, with the 8-foot edge across the joists and the end joints landing on framing members, staggered between rows. For a 12 × 16 foot floor (192 square feet), you need 192 ÷ 32 = 6 sheets by area, so buy 7 to cover cuts at the edges.
Allow extra for cuts around openings, stair holes, and walls that are not square. Typical waste is 5% to 10% for simple floors and 15% for rooms with many cuts.
Walls
Wall sheathing is installed vertically or horizontally over the studs. A wall that is 8 feet tall and 20 feet long has 160 square feet, or 5 sheets of 4 × 8, before subtracting window and door openings. Subtracting openings can reduce the count, but only if the offcuts are not wasted, and large pieces from openings can often be used elsewhere. Many builders do not deduct openings smaller than 2 × 4 feet.
For a house, add up all wall areas, subtract large openings, then divide by 32 and add 10% waste. A 1,500 square foot single-story house with 8-foot walls and a 160-foot perimeter has 1,280 square feet of wall: 40 sheets, or about 44 with waste.
| Project | Area | Sheets (4 x 8) | With 10% waste |
|---|---|---|---|
| 8 x 10 shed floor | 80 sq ft | 2.5 | 3 |
| 12 x 16 floor | 192 sq ft | 6 | 7 |
| Wall 8 x 20 ft | 160 sq ft | 5 | 6 |
| Roof 12 x 24 ft (one side, flat area) | 288 sq ft | 9 | 10 |
| House walls (160 ft perimeter x 8 ft) | 1,280 sq ft | 40 | 44 |
Roofs
Roof decking must cover the sloped area, not the footprint. Multiply the footprint by the slope factor: a 6/12 pitch has a factor of 1.118. A roof with a footprint of 24 × 30 feet (720 square feet) has a sloped area of 805 square feet, which requires 25.2 sheets, so 26, plus waste of about 10%, for 28 sheets. Roof sheathing is laid with the long edge perpendicular to the rafters and joints staggered, with small gaps of 1/8 inch for expansion.
Thickness Choices
Thickness depends on the support spacing and use. Typical choices: 3/4-inch tongue-and-groove for subfloors over 16-inch joist spacing (or 23/32 rated sheathing), 7/16 to 1/2 inch for wall sheathing, and 1/2 inch for roof sheathing with rafters at 24 inches on center, though snow loads and local code may require thicker. Look at the span rating stamped on the panel, such as 24/16, indicating the maximum roof and floor spacing in inches. Follow the manufacturer's nailing schedule: commonly 6 inches at the edges and 12 inches in the field.
Plywood vs. OSB
OSB is generally cheaper and strong in structural applications, but it swells more at the edges when it gets wet. Plywood handles moisture exposure a bit better and holds fasteners well. Both need to be protected from long exposure to weather during construction. Use exterior-rated panels where exposed to moisture, and pressure-treated plywood in contact with the ground.
Frequently Asked Questions
How many square feet is a sheet of plywood?
A 4 × 8 foot sheet covers 32 square feet.
How many sheets for an 8 x 10 floor?
Three, since 80 sq ft needs 2.5 sheets.
How do I allow for waste?
Add 5% to 15%, depending on how many cuts you must make.
Which direction should plywood subfloor run?
Perpendicular to the joists, with the end joints staggered and supported on joists.
Is OSB as good as plywood?
For most structural uses, yes, but it is more sensitive to prolonged wetting.