How to work out plywood sheets
Measure the area to cover, add a waste allowance, and divide by the area of one sheet. Always round up to a whole sheet.
sheets = area × (1 + waste %) ÷ area of one sheet
- Subfloor: the floor length × width, including closets and the space under where cabinets will go.
- Wall sheathing: each wall's length × height. Many framers do not subtract window and door openings, because the offcuts are rarely usable; subtract only large openings such as garage doors.
- Roof deck: the sloped roof area. The roofing calculator works it out from the building size and pitch.
Worked example
A 24 × 16 ft floor is 384 sq ft. With 10 percent extra it becomes 422.4 sq ft. Divided by 32 sq ft per 4 × 8 sheet, that is 13.2, so buy 14 sheets. In metric, a 36 m² floor plus 10 percent is 39.6 m², which also takes 14 sheets of 1200 × 2400 mm (2.88 m² each).
Sheet sizes
| Sheet | Covers | Where you see it |
|---|---|---|
| 4 × 8 ft | 32 sq ft | US and Canada, the standard size |
| 4 × 9 ft and 4 × 10 ft | 36 and 40 sq ft | Wall sheathing on taller walls, fewer horizontal joints |
| 1200 × 2400 mm | 2.88 m² | Australia and other metric markets |
| 1220 × 2440 mm | 2.98 m² | Imported panels made to the inch size |
Common thicknesses (starting points)
| Job | Often used | What decides it |
|---|---|---|
| Subfloor, joists 16 in on center | 23/32 in (sold as ¾ in), tongue-and-groove | Panel span rating, finish flooring |
| Wall sheathing, studs 16 or 24 in on center | 7/16 in OSB or 15/32 in plywood | Stud spacing, bracing rules, wind zone |
| Roof deck, rafters or trusses 24 in on center | 7/16 in to 5/8 in | Rafter spacing, snow load, roofing type |
Panels in North America carry a span rating stamp, such as 24/16 or "24 oc". The rating must match or exceed your framing spacing. These are common choices, not a design; your local code and the panel stamp decide.
Sheathing a wall
For walls, add up the length of every wall and multiply by the wall height. A 40 ft wall that is 8 ft high is 320 sq ft, exactly ten 4 × 8 sheets stood on end. With 10 percent extra for cuts at corners and openings you would buy 11 sheets. If your walls are 9 ft high, 4 × 9 sheets cover the wall in one piece and save a row of horizontal joints and blocking.
Plywood or OSB?
Both are structural panels and both carry the same span ratings, so either can be used where the rating fits. OSB is made from compressed wood strands and is very uniform from sheet to sheet. Plywood is made from thin veneers glued in alternating directions; it tends to be lighter and recovers better if it gets wet during construction. Many builders use OSB for walls and roofs and choose either for floors.
How much waste to allow
Simple rectangular floors and walls usually need 5 to 10 percent extra. Hip roofs, rooms with many corners and walls with lots of openings can need 15 percent. If the floor dimensions fit whole sheets, you may need less; if every row needs a cut, plan for more.
Installation tips
- Run the long side of each sheet across the joists, rafters or studs, and stagger end joints so they do not line up row to row.
- Leave about a 1/8 in gap at panel edges and ends, as panel makers recommend, so the sheets can swell without buckling.
- Glue and screw or ring-shank nail subfloors to reduce squeaks. Follow the fastener spacing on the panel or in your code.
- Working on a roof deck is work at height. Use fall protection and do not walk on unfastened sheets.
Frequently asked questions
How many sheets of plywood do I need for a 12 × 16 floor?
A 12 × 16 ft floor is 192 sq ft. With 10 percent extra that is 211.2 sq ft, so you need 7 sheets of 4 × 8 ft plywood.
How many square feet does a sheet of plywood cover?
A standard 4 × 8 ft sheet covers 32 sq ft. A 1200 × 2400 mm sheet covers 2.88 m².
How many sheets of OSB for a 1,500 sq ft roof?
1,500 sq ft plus 10 percent is 1,650 sq ft, which is 52 sheets of 4 × 8 ft OSB. Use the sloped roof area, not the floor area.
Is OSB or plywood better for a subfloor?
Both are used and both carry span ratings. Plywood tends to recover better if it gets wet during construction; OSB is often cheaper and very uniform. Pick a panel rated for your joist spacing.