Plywood is thin veneers glued in layers, with the grain of each layer turned against the last. OSB, oriented strand board, is strands of wood pressed together with resin in layers. Both come in 4×8 sheets, and the building code calls both of them wood structural panels.
Start with the stamp
Thickness and species matter less than the span rating printed on the sheet. A panel marked 24/16 can go on roof framing 24″ apart and floor framing 16″ apart. Buy to the rating your framing needs, whichever material it comes in.
Exposure 1 on the stamp means the panel can take weather during construction. It doesn't make it a finished exterior surface. For anything that stays exposed, you want siding or a panel rated Exterior.
Where they differ
| OSB | Plywood | |
|---|---|---|
| Price | Usually lower | Usually higher |
| Getting wet | Slow to soak in, slow to dry | Soaks in fast, dries fast |
| Edges | Can swell and stay swollen | Tend to recover better |
| Surface | Consistent, no voids | Can have knots and voids |
| Span ratings | Same system | Same system |
The price gap between the two can be small one month and large the next, so price both at your yard.
Roofs
Either works. OSB is the usual pick on new houses because of the price, and it's fine when the roof gets dried in quickly. If the framing will sit under weather for weeks, or you're reroofing a section in a wet season, plywood is more forgiving.
Leave a 1/8″ gap on all edges, whichever you use. Wood panels grow as they pick up moisture, and the APA recommends that gap to keep them from buckling. Many panels are made slightly under 4×8 for this reason and stamped "sized for spacing."
Walls
Walls are the easiest case, since they shed water and get covered soon. OSB is the common choice. Panels rated 24/16 or better can go over studs at 24″ on center, which is what you'd want on a wider stud layout. The framing calculator counts studs and plates but not sheets, so work those out from the wall length and height.
Subfloors
Wet edges matter most on a subfloor. A floor can sit open to rain before the roof is on, and swollen OSB edges telegraph through thin flooring later. Buy a tongue-and-groove panel rated for floors, in either material, glue it to the joists and keep it covered as soon as you can. Plywood earns its extra cost if the floor will be exposed for a long time.
Which one to buy
- Roof or wall on a normal schedule: OSB, at the span rating your framing needs.
- Long exposure to weather: plywood.
- Shop projects, cabinets, anything with a visible face: plywood, in a sanded grade. Sheathing panels aren't made to look good.
- Outdoors for good: neither sheathing panel. Use exterior-rated plywood with a finish, or solid lumber.