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OSB and CDX plywood are both code-approved for roof sheathing, but they behave differently. OSB is 10-15% cheaper, more dimensionally consistent, and made from sustainable fast-growth wood; plywood is more moisture-tolerant (dries faster), holds nails better, and weighs slightly less. Both meet APA span ratings for 24-inch rafter spacing at 7/16 OSB or 15/32 plywood. In humid coastal climates, plywood is the safer long-term choice; everywhere else, OSB is the modern standard.
For roof sheathing, OSB and plywood with the same APA span rating are interchangeable under the building code — 7/16 in OSB rated 24/16 and 1/2 in plywood both span rafters at 24 in on center. Choose plywood if the deck may sit wet before it is dried in or the climate is damp (it dries faster and recovers from swelling); choose OSB to save money on a roof that will be covered quickly.
The stamp on the panel matters more than the material. The first number of the span rating is the maximum rafter spacing for roof use; the minimum thickness is the same for OSB and plywood at each rating.
| Span rating | Minimum thickness | Max rafter spacing (edges supported) | Without edge support |
|---|---|---|---|
| 24/0 | 3/8 in | 24 in | 20 in |
| 24/16 | 7/16 in | 24 in | 24 in |
| 32/16 | 15/32 in | 32 in | 28 in |
| 40/20 | 19/32 in | 40 in | 32 in |
| 48/24 | 23/32 in | 48 in | 36 in |
Edge support means panel clips, blocking or tongue-and-groove edges between rafters. Many shingle manufacturers ask for at least 3/8 in deck in their installation instructions, and roofers commonly use 7/16 in OSB or 1/2 in plywood at 16 or 24 in rafters because thinner sheets feel springy underfoot and can telegraph through the shingles. Your local code and the shingle instructions win where they are stricter.
Source: IRC Table R503.2.1.1(1) / APA Engineered Wood Construction Guide (E30).
For roof sheathing, OSB and plywood are interchangeable as far as code and span ratings are concerned. The differences that matter in practice are what happens when they get wet, what they cost, and how they behave underfoot.
| OSB | Plywood | |
|---|---|---|
| Cost per sheet | Lower | Higher |
| Weight | Heavier for the same thickness | Lighter |
| Span rating | Equivalent at the same rating | Equivalent at the same rating |
| Wet edge swelling | Swells and does not fully recover | Swells and mostly recovers |
| Drying speed | Slow | Faster |
| Consistency | Very uniform, no voids | Occasional internal voids |
| Holding a nail after a soaking | Weakens noticeably | Holds better |
Both are sold to the same span-rating system, so a sheet rated 24/16 spans the same distance whichever material it is made from. Read the stamp, not the material name.
Installation. A 1/8 inch gap at every edge, staggered end joints, the long dimension across the rafters, and edge support where the span requires it. A well-installed OSB deck outlasts a badly installed plywood one comfortably, and edge swelling — the failure OSB is known for — is very often a symptom of sheets butted tight rather than the material itself.
For span rating and code they are equivalent — read the stamp, not the material name. Plywood recovers better after getting wet and dries faster, which matters if the deck will be exposed or the roof may leak. OSB is cheaper and very flat.
7/16 in OSB is normally stamped 24/16, which allows rafters or trusses up to 24 in on center on a roof. It is the most common roof sheathing on new homes. At 24 in spacing use panel clips if your code or shingle maker calls for edge support.
3/8 in plywood rated 24/0 meets the code minimum for roofs at up to 24 in spacing with edge support (20 in without). It is light and flexes underfoot, so most roofers use 1/2 in plywood or 7/16 in OSB instead, and some shingle warranties ask for more.
Exposure 1 is the bond classification: the adhesive is waterproof and the panel tolerates weather during construction delays, but it is not meant to stay exposed. It is the standard rating for roof decking under underlayment and shingles.
It swells at the cut edges if it gets wet and does not fully recover. Most of the edge swelling seen in the field comes from sheets butted tight with no expansion gap rather than from the material itself.
Yes, as long as both carry the right span rating for the rafter spacing and the thicknesses match so the deck stays flat. Mixing is common when replacing individual bad sheets on an older deck.
Seven sixteenths is common at 16-inch centres and is generally treated as the OSB equivalent of half-inch plywood. Nineteen thirty-seconds or five eighths is used at 24-inch centres and under heavy loads.
Choosing the thickness? See the roof sheathing thickness calculator for the code minimum at 16, 24 or 32 in spacing, with or without H-clips.
Whichever you choose, the sheet count is the same — the roof sheathing calculator works for plywood and OSB.
The longer buying guide: plywood vs OSB guide.