Plywood for roofing must be exterior-grade with a minimum thickness of 1/2 inch, and most building codes require CDX or better rating
The plywood you use as roof sheathing sits between your rafters and your shingles or other roofing material. It carries the weight of everything above it — snow, rain, workers, fallen branches — and it has to resist moisture without swelling or rotting. Interior plywood will fail in weeks. The wrong thickness will sag under load and void your roofing warranty.
Building codes in most jurisdictions specify exterior-grade plywood rated CDX or better, at least 1/2 inch thick for standard rafter spacing (16 inches on center). Some codes allow 3/8 inch if rafters are spaced 16 inches apart and snow load is light, but 1/2 inch is the safer choice and the standard most roofers and inspectors expect. If your rafters are spaced 24 inches on center, you need 5/8 inch minimum. Your local building department can tell you what your area requires based on snow load, wind speed, and roof pitch.
Key Takeaways
- Exterior-grade plywood rated CDX or better is required by code; interior plywood will absorb moisture and fail.
- Minimum thickness is 1/2 inch for 16-inch rafter spacing and 5/8 inch for 24-inch spacing, though local codes may differ based on climate.
- Plywood must be installed with the grain running perpendicular to the rafters and fastened with ring-shank nails or screws spaced 6 inches apart at edges and 12 inches in the field.
- Exposure rating (CDX, BC, or better) matters more than the letter grades; CDX means the face and back are both rated for exterior use.
- Check your local building code before purchasing, because snow load and wind speed in your area determine the exact thickness and grade required.
What the letter grades and exposure ratings mean
Plywood is graded by the quality of its veneer layers. A grade of A means the surface is smooth and nearly perfect. B means it has some knots and repairs. C means larger knots and gaps are allowed. D means the veneer can have knots up to 2.5 inches across. You will see grades written as two letters — the first is the face, the second is the back — so BC plywood has a B-grade face and a C-grade back.
The exposure rating is what matters most for roofing. CDX means both the face and back are rated for exterior exposure, and the glue holding the layers together is waterproof. This is the minimum for roof sheathing. BC plywood has a B-grade face and C-grade back, both rated exterior, and is acceptable but more expensive. AC plywood has an A-grade face (smoother, more cosmetic) and C-grade back, also exterior-rated, and costs more than CDX without adding structural benefit for roofing. Avoid anything marked "interior" or "exposure 1" — those are not designed for the constant moisture a roof encounters.
Thickness requirements based on rafter spacing and snow load
Rafter spacing — the distance from the center of one rafter to the center of the next — determines how much load each piece of plywood has to span. The wider the spacing, the thicker the plywood must be to avoid sagging.
| Rafter Spacing | Minimum Thickness (Light Snow Load) | Minimum Thickness (Heavy Snow Load) |
|---|---|---|
| 16 inches on center | 1/2 inch | 5/8 inch |
| 24 inches on center | 5/8 inch | 3/4 inch |
Snow load varies by region. Areas that receive more than 50 pounds per square foot of snow in winter are considered heavy snow load. Your local building department publishes snow load maps or can tell you the requirement for your address. If you are unsure, use the heavier thickness — the cost difference between 1/2 inch and 5/8 inch is small, and undersized sheathing can cause the roof to sag, shingles to crack, and water to pool.
How to install plywood correctly so it performs
Plywood must be laid with the grain running perpendicular to the rafters. This means if your rafters run north to south, the plywood grain should run east to west. This orientation distributes the load more evenly and prevents the roof from flexing in one direction.
Leave a 1/8-inch gap between sheets to allow for expansion when the plywood absorbs moisture. Do not butt sheets tightly together — they will buckle as humidity changes. Stagger the seams so that the corners of four sheets do not meet at one point; this is called a four-way joint and it creates a weak spot. Fasten the plywood with ring-shank nails (which grip better than smooth nails) or exterior-grade screws, spaced 6 inches apart along the edges and 12 inches apart in the field (the middle of the sheet). Use 8d nails for 1/2-inch plywood and 10d for 5/8-inch.
Do not use staples. Staples do not hold as well as nails under the constant movement and vibration a roof experiences, and many building codes prohibit them for structural sheathing.
When to use plywood versus oriented strand board (OSB)
OSB is cheaper than plywood and has similar strength when new. However, OSB absorbs moisture faster than plywood and takes longer to dry out. Once OSB gets wet, it swells and can lose strength permanently. Plywood is more forgiving — it swells less and recovers better when it dries. For a roof, where moisture exposure is constant and drying time is limited, plywood is the safer choice.
Some builders use OSB to save money on new construction, and it can work if the roof is covered quickly and the building is kept dry during construction. But if you are re-roofing an older house or working in a humid climate, plywood is worth the extra cost. The difference in price is usually $1 to $3 per sheet, and it extends the life of your roof by years.
Checking your existing roof sheathing
If you are replacing shingles on an existing roof, you may find that the sheathing underneath is damaged. Soft spots, rot, or large gaps mean the sheathing has failed and must be replaced before new shingles go on. A roofer will typically discover this during the tear-off and will quote the replacement separately.
If you see water stains or mold on the underside of the sheathing from inside the attic, the roof is leaking and the sheathing is absorbing moisture. This usually means the sheathing is near the end of its life and should be replaced. Patching a small area is possible, but if more than 10 to 15 percent of the sheathing is compromised, replacement is more cost-effective than repair.
Frequently Asked Questions
Can I use 3/8-inch plywood instead of 1/2 inch to save money?
Only if your local building code allows it and your rafters are spaced 16 inches on center with light snow load. Check your code first — many jurisdictions require 1/2 inch minimum regardless of spacing. Using undersized sheathing can void your roofing warranty and cause the roof to sag over time.
What is the difference between CDX and BC plywood for roofing?
Both are exterior-rated and acceptable for roofing. BC has a smoother face (B-grade instead of C-grade) and costs more, but this does not improve roof performance. CDX is the standard choice for sheathing because the face quality does not matter — it is covered by shingles.
Do I need to seal or treat the plywood before installing shingles?
No. Exterior-grade plywood is already treated with waterproof glue and does not need additional sealing. explore sealant can trap moisture and cause rot. The shingles themselves protect the plywood from direct rain.
Can I use plywood that has been stored outside before installation?
If it has been rained on or left uncovered for more than a few weeks, inspect it for swelling, soft spots, or mold. Plywood that has absorbed water should be allowed to dry completely before installation, which can take weeks. If it shows signs of rot or permanent swelling, replace it.
What happens if I install plywood with the grain running parallel to the rafters instead of perpendicular?
The roof will be weaker in one direction and more prone to sagging and flexing. This can cause shingles to crack and water to pool. Always run the grain perpendicular to the rafters, and check that your roofer has done this correctly during installation.