The right plywood thickness depends on your rafter spacing and local building code

Most residential roofs use either 1/2-inch or 5/8-inch plywood as the base layer under shingles or other roofing material. The choice comes down to how far apart your rafters are spaced and what your local building inspector requires. If your rafters are 16 inches apart (the most common spacing), 1/2-inch plywood is usually acceptable. If they are 24 inches apart, you need 5/8-inch to prevent sagging and to meet code.

The plywood sits on top of the rafters and creates a solid surface for nailing down shingles. It also braces the roof structure and helps distribute weight evenly. Using plywood that is too thin for your rafter spacing creates a safety hazard — the roof can flex or sag over time, and shingles may crack or pull loose.

Key Takeaways

  • 1/2-inch plywood works for 16-inch rafter spacing; 5/8-inch is required for 24-inch spacing in most jurisdictions.
  • Thicker plywood (5/8-inch) costs more but provides better support, longer shingle life, and easier walking during installation.
  • Your local building code sets the minimum thickness — check with your inspector or building department before ordering materials.
  • Plywood must be rated for roof use (typically stamped "Roof" or "Exterior") and installed with the grain running perpendicular to the rafters.

How rafter spacing determines plywood thickness

Rafter spacing is the distance from the center of one rafter to the center of the next. Most homes built in the last 40 years use 16-inch spacing, which is why 1/2-inch plywood is the standard choice. Older homes or some newer construction may have 24-inch spacing to save framing lumber, and those roofs require the thicker 5/8-inch plywood to span the longer distance without deflection.

If you are replacing a roof or adding a second layer, measure the distance between two adjacent rafters to know what thickness you need. If you cannot access the attic or roof framing, your roofer can measure it during the inspection. Do not assume — using undersized plywood on 24-inch spacing will fail inspection and create problems that are expensive to fix after installation.

Why building code matters for your choice

Building codes vary by state, county, and sometimes by municipality. Most follow the International Building Code (IBC) or a state-specific version of it, and they all set minimum plywood thickness based on rafter spacing. Some jurisdictions also require thicker plywood in high-wind or high-snow areas to add structural strength.

Before you buy materials, contact your local building department or ask your roofer to confirm the requirement for your address. The inspector will check the plywood thickness during the roof inspection, and if it does not meet code, you will have to remove and replace it — a costly mistake. Your building permit paperwork or the original home inspection report may also list the rafter spacing and required thickness.

The difference between 1/2-inch and 5/8-inch plywood

Beyond code compliance, 5/8-inch plywood offers practical advantages. It is stiffer, so it resists sagging and flexing under the weight of workers, snow, or ice. It also provides a more solid nailing surface for shingles, which can extend shingle life by reducing movement and stress on the fasteners. Roofers often prefer it because it is easier to walk on and less likely to bounce or give way underfoot.

The trade-off is cost and weight. 5/8-inch plywood costs roughly 15 to 25 percent more per sheet than 1/2-inch, and it weighs about 20 percent more, which increases labor time and puts more load on the framing. If your rafter spacing allows 1/2-inch and your code permits it, the thinner option is a reasonable choice. If you are in a snow-heavy climate or want maximum durability, the extra cost of 5/8-inch is often worth it.

Plywood grade and rating for roofing

Not all plywood is suitable for roofs. You need plywood rated for exterior use, stamped with a grade like CDX or BC (the letters indicate the quality of the veneer on each side). The plywood should also be marked "Roof" or "Roof Decking" on the stamp, which means it meets the structural requirements for roof use. Some suppliers also sell OSB (oriented strand board) as a cheaper alternative, but it is less common for residential roofing and does not perform as well in wet conditions.

Check the stamp on each sheet before it is delivered or unloaded. The stamp includes the mill name, grade, thickness, and exposure rating. If the plywood is not marked for roof use or is interior-grade, send it back — it will not hold up under weather exposure and will not pass inspection.

Installation details that affect performance

Plywood thickness alone does not may provide a good roof. The way it is installed matters just as much. Sheets should be laid with the grain running perpendicular to the rafters (across them, not along them) to maximize strength. Leave a 1/8-inch gap between sheets to allow for expansion in humid weather — this prevents buckling and squeaking.

Fasteners should be nails or screws spaced 6 inches apart along the edges and 12 inches apart in the field (the middle of the sheet). Undersized or oversized fasteners, or spacing that is too wide, will allow the plywood to move and shingles to fail prematurely. Your roofer should follow these details, but it is worth knowing them so you can spot problems during the work.

What happens if you use the wrong thickness

Using 1/2-inch plywood on 24-inch rafter spacing will sag over time, especially under snow load or foot traffic. The sag is usually not dramatic — maybe a quarter-inch to half-inch — but it is enough to cause shingles to crack, fasteners to pull loose, and water to pool in low spots. Shingles may also develop a wavy appearance, which shortens their life and looks poor.

If the inspector catches undersized plywood during the rough inspection (before shingles are installed), you will have to remove the shingles, replace the plywood, and reinstall everything. If it is not caught until later, you may have a roof that fails prematurely and requires replacement sooner than expected. The cost of doing it right the first time is far less than fixing it later.

Frequently Asked Questions

Can I use 3/8-inch plywood on a roof?

No. Building codes do not allow 3/8-inch plywood for roof decking on residential homes. It is too thin to span the distance between rafters safely, even on 16-inch spacing. Use 1/2-inch as the minimum thickness.

Is OSB as good as plywood for roofing?

OSB is cheaper but absorbs water more readily than plywood, which can cause swelling and delamination over time. Plywood is the standard choice for roofs, especially in humid or rainy climates. Some building codes allow OSB, but check your local requirements before using it.

Do I need to add a second layer of plywood if I am re-roofing over old shingles?

Not usually. If the existing plywood is in good condition and meets code for your rafter spacing, you can install new shingles directly over it. If there are soft spots, rot, or water damage, those areas need to be cut out and patched with new plywood of the same thickness before new shingles go on.

What is the difference between roof plywood and subfloor plywood?

Roof plywood is thinner and rated for the specific loads and weather exposure of a roof. Subfloor plywood is thicker and rated for live loads (people and furniture). They are not interchangeable — use the right product for the job.

How do I know what rafter spacing I have if I cannot see the attic?

Measure the distance between two nails or fasteners on the underside of the roof from inside the attic, or ask your roofer to measure during the inspection. If the attic is not accessible, the original home inspection report or building permit may list the spacing.