Standard plywood for roof sheathing is either 1/2 inch or 5/8 inch thick, with 5/8 inch being more common in new construction

The thickness you need depends on the spacing of your roof rafters or trusses. If your rafters are spaced 16 inches apart (the most common spacing in residential framing), use 1/2-inch plywood. If they are spaced 24 inches apart, use 5/8-inch plywood. Using thinner plywood than your rafter spacing calls for creates a sag or deflection in the roof deck, which can crack shingles, trap water, and shorten the life of your roofing material.

Plywood for roofing must be rated for exterior use. The grade marking you are looking for is CDX or OSB (oriented strand board), which are the two most common materials for roof sheathing. CDX plywood has a C-grade face and D-grade back, meaning the top surface is acceptable for exposure but the underside has more knots and voids. This is fine for roof sheathing because the top gets covered by shingles and the underside is inside the attic.

Key Takeaways

  • Use 1/2-inch plywood for 16-inch rafter spacing and 5/8-inch plywood for 24-inch rafter spacing.
  • CDX plywood and OSB are the standard materials; both are rated for exterior exposure and cost less than higher grades.
  • Thinner plywood than your rafter spacing requires will sag between rafters and can cause shingles to crack or water to pool.
  • Plywood should be installed with the grain running perpendicular to the rafters and with 1/8-inch gaps between sheets to allow for expansion.

How rafter spacing determines plywood thickness

Rafter spacing is the distance from the center of one rafter to the center of the next. This measurement controls how much weight the plywood has to span without bending. The wider the span, the thicker the plywood needs to be to stay flat.

In most residential homes, rafters are spaced 16 inches on center. This is the standard for single-story homes and the upper floors of two-story homes. In this case, 1/2-inch plywood is sufficient and is what most builders use. Some builders space rafters 24 inches apart to save lumber, especially on roofs with low pitch or in areas with light snow load. When rafters are 24 inches apart, the plywood must be 5/8 inch thick to prevent sagging.

If you are unsure of your rafter spacing, measure from the center of one rafter to the center of the next. If you cannot access the roof framing, ask a roofer or structural inspector to measure for you before you buy materials.

CDX plywood versus OSB for roof sheathing

CDX plywood and OSB are the two materials used most often for roof sheathing. Both are rated for exterior use and both cost significantly less than higher grades of plywood. The choice between them usually comes down to local availability and builder preference rather than performance.

CDX plywood is made from thin layers of wood veneer glued together. The C grade on the face means the surface has some knots and defects but is solid enough to walk on and hold fasteners. The D grade on the back means the underside has more voids and knots, but this does not matter because it faces the attic. CDX plywood is stiffer than OSB and holds fasteners slightly better, which is why some builders prefer it.

OSB (oriented strand board) is made from wood strands glued together in layers. It is slightly less stiff than plywood but performs well on roofs and costs less. OSB can absorb moisture more readily than plywood, so it should be installed with the same 1/8-inch gaps between sheets and should not sit exposed to weather for extended periods before shingles are installed.

Installation spacing and fastening

Plywood sheets should be installed with a 1/8-inch gap between them on all sides. This gap allows the wood to expand and contract with changes in humidity without buckling or splitting. Without this gap, plywood can swell and push against neighboring sheets, causing the roof surface to bulge or crack shingles.

Fasten plywood with 8-penny ring-shank nails or 1 1/4-inch screws spaced 6 inches apart along the edges and 12 inches apart in the field (the middle of the sheet). Ring-shank nails hold better than smooth nails and are the standard for roof sheathing. Screws are stronger but take longer to install and are usually reserved for high-wind areas or when the builder wants extra holding power.

Install plywood 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 reduces the chance of sagging between rafters.

Plywood thickness for different roof pitches and snow loads

Roof pitch (the steepness of the roof) and snow load (the weight of snow the roof must support) can affect the thickness you need. In areas with heavy snow, some building codes require 5/8-inch plywood even with 16-inch rafter spacing. Low-pitch roofs (less than 4:12) are more prone to water pooling, so thicker plywood helps prevent sagging that can trap water.

Check your local building code before you buy materials. Building codes vary by region and are based on climate, wind speed, and typical snow load. A building inspector or your local building department can tell you what thickness is required in your area. If you are replacing a roof on an existing home, the original plywood thickness is usually a safe choice unless your building code has been updated since the home was built.

Specialty plywood products for roofing

Some manufacturers make plywood with tongue-and-groove edges or with a layer of insulation glued to the underside. Tongue-and-groove plywood has interlocking edges that reduce gaps and can make installation slightly faster, but it costs more and is not required for most residential roofs. Insulated plywood can improve attic temperature but adds significant cost and is usually only used in new construction where energy efficiency is a priority.

For most roof replacements and repairs, standard CDX plywood or OSB in the thickness your rafter spacing requires is the right choice. Specialty products offer marginal benefits that do not justify the extra cost in most cases.

Frequently Asked Questions

Can I use 3/8-inch plywood for roof sheathing?

No. Building codes do not allow 3/8-inch plywood for roof sheathing on residential roofs. It is too thin to span between rafters without sagging, even with 16-inch spacing. The minimum is 1/2 inch for 16-inch spacing and 5/8 inch for 24-inch spacing.

What if my rafters are spaced 12 inches apart?

12-inch spacing is uncommon but does occur in some older homes or in areas with very heavy snow load. With 12-inch spacing, 1/2-inch plywood is more than adequate and will not sag. You can use 1/2-inch plywood without concern.

Does plywood need to be installed with a vapor barrier underneath?

No. Plywood itself is not a vapor barrier and does not need one underneath. Ventilation between the plywood and the shingles (called an air gap or ventilation space) is more important than a vapor barrier. Most modern roofing systems rely on ventilation to manage moisture rather than blocking it with barriers.

Can I use interior-grade plywood for roof sheathing to save money?

No. Interior plywood is not rated for exterior exposure and will delaminate (the layers will separate) when exposed to rain and sun. Use only exterior-rated plywood or OSB. The cost difference is small compared to the cost of replacing a failed roof.

How long can plywood sit exposed to weather before shingles are installed?

Plywood can tolerate brief exposure (a few days to a week) without damage, but it should be covered as soon as possible. Extended exposure to rain can cause swelling and delamination, especially with OSB. If weather delays your shingle installation, cover the roof with a tarp to protect the plywood.