Roof Decking & Plywood: Types, Thickness, OSB vs. Plywood & Cost

Roof decking — also called roof sheathing — is the layer of structural panels (plywood or OSB) that covers your rafters and gives your shingles a solid, nailable base. Get the thickness, material, and underlayment right and your roof lasts decades; get them wrong and you risk sagging, leaks, and voided warranties. This guide from the licensed DFW team at 3:16 Roofing covers plywood vs. OSB, the right thickness for your rafter spacing, underlayment, nailing, and cost.

What is roof decking (sheathing)?

Roof decking is the flat structural surface — almost always 4x8 sheets of plywood or oriented strand board (OSB) — fastened across your roof rafters or trusses. Everything above it (underlayment, then shingles or metal) depends on solid decking underneath. If the decking is rotted, sagging, or too thin, new shingles alone will not fix the underlying problem.

What size and thickness of plywood do you need for a roof?

Thickness is driven by your rafter spacing and local building code. As a general guide under the International Residential Code (IRC): for rafters 16 inches on center, 3/8-inch is the bare minimum, but 1/2-inch (15/32) plywood or 7/16-inch OSB is standard practice. For 24-inch spacing, step up to 1/2-inch plywood or 15/32-inch OSB, with 5/8-inch often recommended for added stiffness. Always confirm with your local code and your shingle manufacturer — Tarrant and Denton county jurisdictions can have their own amendments.

OSB vs. plywood for roof decking — which is better?

Both are code-approved. Plywood is stiffer for its weight, more moisture- and impact-resistant, and dries faster if it gets wet — but it costs more. OSB is more uniform, less expensive, and resists splitting, but it can swell and hold water longer if exposed during a re-roof. For most DFW homes either works; we recommend based on your budget, roof design, and how exposed the deck is during the job.

What type of plywood is best for roofing?

For roof decking, use CDX-grade plywood (the X means exterior-grade glue) or APA-rated sheathing panels rated for roofs. Avoid thin or interior-grade plywood. The panels should carry an APA span rating that matches your rafter spacing.

How much does roof decking cost?

Material prices move with the lumber market, so treat any figure as a rough guide: roof-grade plywood generally costs more per sheet than OSB, and full decking replacement is usually priced per square (100 square feet) on top of your shingle install. The biggest cost swing is how much decking actually needs replacing — often you only learn that once the old roof is off. The honest answer is to get a free inspection so the number is based on your actual roof, not a guess.

What roof underlayment types are there?

Underlayment is the water-resistant layer between your decking and shingles. The three common types are felt (15-lb or 30-lb asphalt-saturated paper — the traditional budget option), synthetic underlayment (lighter, stronger, more tear- and moisture-resistant — now the DFW standard), and peel-and-stick (self-adhering, used in valleys, eaves, and high-leak-risk areas, and smart in hail-prone regions). Code requires underlayment under all asphalt shingle roofs.

How many nails per square should shingles have?

Fastening matters as much as the deck. Standard application is 4 nails per shingle — about 320 nails per square (100 square feet). High-wind and steep-slope applications use 6 nails per shingle — about 480 per square. DFW wind and hail exposure often calls for the 6-nail pattern, and using the wrong pattern can void your shingle warranty. Always follow the manufacturer nailing instructions and local code.

Need roof decking or a re-roof in Keller or DFW?

If your roof is sagging, leaking, or due for replacement, the decking underneath is worth a professional look. 3:16 Roofing and Construction is a BBB A+, Google Guaranteed, RCAT-licensed (#03-0246) roofing contractor in Keller, TX serving the Dallas-Fort Worth metroplex, with a 4.9-star rating from 235+ verified reviews. We inspect your decking, underlayment, and shingles for free and give you an honest, written assessment. Call (817) 402-7663 to schedule.