Plywood vs OSB Roof Decking: Which Is Better Under Shingles?

The deck, also called sheathing, is the layer of panels nailed to your rafters. It does three jobs. It carries roof loads across the gap between rafters. It gives every shingle nail something to grip. And it ties the rafters together into one stiff plane that resists wind.

Shingles don't hold a roof down by themselves. They are fastened to the deck, and the deck is fastened to the framing. If the deck is soft, thin or delaminated, a perfect shingle installation is anchored to a weak link. That is why the deck is part of every roof replacement conversation, even though you never see it once the job is done.

Plywood is thin sheets of wood veneer glued together with the grain of each layer crossed at right angles. The crossed grain is what makes it stiff in both directions.

OSB, oriented strand board, is made from wood strands coated with resin and pressed into a panel, with the strands in each layer aligned in a chosen direction. It does the same structural job with a different internal structure.

Both are covered by U.S. product standards. PS 1 is the standard for structural plywood. PS 2 is a performance standard for wood structural panels that sets criteria for specific construction uses, so a plywood or OSB panel can be rated to it.³ APA's position is that, used in their intended applications such as sheathing, the two are made to standards that assure very similar performance.¹

Every rated panel carries a grade stamp. Three items on it matter for a roof:³

• Span rating, two numbers such as 24/16. The left number is the maximum recommended spacing of roof supports in inches. The right number is the same for subfloors.
• Performance Category, a designation tied to the panel's nominal thickness as used in the building codes.
• Bond classification, Exterior or Exposure 1, based on the weather and moisture exposure the panel is built to withstand.

APA's construction guide publishes the allowable roof spans for each rating:⁴

Edge support means blocking, tongue-and-groove edges or panel clips (often called H-clips) at the joints between rafters.⁴ The residential code uses the same span-rating system in Table R503.2.1.1(1). Fort Worth's roofing FAQ, citing the 2021 IRC table, notes that 3/8-inch plywood or OSB is still allowed for rafters at 24 inches on center when the panel is rated 24/0.⁵

The shingle maker can be stricter than the code. Owens Corning's installation instructions call for a minimum of 3/8-inch plywood or 7/16-inch OSB, with OSB sheathing spaced at least 1/8 inch and at most 1/4 inch apart.⁶

Both panels are hygroscopic: they take on and give off moisture like all wood products.² The difference is in how they swell. From APA testing, as summarized in NRCA's trade magazine:²

• OSB's thickness swell is generally greater than plywood's. In testing, OSB swelled about 3.5 times more than plywood.
• Thickness swell is especially sensitive to one-sided wetting, the kind a deck gets during construction. Three days of one-sided wetting produced about the same swell as 24 hours of total immersion.
• Both panels grew about 0.23% in length along their strength direction. Across it, plywood grew about 0.3% and OSB about 0.38%. Movement like that is why panels need an expansion gap.

NRCA has long-term concerns about OSB, citing dimensional changes, ridging and fastener backout, and prefers PS 1 plywood when given a choice.² Ridging is the visible line of raised shingles over swollen panel edges.

The fixes are the same for either panel. Leave the expansion gap: APA recommends 1/8-inch spacing at all edge and end joints unless the panel maker says otherwise.⁴ Keep the deck dry: Owens Corning's instructions say to prevent the wood deck from getting wet before, during and after installation.⁶ And keep the attic ventilated so the deck isn't soaking up moisture from below. See attic ventilation science.

A tear-off is the one time the deck is fully visible. That is the moment to judge it. Fort Worth's roofing FAQ puts the rule plainly: sheathing in good condition does not need to be replaced, and the code does not allow a new layer of decking to be installed over an existing one.⁵ So bad sheets come out, and new sheets go in at the same plane.

Sheets typically get replaced when they are:

• Soft or spongy underfoot, or sagging between rafters.
• Delaminated, with plies separating, or swollen and crumbling at the edges.
• Stained dark and weak from long-term leaks, often at eaves, valleys and around penetrations.
• Thinner than the shingle manufacturer's minimum.

Local rules can set the replacement panel. Universal City's re-roofing bulletin requires damaged or water-soaked sheathing to be replaced with a minimum of 5/8-inch decking.⁷ The IBHS FORTIFIED re-roofing checklist allows roofs with sheathing below the minimum thickness to be redecked.⁸

This is also the strongest argument against an overlay: an overlay never shows you the deck. See roofing over old shingles. On cost, our roof cost guide explains that decking is priced per replaced sheet, so you pay for the sheets that actually need it.

Deck to rafters. APA's minimum schedule for roof sheathing up to 1-inch Performance Category is 8d nails at 6 inches on center along supported panel edges and 12 inches in the field, with increased schedules possibly needed in high-wind zones.⁴ The FORTIFIED high wind and hail re-roofing checklist goes further: 8d ring-shank nails at 6 inches on center.⁸ Panels go with the long dimension, the strength axis, across the rafters, because that is the direction the span rating assumes.⁴

Shingles to deck. Owens Corning requires every shingle fastener to penetrate at least 3/4 inch into the deck, or completely through it by at least 1/4 inch.⁶ On thin decking, that means nail points come through into the attic. That is normal. Rusted nail points seen from the attic are a sign of moisture in the roof assembly worth investigating.

Central Texas is humid, and spring storms can arrive mid-job. A deck opened up in the morning should be dried in with underlayment the same day, because one-sided wetting is exactly the condition that makes panels swell. Ask your contractor how they protect an open deck if weather moves in.

Local requirements vary across the metro. Universal City sets a 5/8-inch minimum for replacement decking.⁷ Other cities follow the IRC table as adopted. If your home is in Universal City or a neighboring city, the replacement sheets should match the local rule, not just the old panel.

Decking is priced per replaced sheet on our roof cost guide, so a quote should say how sheets are counted and priced rather than burying them in a lump sum. Sound sheets stay. Ask to be shown the damaged ones before they go on the trailer.

A small area of soft decking under a leak can be replaced as part of a roof repair, provided the shingles around it are healthy. Widespread deck damage usually means water has been getting in for a long time, and the roof covering is likely near the end of its life too. Our rule of thumb is that repair wins when it costs under 20% of a replacement. See repair vs replacement.