Plywood vs OSB roof decking: what sits under your shingles
Both panels carry the shingles. The real difference shows up only when water gets to the edges, and that is worth knowing before a roofer says the decking is rotten.
Plywood and oriented strand board (OSB) are both accepted as roof decking, and neither is automatically the better roof. Plywood is thin wood veneers glued in layers. OSB is wood strands pressed and glued into a panel. If the roof stays dry, either one lasts for decades. If water reaches the panel edges, they age differently, and that difference is the only part that matters to you as an owner.
What decking is, and why you rarely see it
Decking is the flat layer nailed over the rafters or trusses. The underlayment, the shingles and the flashing all sit on top of it. You cannot inspect it from outside, which is why it usually enters the conversation only when a roofer opens the roof and reports what was underneath. From inside the attic you can see its underside with a flashlight, and that is the safest place to judge it.
How plywood and OSB behave when they get wet
Plywood tends to dry out and keep much of its stiffness after a short soaking. Repeated wetting makes the layers separate at the edges and the surface ripple. OSB is more sensitive at the cut edges: they swell as strands absorb water, and the swelling does not fully reverse once the panel dries. A swollen edge can telegraph through the shingles as a faint ridge along a seam.
Neither behaves well in a chronic leak. That is the honest summary. A panel that has been wet for a season is a panel to replace, whichever kind it is.
Which one is on your roof?
Look from the attic, with a flashlight, at the underside between two rafters. Plywood shows a smooth grain with layered edges at the seams. OSB shows a rough mosaic of flat wood chips. Many homes built since the 1990s have OSB. Older homes often have plywood, or in some regions plank sheathing, which is a third case with its own rules.
Signs the decking has a problem
Some signs are visible from the ground: a sag between rafters, a wavy line of shingles, a soft-looking patch. Others are visible only inside the attic:
- Dark staining or rings on the underside of a panel
- Edges that look puffed, flaky or split apart in layers
- Nail tips with rust streaks running down
- Light showing through a seam that should be tight
Any of these points to a moisture source above or a ventilation problem below. Finding the source matters more than choosing a panel type. New decking under a roof that still leaks will fail the same way.
Why you should not go up to test it
Some guides suggest walking the roof and pressing on it to check for soft spots. This site does not, and here is why. A weakened panel can give way under body weight, and the fall from a roof edge is the injury that ends up in hospital statistics. The information you would gain from a boot press is available from the attic and from a roofer who works with fall protection. Stay on the ground or inside the attic.
What to ask a roofer about decking
Ask them to state in writing how they will handle rotten decking they find once the shingles come off. The usual arrangement is a per-sheet replacement clause, agreed before work starts. That way, the discovery of bad panels does not turn into an argument in the middle of the job. Ask which panel they plan to use, and ask that the new panels match the thickness of the existing ones so the roof plane stays flat. Also ask them to photograph what they find, before and after.
The bottom line
Do not treat plywood versus OSB as a reason to reject a house or to argue with a roofer. Treat it as background. The decision that changes the outcome is whether water is getting in, and whether the panels have already paid for it. If they have, replace the wet ones and fix the source. If they have not, leave them alone.
Frequently asked questions
Is plywood or OSB better for roof decking?
Neither wins outright. Both are approved and both last for decades when dry. Plywood usually tolerates short wetting better, while OSB edges swell and stay swollen after repeated soaking.
How can I tell which one my roof has?
Look at the underside from inside the attic with a flashlight. Plywood shows smooth grain and layered edges. OSB looks like a mosaic of flat wood chips.
Can I check the decking by walking on the roof?
No. A weakened panel can give way under body weight. Inspect from the attic instead, or have a roofer with fall protection look at it.
What does rotten decking look like from the attic?
Dark stains or rings, puffed or flaking edges, rust streaks on nail tips and light showing through seams. Any of these means moisture has been reaching the panel.
Should decking be replaced during a re-roof?
Only the panels that are wet, soft or delaminated. Agree a per-sheet clause in writing before work starts so hidden damage is not a surprise.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.
- NRCA. National Roofing Contractors Association
- US OSHA. 1926.501 — Duty to have fall protection (OSHA)