Both are code-compliant. Both are used constantly. The difference between them matters more in Seattle than in most of the country, and the reason is entirely about how each material behaves when it gets wet and then has to dry.
Plywood is thin wood veneers laminated with the grain of each layer running perpendicular to its neighbours. That cross-graining is what gives it dimensional stability and strength in both directions.
OSB is engineered from wood strands oriented in layers and bonded under heat and pressure with resin. It uses smaller, faster-growing timber, which makes it cheaper and more resource-efficient, and structurally it is entirely capable — it has been the dominant sheathing material in North American construction since the 1980s for good reason.
Moisture behaviour, specifically what happens after wetting.
Plywood absorbs water relatively slowly and, importantly, releases it relatively evenly. When it dries it returns close to its original dimension. Repeated wetting eventually causes delamination between the veneers, but the process is gradual.
OSB absorbs water more readily at its cut edges, where the strand structure is exposed, and it swells there. The critical property is that this swelling is largely irreversible — when the panel dries, the edges do not return to their original thickness. The result is the characteristic failure you see on water-damaged OSB roofs: intact-looking sheets with raised, spongy, crumbling perimeters, producing visible ridges telegraphing through the shingles above.
Two exposures matter, and we have both.
The installation window. A tear-off leaves the deck exposed. In a climate with roughly 150 days of measurable precipitation, scheduling a re-roof that is guaranteed dry from tear-off to dry-in is not realistic for much of the year. Decking gets rained on. Plywood tolerates that episode better.
The service environment. More importantly, a large share of Seattle deck damage comes from attic condensation wetting the sheathing from below, persistently, through an eight-month wet season. That is not a single soaking, it is chronic low-grade moisture, and it is precisely the exposure where the difference in recovery behaviour compounds.
OSB is not a bad product and treating it as one would be wrong. On a properly dried-in roof, with functioning attic ventilation and no chronic moisture source, OSB performs well for decades and is meaningfully cheaper — on a full roof, the material saving is real.
Modern OSB also comes in enhanced versions with edge sealing and improved moisture resistance that substantially narrow the gap, and some products carry extended warranties against edge swell. If cost is a genuine constraint, OSB with sound detailing is a defensible choice, not a compromise you should feel bad about.
Our preference for plywood in this market is a lean, not a rule. Where it becomes closer to a rule is on repair and patch work: matching thickness matters, and mixing an OSB patch into a plywood deck creates a plane discontinuity that telegraphs through the finished surface.
Three details affect deck performance more than plywood versus OSB does.
Thickness relative to rafter spacing. Seven-sixteenths at 24 inch centres is common, code-compliant, and noticeably flexible underfoot. Half-inch or five-eighths gives a stiffer deck and better fastener holding. When someone is already sheathing your roof, stepping up a thickness is a small incremental cost for a durable improvement.
Expansion gaps. Panels expand with moisture uptake. Butted tight, they have nowhere to go and buckle, producing ridges that telegraph through the shingles. A small gap at panel ends and edges is standard practice and gets skipped by crews working fast.
Edge support. Where panel edges fall between rafters, clips support the unsupported edge and prevent differential movement between adjacent panels. Inexpensive, frequently omitted.
When comparing quotes, ask what material, what thickness, whether edge clips are included, and what the plan is if it rains between tear-off and dry-in. That last question is the one that separates contractors who have thought about working in this climate from those who schedule as though they were in Denver.
Every structural panel carries a grade stamp, and two numbers on it determine whether it is right for your roof.
The span rating appears as two numbers separated by a slash, such as 24/16. The first is the maximum recommended spacing in inches when used as roof sheathing; the second is the maximum spacing for subflooring. So a panel rated 24/16 is approved for roof rafters up to 24 inches on centre. Panels rated 32/16 or 40/20 span further.
This matters because rafter spacing varies, particularly on older homes where framing was not always laid out to modern conventions. A panel adequate at 16 inch spacing is not automatically adequate at 24.
Exposure durability. Look for Exposure 1, which indicates the adhesive can tolerate moisture during construction delays. In a climate where a roof may sit open through a rain event between tear-off and dry-in, that classification is doing real work rather than being a formality.
If a contractor cannot tell you the span rating of what they are installing, that is worth pressing on. It is printed on every sheet.
The common configurations and what to expect from each:
7/16 inch at 24 inch spacing. Code-compliant with an appropriate span rating, and the most common new-construction default because it is the cheapest compliant option. Noticeably flexible underfoot, and it benefits substantially from panel edge clips where edges fall between rafters.
1/2 inch at 24 inch spacing. A meaningful improvement in stiffness and fastener holding for a small material cost increase.
5/8 inch at 24 inch spacing. Stiff, holds fasteners well, tolerates foot traffic during future maintenance without flexing. Where a homeowner is already paying for a skip sheathing overlay across a whole roof, the incremental cost of stepping up from 7/16 to 5/8 is one of the better value upgrades in the entire project, because the labour is identical and it is a one-time decision for the life of the structure.
Matching existing thickness on patch work. Non-negotiable. A thickness mismatch creates a step in the roof plane that telegraphs through the finished shingles as a visible line, and it will be visible from the street for the life of the roof.
Small metal H-shaped clips fitted between panels where an edge falls between rafters with no framing beneath. They transfer load between adjacent panels so the unsupported edges cannot move independently.
Without them, adjacent panel edges deflect differently under load, and over time that differential movement telegraphs through the roof surface and can crack shingles along the line. They cost very little and are omitted routinely. Ask whether they are included.
Sheathing fastening pattern is a code requirement, not a preference, and it is what transfers wind uplift from the roof surface into the structure. Under-fastened sheathing is a structural weakness that is invisible once the roof is on.
Related and worth asking about on any older home: re-nailing the existing sheathing during tear-off. Original fastening on a 1920s house may be sparse, corroded, or both. Adding fasteners while the deck is exposed costs almost nothing in labour because someone is already standing there, and it is impossible afterwards without removing the roof again. It is the cheapest structural improvement available during a re-roof and it is almost never volunteered.
This post is part of our in-depth coverage of roof replacement in Seattle.
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