Plywood vs. OSB for Roof Decking: What Bay Area Homes Actually Need

Walk into any reroof estimate in Los Gatos and at some point the conversation comes around to the wood. Plywood or OSB. Half-inch or three-eighths. Radiant barrier or standard. The bid sheet uses words most homeowners have not had to think about since their last home improvement project, and a lot of it sounds interchangeable. It is not. The decking layer — that flat sheet of wood between your rafters and the shingles you can actually see — is the foundation every other part of the roof depends on, and the choices we make there shape how the roof performs for the next 30 years. Here is the conversation we have with most homeowners, written down.

Why Bay Area Roofs Have a Decking Conversation at All

Almost every original home in Los Gatos, Saratoga, Monte Sereno, and the older neighborhoods of San Jose was built with a wood shake or wood shingle roof. Those roofs sat on what the trade calls space sheathing or skip sheathing — 1×6 boards run across the rafters on 10-inch centers, with gaps in between. The gaps were intentional. Wood shakes need air on both sides to dry out, and the open sheathing let the roof breathe.

That worked beautifully for shake roofs. It does not work at all for asphalt shingles. A composition shingle cannot bridge the gap between two pieces of skip sheathing — the nails would never hit anything solid, and the shingle would deflect under load. So when the valley converted from shake to composition over the last 40 years, every reroof had to add a layer of solid sheathing on top of the original 1x6s before the new shingles could go down. That layer is what we are talking about when we say plywood or OSB.

A second housing type runs into a similar problem. Many homes built in the 1940s and 1950s went up with composition shingles from day one but used solid 1×6 decks instead of plywood, because plywood was not a common construction material yet. Those decks looked fine when they were new, but 70 years later the boards have cracked, the knots have fallen out, and the gaps between boards have widened. Drive a roofing nail through one of those gaps and the nail wiggles every time the roof heats and cools. That wiggle eventually leaks. Look at the underside of a 70-year-old composition roof in your attic and you will see rusted nail tails sticking through every gap in the sheathing — a roof that has been failing slowly for decades.

For both of those situations, the fix is the same: install a continuous solid deck on top of the original sheeting before the new roofing goes down. Which brings us to the question of what wood to use.

Plywood: The Original Answer

Plywood is laminated. A log gets peeled into thin, perfect veneer sheets, and those sheets get glued together with the grain alternating direction layer by layer. A 3-ply panel has three veneers, a 5-ply panel has five. The cross-laminated grain is what gives plywood its strength — load applied in any direction is resisted by veneers running both ways.

For roofing, the plywood you actually see on a job is almost never half-inch, even if that is what the homeowner asks for. The trade standard is 15/32-inch CDX, which is a 3-ply panel just slightly under half an inch. (Lumber is full of these “nominal vs. actual” measurements — a 2×4 is actually 1.5×3.5, and 15/32 is the real number behind what people call “half-inch plywood.”) The thinner 3/8-inch plywood that was common on 1960s decks does exist but it is not industry standard, and we usually have to re-sheath over it during a reroof.

Plywood has two real advantages. It is roughly 10% stiffer than OSB sheet for sheet, and the laminated veneer holds nails slightly better. Where it loses ground is on the cost side and — more importantly for a roofer — on consistency. A piece of CDX plywood has knots and voids in it. When our crew is nailing off a roof and the air gun is calibrated to drive a nail through the shingle without piercing the surface, hitting a knot in the ply will overdrive that nail. It is a small thing on any one nail. Across 30 squares of roof, it adds up.

OSB: The Newer Standard

Oriented strand board entered the residential market in the early 1990s and quickly became the default for new construction. The product is exactly what it sounds like: long strands of wood, oriented in a controlled direction, glued together under heat and pressure. It is not particle board, which is a much weaker shelving material. OSB is engineered structural sheathing rated to the same standards as plywood for roof and wall applications.

The advantage that pushed OSB to the front of the market is yield. A log gets peeled for plywood and the leftover material that does not make a perfect veneer becomes scrap or chips. OSB uses the strands and chips. One tree produces dramatically more sheathing as OSB than as plywood. As public attention turned toward forestry practices, that efficiency mattered, and the price reflected it — OSB was historically 10–15% cheaper than plywood, though the gap has narrowed in recent years.

For roofing specifically, OSB has one practical advantage that plywood cannot match: density. An OSB panel is consistent corner to corner. There are no knots to overdrive a nail through. When our crew is shooting nails into 30 squares of decking, every fastener seats the same way, every shingle lays flat, and the finished roof has the smooth, uniform substrate that asphalt shingles need to perform across a 30-year warranty period.

What About Swelling — Does OSB Hold Up?

This is the most common pushback we hear. Homeowners read online that OSB swells when it gets wet, and they want plywood instead. There are two things worth saying here.

The first is that both products are designed not to be exposed to weather. Plywood gets wet, plywood delaminates. OSB gets wet, OSB swells. They are both engineered wood products and they both need to be protected. If your sheathing is getting wet under your roofing system, the conversation we are having is no longer about sheathing material, it is about the roofing product and installation.

The second is that OSB does swell more permanently at the cut edges than plywood does. When OSB gets soaked, the edges expand and stay slightly expanded even after drying. Plywood, by comparison, returns closer to its original thickness as it dries out. In high-humidity coastal climates or in an attic with active condensation problems, that difference can matter. In the dry summer climate of the South Bay, with a properly installed roof over the right underlayment, it almost never does. Our standard reroof uses 7/16-inch OSB and we have been watching those decks perform across thousands of Los Gatos and San Jose homes for over 25 years.

What Each Product Weighs

Homeowners occasionally worry about the weight of adding sheathing to an old roof. The math works strongly in your favor. A wood shake roof — the original roof on most South Bay homes — weighs about 2.5 pounds per square foot when dry, and roughly 5 pounds per square foot after a few days of rain because the wood absorbs water. OSB adds about 1 pound per square foot. Composition shingles add roughly another 3. Even with a full new sheathing layer plus modern composition, the finished assembly typically weighs less than what was up there before. Weight is almost never the constraint. A lightweight roof is normally classified as being under six pounds per square foot, or “Sub Six”.

Where Radiant Barrier Fits Into This Decision

If you have decided to upgrade your sheathing as part of a reroof — and on most Bay Area homes you will have to — the radiant barrier upgrade is a small additional cost for a meaningful comfort and energy benefit. Radiant barrier OSB is the same 7/16-inch panel we already install, with a thin perforated aluminum foil bonded to the bottom face at the factory. The foil reflects up to 97% of radiant heat trying to enter the attic, drops attic temperatures by as much as 30°F on a hot afternoon, and trims cooling costs by 5–17% depending on the climate and the home.

The reason it pairs naturally with a Bay Area reroof is the same reason the decking conversation exists in the first place: skip sheathing. A radiant barrier needs an air space on the foil side to do its job. New construction puts the OSB on open rafters with empty bays underneath, which is where you get the 97% performance number. On a reroof over existing 1×6 skip sheeting, the foil contacts the boards in some places and bridges the gaps in others. Real-world effectiveness lands in the 40–60% range — not the published spec, but a real reduction in attic temperature for a small upcharge on material we were going to install anyway.

Radiant barrier is also one of the three accepted compliance paths under California’s Title 24 energy code, which applies to most reroofs that replace more than half the roof surface. The other two paths are upgraded attic insulation and a high-reflectance cool-roof shingle. Radiant barrier OSB lets you satisfy the code without changing your shingle color or opening up the attic. We learn the local building department’s preferences and handle the documentation as part of the permit.

What We Actually Specify on Most Reroofs

On a typical residential reroof in Los Gatos, San Jose, Cupertino, or any of the surrounding South Bay neighborhoods, our standard spec is 7/16-inch radiant barrier OSB installed over the existing skip sheathing. We tear off the old roofing down to the original deck, inspect every rafter and every sheathing board, replace anything rotted, or structurally compromised, lay down the new Radiant Barrier OSB foil-down, and nail it off to code. From there the roofing system goes on top: Type 30 felt underlayment, cap sheet in the valleys, then the shingles. The whole assembly is a single coordinated system rather than a stack of independent products, and the decking is the foundation it all sits on.

The choice of plywood or OSB for that decking layer is no longer a close call for most reroofs. OSB gives the crew a more consistent nailing surface, performs identically under code, and pairs naturally with the radiant barrier upgrade. Where plywood still earns its place is in specific situations — repair work tying into existing plywood, exposed eave details where the underside will be visible, or premium custom applications where 5/8-inch material is specified for span or wind requirements. We use whichever the project actually calls for.

A Note on the One-Inch Solid Decks

If your home was built in the 1940s or 1950s with a solid 1×6 deck and the original composition shingles, the reroof conversation is slightly different. Those decks are usually too far gone to nail directly to — the boards have cracked, the spaces have widened, and the knots have fallen out. The fix is the same as the skip-sheeting case: a continuous OSB layer goes over the top before the new roof system goes down. The benefit, if there is one, is that you do not need a radiant barrier as urgently, because solid 1×6 decks already block more radiant heat than open skip sheeting does. We will look at your specific decking during the inspection and tell you what makes sense.

What Honest Roof Decking Looks Like

The best decking for your home is the one that gives the roofing system above it a flat, consistent, code-compliant nail base — that is the whole job. For most Bay Area reroofs, that comes down to 7/16-inch OSB, with a radiant barrier upgrade if you want the energy benefit and a Title 24 compliance path. The marketing differences between manufacturers matter less than installation quality, and installation quality is where most roofs actually fail.

If your roof is approaching the end of its life and you are starting to wonder about decking, an honest inspection now is always less expensive than an emergency tear-off later. Our team has been doing this exact work on Bay Area homes since 1978 — family-owned, no subcontractors, 400-plus five-star reviews — and we are happy to come walk the roof with you.