Overview
Structural panels are the skin of a wood-framed building. The same two products, plywood and oriented strand board (OSB), get nailed down as subfloor over the joists, stood up as wall sheathing over the studs, and laid across the rafters as roof sheathing. They do three different jobs. The subfloor is the walking surface and the base for your finish floor, so it carries gravity load and has to stay flat and quiet. Wall sheathing braces the frame against racking and wind, which is a shear job. Roof sheathing spans between rafters or trusses and takes snow, wind uplift, and a roofer's boots. Every code-rated panel carries an APA stamp that tells you what it is allowed to do. The span rating on that stamp, not the species or the brand, is what matters when you spec a sheet. For sheathing duty, OSB and plywood are rated the same way and substitute for each other freely. I treat the stamp as the spec and the panel name as a footnote. Get the rating and the thickness right for your framing spacing, install it correctly, and the panel disappears into the assembly the way it should.
OSB vs Plywood
Here is the honest version. OSB is cheaper per sheet, comes more uniform with fewer voids and no football-shaped patches, and is available in larger formats for fewer seams. Plywood is lighter to throw around, holds a screw or nail a little better right at the panel edge, dries out faster after it gets wet, and is less prone to permanent edge-swell. Both are manufactured to the same APA performance standards, both are code-approved, and both carry the same span ratings. A 7/16 OSB wall sheathing panel and a 15/32 plywood panel rated for the same studs will do the same structural work. So the choice comes down to context. For wall and roof sheathing that gets covered quickly and stays dry, OSB is the default and there is no real penalty. For a subfloor that might sit open through a few rain delays, or a roof deck in a wet climate, I lean plywood for the faster drying and lower swell risk. Plywood also wins where you want the lighter sheet for solo work up on a roof. Neither is the wrong answer. Spec to the rating, then pick based on exposure and how fast you can dry it in.
Span Rating and Thickness
The APA stamp is the whole ballgame. On sheathing panels you will see two numbers like 24/16. The left number is the maximum on-center spacing in inches the panel can span as roof sheathing; the right is the max spacing as subfloor. So a 24/16 panel spans rafters up to 24 in oc and floor joists up to 16 in oc. Match the number to your framing, not the other way around. Dedicated subfloor panels carry a single Sturd-I-Floor rating instead, written like 20 oc or 24 oc, meaning the panel works as a combined subfloor over joists at that spacing. For floors, the workhorse is 23/32 or 3/4 in tongue-and-groove over joists at 16 in oc. Drop to 19/32 or 5/8 only if the rating and spacing back it up. Wall sheathing is commonly 7/16 OSB or 15/32 plywood. Roof sheathing usually runs 15/32 to 19/32 depending on rafter spacing and load. Read the full stamp: it lists the span rating, the thickness, the exposure durability classification (Exposure 1 handles construction-stage weather), and the mill number. If the stamp does not match the job, the panel is wrong no matter how good it looks.
Subfloor Install: Glue, Gap, Fasten
A quiet subfloor is built, not bought. Use tongue-and-groove panels so the long edges lock into the adjacent sheet and share load without a joist underneath them. Run the panels perpendicular to the joists and stagger the end joints so seams do not all land in one row. Then glue and screw, every time. Lay a continuous bead of construction adhesive on the top of each joist right before you set the panel, set the sheet into the wet glue, and fasten with ring-shank nails or subfloor screws. The glue bonds the panel to the framing so the two move as a unit; the mechanical fasteners hold it down while the glue cures and forever after. Nail-only floors squeak because the panel works loose against the shank as the wood cycles through humidity, and you get that pop every time someone walks the hall. Leave roughly a 1/8 in gap at panel ends and edges. Panels grow when they pick up moisture, and a tight-butted floor has nowhere to expand, so it buckles and telegraphs ridges through the finish floor. Most tongue-and-groove panels have spacer nibs molded into the edge to set the gap for you. Snap chalk lines for your fastener rows so you actually hit the joist.
Wall and Roof Sheathing
Wall sheathing does two things: it gives the wall its racking resistance, the shear strength that keeps the frame from folding over in wind or seismic load, and it provides a nailable backing for the weather-resistive barrier and cladding. The water control layer goes over the sheathing. With traditional sheathing you wrap it in housewrap or building felt, then flash and install siding. With ZIP System panels the WRB is integrated into the panel face and you tape the seams instead of wrapping, which saves a step but demands clean taping. Roof sheathing is sized to the rafter or truss spacing per the span rating, same as everything else. On the roof, install panel edge clips, the little H-shaped metal clips, on the unsupported long edges between rafters. They transfer load between adjacent panels and keep edges from deflecting under a load or a footstep. Gap the roof panels the same 1/8 in for expansion. The nailing schedule is what actually delivers the shear and uplift numbers the engineer specified: typically 6 in on center along panel edges and 12 in in the field, tighter in high-wind or braced-panel zones. Underdrive or overdrive the nails and you lose capacity, so set your gun depth and check it.
Common Mistakes
The same handful of errors show up on inspection over and over. Butting panels tight with no edge gap, which buckles the floor and starts the squeaks the day the building dries out. Nailing the subfloor down without glue, then chasing pops with a screw gun for the life of the house. Using the wrong span rating for the joist or rafter spacing, usually a thin panel that passed at the lumberyard counter and fails under load or feels spongy underfoot. Skipping H-clips on a roof deck, so the unsupported panel edges sag between rafters and you see waves in the shingles. Leaving panels stacked or installed and exposed to weather for weeks, which swells the edges, especially on OSB where the swell is permanent and shows as raised seams forever. And running seams without staggering them or landing panel ends in mid-air off the framing, which kills the shared-load behavior the whole system depends on. None of these are subtle, and none cost extra to avoid. They come from rushing the deck to stay on schedule. Slow down for the part that everything else sits on.
FAQ
Is OSB or plywood better for a subfloor? For the rating, they are equal. I lean plywood for a subfloor only because it dries faster and resists permanent edge-swell if the deck sits open through rain. If you can dry it in quickly, OSB is fine and cheaper.
What does 24/16 mean on a panel? It is the APA span rating. The first number, 24, is the max rafter spacing in inches for roof use; the second, 16, is the max joist spacing for subfloor use. Match it to your framing.
Do I really need to glue the subfloor if I screw it? Yes. Screws hold the panel down but the glue bonds it to the joist so they move as one. Glue-and-screw is what keeps a floor quiet; screws alone still work loose and squeak.
Why do roofers use H-clips? The little H-shaped clips sit on the unsupported panel edges between rafters and transfer load across the seam, so the edges do not sag or telegraph waves through the shingles. They also help hold the expansion gap.