Structural sheathing is the layer that turns a pile of framing lumber into a stable building. Panels brace walls against wind, give roofs a surface for shingles, and carry the daily load of people walking across a second floor. Most homes built today use engineered wood panels: oriented strand board (OSB) or plywood. Both do the job, but they differ in cost, moisture behavior, and fastener holding. Choosing well starts with knowing how each panel is made, how to read the grade stamp, and how installation rules change the performance. The rest of the wall stack matters just as much, and the rigid foam sheathing placement guide covers whether that insulation belongs inside or outside the framing.
What Engineered Wood Sheathing Is
Engineered wood panels are built from layers of wood bonded with adhesives under heat and pressure. OSB starts as wood strands, typically 3 to 4 inches long, laid in crossing layers and fused with wax and resin. Plywood starts as thin veneers peeled from logs, stacked with the grain running in alternating directions, and glued together. The cross-lamination gives both products their strength: a load pushing on the panel in one direction is carried by strands or veneers running in the other.
Because the layers run in opposite directions, the panels resist racking, the sideways force that tries to turn a rectangular wall into a parallelogram during a windstorm or an earthquake. Panel thickness, span rating, and fastening schedule determine how much of that force a wall can carry, and building inspectors check those numbers before the structure is approved.
The panel is only half of the wall assembly. The insulation layer can sit inside the cavities or outside the panels, and the choice changes the moisture profile of the building. The guide on foam sheathing should be installed inside or outside the framing walks through the trade-offs for each arrangement.
OSB vs Plywood: How the Panels Compare
Both panels meet the same structural standards, so the practical differences come down to handling, moisture, and price. The numbers below are typical for 4-by-8-foot panels at 7/16-inch and 15/32-inch thickness, and the table summarizes the points that matter on a jobsite.
| Characteristic | OSB | Plywood |
|---|---|---|
| Raw material | Cross-layered wood strands | Cross-laminated veneers |
| Surface | Smooth and uniform, no knots | Visible grain, occasional patches |
| Strength | Comparable at equal thickness | Comparable at equal thickness |
| Edge behavior when wet | Cut edges swell noticeably | Less edge swelling |
| Fastener holding | Good | Good |
| Typical price per sheet | Lower | Roughly 20 to 40 percent higher |
| Weight | Slightly heavier | Slightly lighter |
Reading the Grade Stamp
The grade stamp carries the information that matters. Look for the APA trademark, the panel grade (Structural I is the strongest common grade), the span rating (a wall panel is often rated 24/16, meaning 24 inches on center for roof use and 16 inches for subfloor use), and the exposure rating. Exposure 1 panels tolerate construction moisture, not permanent wetting, and that distinction decides how long panels can sit in the weather during a build.
The most common leak path is not the panel face but the bottom edge, where sheathing meets the foundation. Water splashing off the ground wicks up the cut edge, so the sheathing to foundation connection needs a capillary break and clearance above the soil. The Fine Homebuilding guide to installing ZIP system sheathing covers that connection detail in practice.
Installing Wall Sheathing: Layout, Gaps, and Fasteners
Panel installation follows a handful of rules that have outsized effects on strength and moisture.
- Start at a corner and work across the wall, keeping panel edges centered on studs.
- Leave a 1/8-inch gap between panels so they can expand and contract without buckling.
- Fasten the panel to every stud around the perimeter and across the field.
- Stagger vertical joints from one row to the next, the way brick courses stagger.
- Cut openings for windows and doors after the panel is fastened, using the marked frame layout.
Panel orientation matters more than many installers realize. On walls, the long dimension runs vertically so the panel spans from the top plate to the bottom plate without a horizontal joint. Horizontal joints are allowed, but they need blocking or a double row of fasteners, because a joint with nothing behind it flexes and later cracks the finish. On roofs, the long dimension runs across the rafters so the panel bridges at least two rafter bays.
Fastener Schedules
For 7/16-inch and 15/32-inch wall panels, common nails spaced 6 inches on center along panel edges and 12 inches on center in the field are standard practice. Screws of the same length follow the same spacing and resist pull-out better. Fasteners must land in framing rather than in the gap between panels, and they should sit about 3/8 inch from the panel edge so they do not split it. Where panels bridge a floor line or a wall-to-wall corner, add blocking so every panel edge has solid wood behind it.
Cutting and Working Around Services
Cut panels with a circular saw set to the panel thickness, and support the sheet so the blade does not bind. When plumbing, wiring, or ducts pass through a wall, cut the openings cleanly and seal them. Ducts are especially vulnerable: metallic sheathing on ducts protects the metal from puncture and keeps insulation from being crushed, and the same care applies when you cut the panel around a duct chase. Never leave an opening without a patch or blocking, because holes cut the panel’s shear strength and open the wall to pests.
Subfloor and Roof Applications
Subfloor panels are thicker than wall panels. The standard floor panel is 23/32-inch tongue-and-groove, sold as 3/4 inch, for joists spaced 16 or 24 inches on center. The tongue-and-groove edges lock the panels together so the loaded floor acts as one surface instead of separate boards. Installers run a bead of subfloor adhesive on the joist, set the panel, and fasten it with screws or ring-shank nails, typically 6 inches apart on the edges and 12 inches in the field.
On roofs, 7/16-inch or 15/32-inch panels span rafters spaced 24 inches on center. Lay the long dimension across the rafters, stagger the joints, and keep the same 1/8-inch gap rule. Roof panels need to be dry before the underlayment goes down.
Span Ratings and Thickness by Use
- 7/16 inch: wall sheathing and roof panels over framing 24 inches on center.
- 15/32 inch: walls and roofs where higher wind resistance is specified.
- 19/32 inch: subfloor over joists 16 inches on center in some assemblies.
- 23/32 inch: standard subfloor panel over joists up to 24 inches on center.
Subfloor panels go down after the framing is dried in, but the deck is still exposed to weather on many job sites. If a panel gets rained on, let it dry before it is covered with underlayment or tile. Covering a wet deck traps the moisture, the floor cups, and the problem telegraphs through the finish floor. Fastener heads should be driven flush rather than buried, because a countersunk head weakens the panel face.
Under the floor or behind the wall, the insulation can be continuous foam rather than cavity fill. The technical guide to EPS, XPS, and polyiso rigid foam boards explains the R-values and vapor behavior of each type, which matters when the foam sits directly against the sheathing and changes how the panel dries.
Shear Walls and Lateral Bracing
On a windy site or in a seismic zone, some walls are designed as shear walls that carry the lateral loads trying to push the building over. The sheathing does the work, so the details come straight from the engineer’s drawings: panel grade, fastener spacing, and hold-down anchors. Nailing on a shear wall is often tighter than standard framing, sometimes 4 inches on center at panel edges, and the wall needs a continuous load path from the roof down to the foundation. The shear panel field guide walks through the layout, fastening, and inspection details that make a shear wall actually work.
Moisture Management and Weather Exposure
Engineered wood panels are moisture-resistant, not waterproof. Exposure 1 panels tolerate rain during construction, and premium panels with enhanced resin systems last longer in the weather, but every panel has limits. Store panels flat, off the ground, and covered. Install them only when the framing is dry, and cover the walls with housewrap or foam as soon as the panels are up.
Ventilation gives the panels a second line of defense. Roof panels dry from the attic side when ridge and soffit vents move air across the underside, and wall panels dry through the cavity when the assembly has the right vapor profile. Sealing every gap with foam and caulk sounds efficient, but a wall that cannot dry on either side holds moisture inside the panel.
The weather barrier works with the panel rather than against it. Water that gets past the siding runs down the face of the panel and out of the wall, but only when the layers are sequenced correctly. The installation guide for foam sheathing covers thickness requirements, vapor barrier placement, and housewrap integration, the details that keep the sheathing dry for the life of the building. When water stays out of the panel, the structure stays straight and strong, and the sheathing does the job it was engineered for.