Plywood and OSB are the structural panels most often specified to sheathe wood framing and increase its shear strength. For example, a 20-ft. wall sheathed with %6-in. plywood can withstand more than a ton of lateral force pushing against the top of the wall.
Structural plywood is made by laminating softwood plies. Each panel is stamped to indicate veneer grade, species group or span rating, thickness, exposure durability, mill number, and certifying agency.
Veneer grades. Veneer grades range from A to D, with letters appearing in pairs to indicate the front and back veneers of the panel. "A/B Exterior,” for example, has a grade A front veneer, a grade B back veneer, and grade C inner plies. When you buy CDX (C/D exterior-grade), it’s advisable to place the grade C side toward the weather—or up, if used as subflooring.
Most roof and wall sheathing and subflooring is CDX. If a panel is also stamped PTS, its imperfections have been plugged and touch sanded. Lower veneer grades have more plugs and bigger knots.
Grade D is the lowest grade of interior plywood panels; it should not be exposed to weather.
Species grade or span rating. Plywood’s strength may be indicated by two marks. One is a species group number (1-5). Group 1 is the strongest and often contains Douglas fir or southern yellow pine.
The second mark, a span rating, is more common. The two-digit rating looks like a fraction, but it’s not. Rather, a rating of 24/16 indicates that a panel can sheathe rafters spaced 24 in. on center and studs spaced 16 in. on center.
Another common stamp is Struc I, which stands for Structural I sheathing, a five-ply CDX that’s tested and guaranteed for a given shear value. If an engineer specifies Struc I, it must be used. Note: Plywood used for structural sheath-
ing must have a minimum of five plies. Avoid three-ply, h-in. CDX. Although it is widely available and cheaper than five-ply, it’s vastly inferior.
Thickness and length. APA (American Plywood Association) panels rated for Struc I wall sheathing, roof sheathing, and subflooring range from 3з8 in. to 23/з2 in. thick. Although 4×8 panels are the most common, 4×9 or 4×10 sheets enable you to run panels vertically from mudsills to the rim joists atop the first floor, thereby reducing the shear-wall blocking you might need behind panel edges and greatly improving the shear strength of the wall. (Shear walls are specially engineered walls that brace a building against lateral seismic and wind forces.) Although the square-foot price of 4×9 and 4×10 panels is higher than that of 4x8s, the larger panels enable you to work more quickly.
Exposure durability. How much weather and moisture a wood-based panel can take is largely a function of the glues used. Exterior-grade panels can be exposed repeatedly to moisture or used in damp climates because their plies are bonded with waterproof adhesives. Exposure 1 is suitable if there’s limited exposure to moisture—say, if construction gets delayed and the house doesn’t get closed in for a while. Exposure 2 panels are okay for protected applications and moderate construction delays. Interior-grade panels will deteriorate if they get wet; use them only in dry, protected applications.
OSB and plywood have almost exactly the same strength, stiffness, and span ratings. Both are fabricated in layers, and they weigh roughly the same. Both can sheathe roofs, walls, and floors. Their installation is almost identical, down to the blocking behind subfloor edges and need for H-clips between the unsupported edges of roof sheathing. Exposure ratings and grade stamps
tongue and groove good two sides good one side
000: performance-rated panel (number follows)
select tight face uniform surface, acceptable for underlayment sheathing
are also very similar. But in some respects, OSB is superior to plywood. It rarely delaminates, it holds screws and nails better, and it has roughly twice the shear values. (That’s why I-joists have OSB webs.) So given OSB’s lower cost (10 percent to 15 percent cheaper, on average), it’s not surprising that OSB grabs an increasing market share every year.
But OSB has one persistent and irreversible shortcoming: Its edges swell when they get wet and appear as raised lines (ghost lines) through roofing. To mitigate this swelling, OSB makers seal the panel edges; but when builders saw panels, the new (unsealed) edges swell when wet. Buildings under construction get rained on, so edge swelling is a real problem. Swollen edges can also raise hell in OSB subflooring or under — layment if it absorbs moisture, as commonly occurs over unfinished basements and uncovered crawl spaces. Thus many tile and resilient-flooring manufacturers insist on plywood underlayment.
Given the huge market for OSB, however, count on solutions before long. At this writing, J. M. Huber AdvanTech®, Louisiana-Pacific Top Notch®, and Weyerhaeuser Structurwood® are all tongue-and-groove-edged OSB panels purported to lie flat, install fast, and have minimal "edge swell.” Stay tuned.
