Plywood long ago lost its market leadership to oriented strand board (OSB), which is now the most-used sheathing and subflooring material. Estimates put its market share as high as 75%, with much of that dominance thanks to cost-conscious production builders.
Prices for commodities like structural panels are notoriously volatile, but at this writing, OSB sheathing is selling for around $3 per sheet less than plywood. That's a savings of several hundred dollars per home.
Industry voices from panel manufacturers or their main trade group—APA - The Engineered Wood Association—say that there's no real difference between the two panels: OSB's and plywood's structural characteristics are equivalent, and they can be used interchangeably. That's true: Both are rated Exposure 1 for temporary vulnerability to the weather; they have equivalent nail withdrawal resistance; and they're installed using the same methods and construction details. However, there are differences that builders should be aware of.
OSB has more going for it than just cost. Resource-efficient builders appreciate that it can be made from small, fast growing trees, many of which come from tree farms rather than forests. It can also be bought in 9 foot sheets, which means you can sheathe a wall from the top plate to the bottom of the floor joists with single, vertical sheets, leaving no horizontal seams. OSB panels can be manufactured in lengths up to 16 feet (or sometimes even higher), while plywood is generally limited to 8 to 10 feet.
OSB boasts a more consistent density. While a sheet of plywood might be 5 to 7 plies thick, a sheet of OSB is made from as many as 50 strand layers packed and compressed into the same thickness. There's no equivalent of the weak spots that can be left in plywood when knotholes in adjacent plies overlap.
On the downside, the material is a bit heavier than plywood—two pounds or more per sheet depending on its thickness and intended use—but this difference has no effect on the panels' performance. It just takes a bit more muscle to handle on the jobsite.
Plywood's Superior Wet Performance
The biggest difference between the two panels is how they react when exposed to large amounts of moisture over extended time periods. With the exception of projects in very arid regions like the Southwest, sheathing and flooring panels are routinely covered with rain, snow, and ice during construction delays. It's here that plywood has the edge.
When plywood gets wet, it tends to swell consistently across the sheet, and then returns to its to normal dimensions as it dries out. It dries out relatively quickly, and the swelling is usually not enough to affect floor or roof finishes.
OSB takes longer to get wet than plywood but also takes longer to dry out. When used as a roof sheathing, this tendency to hold moisture means it can degrade faster than plywood when exposed to chronic leaks.
When OSB does get wet it also tends to swell along the edges, and those edges stay swollen even after the material has dried out. Swollen edges have been known to telegraph visible ridges called "ghost lines" through asphalt roof shingles.
Manufacturers insist that OSB's moisture problems have been corrected, thanks to the development of water-resistant edge seals. But of course that edge seal is lost when panels get cut on site, as they often do.
Although APA says the products are structural equals, the way they handle moisture does affect recommendations for their use. The National Tile Contractors Association and the Resilient Floor Covering Institute both recommend plywood for subflooring and underlayment, because it doesn't have the risk of swollen edges that OSB does.
Plywood also has a slight advantage in stiffness, which means that subflooring panels need not be quite as thick. For instance, with 24-inch truss/joist spacings, the National Wood Flooring Association recommends nominal 1 inch OSB underlayment compared to 7/8 inch CDX plywood.
Perhaps the biggest thing plywood has going for it is the perception of higher quality among homeowners. Some consumers still perceive plywood as a better quality product—unlike OSB, it actually looks like wood and most people are familiar with it—which is one reason plywood dominates the market for DIY projects. True or not, that perception generally makes plywood a good choice for remodeling or custom home projects.
More from ProSales: