Oriented strand board (better know as OSB) is one of the main materials used in parts of our house construction. It is a type of engineered wood, formed by adding special adhesives and then compressing many layers of wood strands, in specific orientation. OSB looks like (and is) a bunch of wood chips glued together.
Many constructors talk a lot about the differences between OSB and plywood, another important product that can be used in sub floors and walls. Building codes typically use the phrase “wood structural panel” to describe the use of plywood and OSB. Codes recognize these two materials as the same. Likewise, APA the Engineered Wood Association, the agency responsible for approving more than 75% of the structural panels used in residential construction, treat OSB and plywood as equals in their published performance guidelines. And wood scientists agree that the structural performance of OSB and plywood are equivalent.
Nonetheless the composition of each material is different. Plywood is made from thin sheets of veneer that are cross-laminated and glued together with a hot-press. Imagine the raw log as a pencil being sharpened in a big pencil sharpener. The wood veneer is literally peeled from the log as it is spun. Resulting veneers have pure tangential grain orientation, since the slicing follows the growth rings of the log. Throughout the thickness of the panel, the grain of each layer is positioned in a perpendicular direction to the adjacent layer. There is always an odd number of layers in plywood panels so that the panel is balanced around its central axis. This strategy makes plywood stable and less likely to shrink, swell, cup or warp.
In order to produce oriented strand-board logs are ground into thin wood strands. Dried strands are mixed with wax and adhesive, formed into thick mats, and then hot-pressed into panels. Don’t mistake OSB for chipboard or wafer-board. OSB is different. The strands in OSB are aligned. “Strand plies” are positioned as alternating layers that run perpendicular to each other. This structure mimics plywood. Wafer-board, a weaker and less-stiff cousin of OSB, is a homogeneous, random composition. OSB is engineered to have strength and stiffness equivalent to plywood.
Performance is similar in many ways, but there are differences in the service provided by osb and plywood. All wood products expand when they get wet. The Structural Board Association (SBA), a trade association that represents OSB manufacturers in North America, has issued a technical bulletin outlining a plan to prevent this phenomenon. SBA correctly indicates that dry storage, proper installation, adequate roof ventilation and application of a warm-side vapor barrier will help prevent roof ridging, for example.
For the majority of the sub floor applications, both plywood and OSB are fine. Take care if you are thinking about using OSB as a sub-floor for tile. For wall sheeting both are good materials, and different builders will have different preferences. We prefer the use of OSB.
OSB products are improving a lot. Many companies are now reaching an excellent level of manufacturing, and you can avoid the problems from the past because of the new techniques involved in the process. But, until now, the worst enemy for the wood will continue to be the same: moisture and wet climates always will represent a threat for your house if your builder do not control the water exposure during the building process.