Used for sub floors, exterior walls, and roof decking, OSB sheathing is a structurally-engineered board that is made up of compressed wood strands. These strands of wood are arranged in layers that are bonded together with resin, which helps produce a strong product. OSB, or oriented strand board, is unlike traditional plywood in that it has no laps, gaps, knots or voids. In most applications, this sheathing is dimensionally stronger and stiffer than comparable dimensional plywood boards. Many builders gravitate toward it not only for the increased strength of the finished product, but for the cost effectiveness as well. Usually, the price difference between OSB and plywood is significant enough to warrant common usage.
While custom boards can be requested from a laminate mill, most OSB boards have some standard features. Finished ones are smooth and often have pre-marked nail lines to make installation easier. Boards are 0.25 inch (about 6.35 mm) thick to 0.75 inch (about 1.9 cm) thick and usually don't cup as easily as plywood.
Another advantage of OSB sheathing can be found in the way it's produced. Most manufacturers use wood from small, fast-growing trees, which translates to a shorter growing time. Some trees that are used for this material include aspen, poplar, southern yellow pine, and mixed hardwoods.
There are no special handling instructions for OSB sheathing, other than protecting it from the weather. Unsealed edges can swell if left unprotected, taking on water and warping the boards. Edges can expand up to 15%, especially if they are cut edges. On the other hand, plywood expands more evenly and shrinks more rapidly than OSB. For this reason, some builders will use plywood on the outside, exposed edges of roof decking and then use OSB boards for the rest.
When installing the sheathing, experts recommend leaving a 0.125 inch (about 3 mm) gap between boards and around window and door frames to allow for seasonal expansion and contraction. Edge clips can be used between boards to gain the necessary amount of spacing. Panels can be installed with nails, staples or screws, although it is best not to use drywall screws as they are not considered to be strong enough. Panels usually are installed with the textured side out or in, with their long direction horizontally across the beams or studs.
Local building codes should always be considered when engaging in construction activities, and this is certainly the case when using OSB. Some regions require different thicknesses to be used for different applications. For example, a board of a particular thickness might be required for sub floors, while wall sheathing may be raised with a thinner board. Some regions are also restricted from using staples as OSB fasteners.