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An OSB subfloor is a subfloor made from oriented strand board (OSB), a manufactured wood product. OSB is often used for sheathing in floors and walls, and in some regions, it is the building material of choice. As with other engineered wood products, OSB comes in a range of thicknesses and styles, and it is important to make sure that the appropriate product is used.
A subfloor is a rough floor made from sheathing which is attached to the joists of the floor. Other flooring such as tile, carpet, wood, or stone is fixed on top of the subfloor. The subfloor needs to be durable and strong, as it is part of the system which supports and distributes the weight of the house, but it does not need to be particularly attractive, because no one is going to see it. OSB subfloors may not look pretty, but they certainly do the job.
At first glance, a sheet of OSB looks sort of like someone took a bunch of wood chips, glued them together, and compressed them into a sheet. That's actually pretty much exactly what happens; OSB is made from strands of wood which are laid across each other at various orientations to increase tensile strength, bonded with a wax and resin adhesive, and compressed to create a dense board. OSB is strong, durable, and somewhat water resistant, with some products being more water resistant than others.
The alternative to an OSB subfloor is a plywood subfloor; solid wood sheeting is rarely used for subflooring today, because it's very expensive and it may not perform as reliably as an engineered wood product. OSB is less expensive than plywood, as a general rule, which is one reason it's a preferred subflooring material. When installing an OSB subfloor, people need to be careful to avoid exposing it to moisture, as it can swell. If the subflooring will be exposed to moisture during construction, a product rated for moisture exposure should be used.
Almost anything can be installed over an OSB subfloor. Some professional trade organizations recommend that OSB be avoided in kitchens and bathrooms, or in installations with tile, due to concerns about moisture compromising the floor's integrity. However, the technology behind OSB is constantly improving, and these recommendations may not always hold true. A contractor should be consulted for a recommendation before installing an OSB subfloor. As always, when building a new home or making significant renovations to an existing one, the building code should be consulted to confirm that the construction will meet the standards of the building department.