OSB (Oriented Strand Board) Sub-Flooring Basics

OSB Subflooring
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OSB is one of those divisive building products that some people swear by and others avoid. But love it or not, OSB is here to stay. It works well for walls that stay dry or for roof sheathing. But how well suited is OSB for flooring or subflooring?

What OSB Is

OSB is short for oriented strand board. OSB is a synthetic wood product made from chipped wood. Up to 50 layers of strands are compressed under high pressure to form OSB.

From the side, OSB has the appearance of giant cornflakes pressed together to form structural panels in sizes similar to plywood, such as 4-foot by 8-foot.

From the front or back, OSB is flat and sometimes has a waxy feeling.

OSB Floor Covering

Floor covering (or finish floor) is the term for the topmost, eventual floor—the floor that you walk on and see. Laminate, luxury vinyl, and ceramic tile are examples of floor coverings. 

OSB is generally unsuited as a floor covering, though it may work as a floor covering in sheds or shops. OSB doesn't work as a floor covering because of:

  • Appearance: OSB is not attractive enough to serve as a floor covering. Not only is the chip-board appearance unattractive, but the surface is also often marked with grid lines and manufacturer markings.
  • Moisture: OSB, when exposed to the weather, will eventually swell and bubble up. Minor instances of bubbling can be sanded, but for the most part, the OSB is irreparable and should be replaced.
  • Painting Difficult: Paint does not take well to OSB. Nonetheless, OSB can be painted.

OSB Subflooring

OSB subfloor serves as a base under the finish floor (hardwood, engineered wood, vinyl tile, etc.) and even below the underlayment, in some cases. Plywood has long been used for subflooring. OSB was introduced in the 1970s, but mainly was used for walls. Around the 1990s, OSB began to make in-roads into the plywood market for subflooring.

Most local codes allow OSB to be used for subflooring. It's always best to check with your local permitting office before using OSB for your subfloor.

One advantage that OSB has over plywood is its larger formatting. For plywood, 8-foot and 10-foot long sheets of plywood are standard. For OSB, it's possible to find super-sized sheets that are up to 8 feet wide and 16 feet long.

Pros

  • Some builders prefer OSB over plywood for flooring because the surface, when new, is always clean, flat, and knot-free.
  • Since OSB is so dense, it's a good soundproofing product.
  • OSB is a more consistent product than plywood or other types of material for subfloor: every sheet is the same.
  • OSB tends to be less expensive than plywood, so builders and contractors can squeeze a bit more profit out of the project. When OSB goes beyond flooring use and into wall and roof sheathing, the profit margin will increase.
  • OSB makes good use of waste products. The trees used to make OSB are often grown from seedlings in farms, thus reducing the need to log old-growth forests.

Cons

  • OSB's materials are held together with resin binders and wax. The wax makes it difficult, if not impossible, to adhere to some types of flooring directly to the OSB subfloor, requiring the installation of a plywood or cement board underlayment.
  • OSB is heavier than plywood, weighing about 2.4 pounds per square foot. A 4-foot by 8-foot sheet of OSB weighs about 77 pounds, which is heavy enough to require two workers to handle it.
  • Once wet, OSB stays wet for a long time—slowing down the project. Once swollen, OSB will not compact to its original size. Edges of OSB are especially prone to moisture damage.
  • Base materials for making OSB—poplar, aspen, and the like—are rot-prone woods. This means that the resin and wax additives do most of the work of keeping the material from rotting away.
  • Nails pull out more readily from OSB than plywood—not a favorable quality when nailing down flooring.

Bottom Line

OSB will work as a floor covering only in a pinch. Even then, it could only be used for outbuildings or workshops. Eventually, though, the OSB will begin to break down after repeated use and traffic.

If you are a homeowner laying your own subfloor in your house, consider purchasing 5/8-inch CDX plywood or comparably sized OSB.