OSB (Oriented Strand Board) Sub-Flooring

OSB Subflooring
OSB Subflooring. BanksPhotos / Getty Images

Love it or despise it, OSB is here to stay.  But how well suited is OSB for flooring or subflooring? Are there any better options available?

OSB Defined

OSB stands for oriented strand board. OSB has the appearance of giant cornflakes pressed together to form structural panels in sizes similar to plywood, such as 4' x 8' x 5/8".

OSB:  Floor Covering vs. Subfloor

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:

  • 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, it can be painted.

OSB subfloor is mostly serving as a base under the finish floor (hardwood, engineered wood, vinyl tile, etc.) and even below the underlayment, in some cases.

Other OSB Negatives

Contractors, builders, and home remodelers have mixed opinions about the use of OSB for flooring, mostly tending toward the negative:

  • Adhesion: OSB's materials are held together with resin binders and wax. The wax makes it difficult, if not impossible, to adhere some types of flooring directly to the OSB subfloor, requiring the installation of a plywood or cement board underlayment.
  • Weight: OSB is heavier than plywood.
  • Drying Ability: Once wet, OSB stays wet for a long time—slowing down the project.
  • Low-Grade Wood: Base materials for making OSB—poplar, aspen, etc.—are very rot-prone woods. This means that the resin and wax additives "pull the weight" of keeping the material from rotting away.
  • Nail Withdrawal: Nails pull out more readily from OSB than plywood. Not a favorable quality when nailing down flooring.

Why Some Contractors Like OSB For Subflooring

A few builders say that they prefer OSB over plywood for flooring because the surface (when new) is always clean, flat, and knot-free.

Simply put, many contractors find OSB to be a more consistent product than plywood or other types of material for subfloor: every sheet is the same.

The other motivation for builders to use OSB is money. 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 Flooring Myths Debunked

Is there anything to be said in favor of OSB for subflooring? Debunking a few myths about OSB is helpful.

Myth: OSB is made from waste products

Truth: No. Trees are logged expressly for the manufacture of OSB. No bark is used.

Myth: OSB and plywood are vastly different in term of construction

Truth: Not really. Both products are "sandwiches" of many layers of perpendicularly-grained wood. The main difference is that OSB mats, as they are called, are composed of smaller segments of wood that are plywood layers.

Myth: Plywood is stronger than OSB

Truth: Yes and no. For flooring applications, this is most likely true. OSB has excellent shear strength; that's why OSB is used for wooden I-beam joists. Shear is not an issue with flooring. OSB deflects more than plywood, which is a big reason why tile contractors unequivocally say that OSB should not be used under tile. OSB deflects 10% more than plywood underfoot.

Myth: OSB is responsible for thinning out our forests and is not considered "green."

Truth: OSB uses smaller-diameter, faster-growing wood such as aspen poplar, southern yellow pine, and mixed hardwoods. In some instances, the waste bark is used as an energy source in the manufacture of OSB.

Myth: OSB instantly falls apart when it comes in contact with moisture

Truth: It is true that OSB isn't as good against moisture as other materials. The resin binders and wax in OSB help it perform adequately well in light water conditions.

Bottom Line

OSB will work as a floor covering only in a pinch. Even then, it could only be used for outbuildings or workshops.

If you are a homeowner building your workshop or laying own your subfloor in your house, consider purchasing 5/8" CDX plywood.