Plywood Sheathing, Sub-Floors & Plywood Underlayment

May 22nd, 2014

Plywood sheathing (OSB or plywood) is often mistaken for plywood underlayment.  They are not the same.  Plywood sheathing is most often used to construct the structural sub-floor.  Plywood sheathing can either be OSB or all plywood.  We will discuss these types of plywood sheathing, sub-floors and plywood underlayment in this blog post.

OSB or oriented strand board was developed in the 1970’s as a better substitute for waferboard (aka chipboard) and to compete with plywood in the construction industry.  OSB is constructed using long strips (strands) of wood that are held together with wax and adhesive.  The strands are then placed strategically so that the sheets are strong and uniform.  You will not find knots or voids in OSB; each sheet is consistent.  The same cannot be said for its plywood counterpart.  One drawback to OSB is that is has a tendency to swell when cut edges are exposed to moisture.  If sheets are not cut then they maintain their waterproofing capabilities.

Plywood sheathing is typically more expensive than OSB and is constructed using plies.  The plies are constructed when sheets of veneer are cross-laminated and glued together using a hot-press.  The grain of each layer of veneer is positioned in a perpendicular direction to the adjacent layer.  When exposed to moisture, plywood will get saturated much more quickly than OSB but it does dry out more quickly.  Plywood is not prone to edge swelling like OSB.

As you can see, both OSB and plywood sheathing are good options for the construction of structural sub-floors.  Both are strong, durable and long-lasting. Wood scientists agree that the structural performance of plywood and OSB are equivalent.

Sub-floors are actually constructed of either OSB or plywood sheathing, and rest on floor joists.  Sub-floors serve as structural platforms for your plywood underlayment and finished flooring.  In essence, your finished floor is actually made up of three separate and distinct layers.

1)      Sub-floor

2)      Plywood underlayment

3)      Finished flooring (carpet, vinyl, VCT, ceramic tile, engineered wood floors, etc.)

Once the sub-floor has been constructed, it’s time to choose a plywood underlayment that is suitable for your finished flooring.  There are many plywood underlayment panels on the market.  They are not all created equal.  Check out our webpage dedicated to plywood underlayment at

For more information on the Patriot Timber Family of Panel Products, including our line of warranted plywood underlayment panels, visit



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