Plywood Underlayment - Basics To Get You Started

Finish flooring--the visible portion of your flooring which you see and walk on--usually has a lower portion called an underlayment. And plywood is the most popular choice for underlayment.

Why Plywood is Ideal for Underlayment

Plywood is easily available, cheap, and dimensionally stable. By this, I mean that plywood's perpendicularly-aligned layers make this product highly resistant to sharp changes in humidity and even limited direct contact with water.

While plywood is not waterproof by any means (it will eventually delaminate under prolonged contact with water), it is considered to be resistant to some water contact.

Is Underlayment a Subfloor?

At the very base of most floors is a subfloor. The subfloor, often plywood itself, rests on the house's joists. The next layer may be the underlayment (which we are discussing in this article). Finally, there is the finish floor on the top. Examples of finish flooring are ceramic tile, hardwood, and laminate.

Plywood vs. Other Types of Underlayment

Plywood isn't the only kind of underlayment you can purchase. Before you land on plywood as your choice, assess the merits of other types of flooring underlayment:

  • Cementboard - Typically used for tile. Tilework requires a lot of water, and this water may compromise plywood's strength. Cementboard works well for tile. Wonderboard is one brand name of cementboard.
  • Fiber Cement Board - A smoother type of cement board. USG's Fiberock brand Aqua-Tough is one example.
  • OSB - Orient-stranded board
  • - Single-layer composite wood that also can be used as underlayment, though plywood is the preferred choice. OSB stands up well against moisture.

Type of Plywood Underlayment You Need

Don't just grab any plywood off the shelf at The Home Depot or Lowe's.

Because plywood comes in various grades, you need to buy the right kind of plywood for underlayment usage.

  • Resilient (Vinyl) Flooring: Use 1/4" exterior-grade AC plywood. Make sure that the smooth side of the plywood is facing up.
  • Wood Flooring: 1/4" to 1/2" exterior grade plywood. No need to choose AC grade plywood, since smoothness is not a prime concern with wood flooring. Even a rougher finished plywood, such as CD plywood, would work.
  • Ceramic Tile: If you do choose to use plywood as underlayment for ceramic tile rather than cementboard or fiber cement board, use 1/2" AC exterior grade plywood.

AC plywood means that one side is graded as "A" (smooth and sanded), which the other side is graded "C" (rough and exhibiting knots as wide as 1 1/2"). Since the inferior "C" side is facing downward, knots and roughness do not matter.

With any type of plywood, you should look for shiplapped or tongue-and-grooved joints. While not absolutely necessary, these joints do make for cleaner seams between the boards.

Brands of Plywood to Choose

For buying plywood as underlayment, I find that brand choice does not matter much. However, if moisture is of great concern, you may want to consider Dry Ply, a brand of coated plywood from Georgia-Pacific that works well as flooring underlayment.

If you are installing vinyl flooring, you can buy brand-specific plywood. Halex plywood underlayment is tooled specifically to Armstrong, Mannington, Congoleum, and Domco/Tarkett floors. Not only do you have plywood underlayment made just for a particular brand of flooring, but you have brand-specific warranties for each.

Installing Plywood Underlayment

One advantage to plywood underlayment is that it is easy to install. There are two reasons for this. First, plywood comes in large (4' x 8' or even larger) sheets, which quickly cover your floor space. Second, plywood is easy to cut with a circular saw.

To install, use 1" screws driven every 8 to 12 inches along the edges of the board. Sink the heads of the screws slightly below the surface of the plywood. Pepper the field of the plywood with 1" screws about 12 inches apart from each other.

Make sure to allow for 1/4" between each sheet of plywood and long the walls to allow for expansion.

If installing resilient flooring (vinyl), patch the screw divots and seams with floor patching compound and later sand down the dried compound and any irregular spots in the plywood.