All floor coverings need a supporting layer beneath them. For decades, plywood has served that purpose well. Other types of underlayment have grown in popularity, notably orient-stranded board (OSB). But plywood is still widely installed because it is easily available, inexpensive, and dimensionally stable.
What Is Plywood Underlayment?
No interior flooring is installed as a single layer. Multiple layers contribute to a floor's strength and make it possible to use costlier materials such as exotic hardwoods or tile as the finish surface.
The top level of flooring that is seen is called the floor covering or finish floor. Examples of floor coverings are ceramic tile, hardwood, luxury vinyl, and laminate. The lower level is never seen and it may be either a subfloor or a subfloor plus an underlayment.
The terms "subfloor" and "underlayment" are often used interchangeably. Strictly speaking, the subfloor is the relatively thick, structural layer that rests directly on the house's joists. The next layer up might be an intervening section of thinner underlayment or it might be the floor covering itself.
Underlayment is essential when installing certain types of flooring that are thin or not very strong by themselves. Resilient or vinyl flooring is a classic example of a top floor covering that benefits from the smooth surface provided by underlayment.
Plywood Underlayment: Pros
- Dimensionally Stable: Plywood's perpendicularly-aligned layers make this product highly resistant to sharp spikes 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.
- Easy to Obtain: Plywood is found at every Home Depot, Lowe's, and other types of consumer-level home center, as well as at lumber yards.
- Inexpensive: Plywood in 4-foot by 8-foot sheets is relatively inexpensive, at least compared to a product like prefabricated subfloor tiles.
Plywood Underlayment: Cons
- Difficult to Handle: Plywood in large sheets are heavy and unwieldy, requiring either store delivery or a full-size pickup to get it to your home.
- Installation Schedule: Because plywood has a dominant grain direction, you need to be aware of how it is laying across the joists. The grain should always run perpendicular to the joists. By contrast, OSB has no grain direction, so it can run in any direction relative to the joists.
Types of Plywood Underlayment
For a tight, stable installation, avoid grabbing just any type any plywood off the shelf. Because plywood comes in various grades, you need to buy the right kind of plywood for underlayment:
- Resilient (Vinyl) Flooring: Use 1/4 inch exterior-grade AC plywood. Make sure that the smooth side of the plywood is facing up.
- Wood Flooring: Use 1/4 inch to 1/2 inch exterior grade plywood. There is 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, will work with most applications.
- Ceramic Tile: Cement board is the preferred underlayment for tile. If you do choose to use plywood as underlayment for ceramic tile rather than cement board or fiber cement board, use 1/2 inch AC exterior grade plywood.
AC plywood means that one side is graded as "A" (smooth and sanded), while the other side is graded "C" (rough and exhibiting large knots).
Since the inferior "C" side is facing downward, knots and roughness do not matter.
With any type of plywood, it is beneficial to choose ship-lapped or tongue-and-grooved joints. While not absolutely necessary, these joints do make for cleaner seams between the boards.
Brands of Plywood to Choose
Most home centers like Home Depot, Lowe's, Menard's, and others have favored-brand arrangements with certain suppliers. For example, Columbia Forest Products is one such favored brand found at Home Depot. However, for special uses you may wish to look for some of these brands:
- GP Dry Ply: 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.
- Halex: If you are installing vinyl flooring, you may wish to look for Halex plywood underlayment, as it is marketed as being tooled specifically for 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
Plywood underlayment is easy to install. It comes in large sheets which quickly cover your floor space. Plus, it is easy to cut with a circular saw.
To install, use 1-inch 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-inch screws about 12 inches apart from each other.
Make sure to allow for 1/8-inch between each sheet of plywood and along the walls to allow for expansion.
If installing resilient flooring, fill the screw divots and seams with floor patching compound and later sand down the dried compound and any irregular spots in the plywood. Because resilient flooring is so thin, imperfections such as divots will quickly telegraph to the surface of the floor covering.
Plywood vs. Other Types of Underlayment Materials
Plywood is not 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:
- Cement Board: Cement board is typically, but not always, used for tile. Tilework requires copious amounts of water, and this water may compromise plywood's strength. Cement board works well for tile. Wonderboard is one brand name of cement board.
- Fiber Cement Board: Fiber cement is a smoother type of cement board. USG's Fiberock brand Aqua-Tough is one example.
- OSB: OSB is a single-layer composite wood that also can be used as an underlayment. OSB stands up well against some moisture.