Underlayment Is Not Subflooring
Often, the terms “subfloor” and “underlayment” are used interchangeably. Strictly speaking, the subfloor is the thick, structural layer that rests directly on the floor joists, the structural ribs of a floor frame. Underlayment is a thinner material that lies over the subfloor. While underlayment may add a modest amount of stiffness to a floor assembly, it does not strengthen a floor or enhance it structurally. Plywood is only one type of underlayment. Other common types include cement board or tile backer (used under ceramic or stone tile), foam underlayment (used under laminate and engineered wood “floating” floors), and rosin paper (used under solid hardwood flooring). There are also engineered subfloor panels designed for covering concrete slabs in basements. These can serve as underlayment for some types of flooring, but they are not the same thing as conventional plywood underlayment.
When to Use Plywood Underlayment
It’s easy to find conflicting advice about underlayment, especially plywood underlayment. Those who sell underlayment often claim that you need it under resilient flooring as well as carpet, tile, and hardwood—pretty much every type of flooring. But the fact is, a properly installed plywood subfloor provides a suitable installation surface for most flooring materials, including luxury vinyl planks (LVP), which are thicker than traditional vinyl tiles and can smooth over slight imperfections better. Plywood underlayment may not be a good choice for ceramic or stone tile because it is not water-resistant. For floor tile in any room, use cement board or a similar tile backer instead. To simplify your decision, consider plywood underlayment if it is recommended by the flooring manufacturer. In some cases, the use of underlayment may be required to satisfy the product warranty. Plywood underlayment is typically needed when a floor material must have a very smooth, flat surface. For example, if you install resilient tiles or sheet flooring over a subfloor, any bumps, dips, or voids in the subfloor may be evident in the finished flooring. Underlayment covers subfloor flaws for the best possible results.
Types of Plywood Underlayment
Most plywood underlayment is about 1/4 inch thick and has square edges and a smooth, knot-free top face. It typically comes in 4 x 8-foot sheets. For many years, the standard option was lauan plywood, also called Philippine mahogany or luan plywood, which is made with an inexpensive tropical hardwood, and therein lies its problem. Lauan products have been responsible for massive deforestation in the Philippines, Indonesia, Malaysia, and other countries. Today, a much better option is a plywood specifically designed as underlayment and made with sustainably forested wood. You can also use AC exterior-grade plywood, which has one smooth face and one rougher face. Always install underlayment with the smoother face up. If you have any questions about the suitability of an underlayment material, contact the flooring manufacturer for recommendations.
Installing Plywood Underlayment
Plywood underlayment is easy to install. The sheets are lightweight and have perfectly uniform dimensions and edges. You can cut sheets with a circular saw, jigsaw, table saw, or handsaw, just like any wood sheet material. While underlayment traditionally was glued down to wood subflooring, most manufacturers today recommend installing it without glue, which greatly simplifies the installation. Thorough fastening of the underlayment panels is key to a successful installation. The easiest and best fastening option is galvanized staples driven with an air-powered stapler (available for rent at home centers and rental outlets). You can also fasten underlayment with galvanized or coated screws or nails. Do not use standard, uncoated fasteners because they are prone to rust, which can discolor some types of flooring. The installation process starts with storing the underlayment panels in the room where they will be installed for at least 72 hours. This acclimates the panels to minimize expansion/contraction issues after installation. Once the subfloor is thoroughly cleaned, the underlayment panels are installed one at a time directly over the subfloor. The underlayment sheets usually run in the same direction as the plywood subflooring, but it is critical that the seams of the underlayment are offset from those in the subflooring, so the underlayment bridges over the subfloor joints. When installed with staples, the underlayment sheets are fastened every 2 inches along the edges of the sheet and every 4 inches in the field area. Screws and nails may be spaced farther apart. Sheets can be butted together (just touching, not forced tightly together) or they can be gapped about 1/8 inch, or so. If they are gapped, usually the gaps must be filled with a seam filler.
