How to Install Plywood Underlayment

The appearance of a floor is determined by the quality of subflooring, and plywood underlayment is often the blessing or curse of an attractive room. Poorly installed underlayment leaves obvious blemishes that show through vinyl, laminate, hardwood and other floor coverings. The choice of materials and the quality of workmanship can ensure that your plywood underlayment contributes to the professional look of your floor. Following a few suggestions, anyone can install plywood underlayment correctly.

Step 1: Prepare the Subfloor

Sweep the subflooring clean of all debris. Examine the entire room and remove or set any nails that protrude from the material. The underlayment must rest flat over the entire floor. Use wood filler to repair any uneven areas.

Step 2: Measure the Panels

Measure the room and determine how many 4 X 8 foot panels of ¾ inch CDX plywood will be needed to cover the floor. Select the smooth side of the plywood to be facing up. Lay the panels out flat to cover the subflooring. Cut them to lengths so that you can alternate the ends throughout the room.

Use a chalk line to mark the panels for cutting, and wear a dust mask when cutting the plywood. The wood grain of the panels should be perpendicular to the floor joists. After cutting the panels to the proper sizes, stand them on end in the room for a day or more if possible. This allows them to adjust to the heat and moisture conditions of the room and avoids warping after they are installed.

Step 3: Install the Underlayment

Ensure that the subflooring is still free of debris and completely dry. Underlayment must fit flat against the subflooring with no moisture trapped beneath. Some builders suggest adding an adhesive between the subflooring and the plywood, while others advise against it. Unless applied uniformly, the adhesive could keep the plywood from fitting flat. Lay the panels across the room alternating the ends so no joints extend continuously across the floor. The CDX plywood should have tongue in groove edges to interconnect.

Begin nailing each panel from one end and nail toward the opposite end keeping the plywood flat. Use ring shank nails so they will be less likely to back out of the wood after the flooring is laid. Put each nail about 4 inches apart along the edges and 6 inches apart across the remainder of the plywood. Try to create a random pattern with the nails so they will not appear in a regular pattern through the flooring.

Wear safety glasses when nailing the plywood to protect your eyes from any debris. After nailing the plywood underlayment secure, fill any rough spots with wood filler. Your floor is ready for a vinyl covering at this point. If you plan to use laminate, you will need to use the recommended pad to cover the underlayment. A layer of felt would be added over the plywood underlayment if you plan hardwood floors.