Plywood Subfloor Underlayment

A plywood subfloor, when properly installed, gives excellent dimensional strength, stability, and surface smoothness for the subsequent installation of the underlayment as well as for any number of finished flooring materials. Different grades of plywood, however, are used for different types of flooring.

For instance, when sheet vinyl or vinyl tile is to be used in a typical in-home setting, the grade of plywood varies from that of where unusual moisture may accumulate, such as in a basement. Plywood purchased for basement subfloors and underlayments should be rated for exterior use for this reason.

Pre-Installation Handling

Protection from moisture and other damage must be provided for any plywood subfloor and underlayment before installation. But to avoid any surprises, such as buckling or warping after installation, allow the plywood to sit for several days in the room in which it is to be installed. This gets the material used to the conditions prevalent, adjusting to the warmth and moisture that is common to the room. When doing this, always stand the plywood panels on their edges.

Inspection and Installation

The plywood subfloor joints need to be inspected to make sure they are even and that they are flat between the joists. You may need to do some sanding near the joints, add some extra blocking, and fasten the plywood more securely to flatten out any uneven spots. The plywood underlayment should always be installed on a dry subfloor, the smooth side facing upward, and just before laying the final finished flooring, i.e., the carpet, tile, sheet goods, etc. Install the underlayment panels with the face grain at right angles to the floor joists and stagger the end joints. Offset underlayment joints from subfloor joints by at least two inches and, at all edges, space underlayment panels at 1/32 inch.

Hardware Fasteners and Filling

Use 1 1/4 incg screw-shank or ring-shank nails to install the underlayment beneath a plywood subfloor. With the panel kept flat as possible, begin nailing at one edge and move toward the edge at the opposite side. Nail heads should be driven flush with the surface or slightly below (countersunk).

Avoid the temptation to use some sort of construction adhesive to glue the underlayment to the subfloor. This can cause problems with either the installation of the finished flooring product or with its appearance, or both, especially with resilient sheet-flooring materials. Use filler that sets hard and quick, and does not shrink to take care of any gaps in the edges, splits, or areas that are damaged.

Finished Flooring Installation

To protect the underlayment and the plywood subfloor, always use the highest quality adhesives when installing resilient sheet-flooring products. These premium adhesives contain more solids and less water content, which provides better protection. Otherwise, use only loosely laid sheet flooring that is attached only at the perimeters.

This minimizes the underlayment and subflooring to the moisture found in average-quality adhesives that are water-based. For all other types of finished flooring, carpet, laminate, hardwood, ceramic or vinyl tile, refer to the manufacturers directions for installation.