Subfloors are the foundation for wood floors, and as a result, any wood floor installation will only be as good as the subfloor beneath it. This is why it is important to properly evaluate a wood subfloor to ensure a successful project.
Wood subfloors often are supported by a joist or truss system. Because they are the structural components of any floor framing system, they determine how much give and take a subfloor will have. Subflooring systems that lack adequate stiffness can contribute to performance problems in wood floors, such as fasteners pulling out of the subfloor, excessive noise, and potential damage to the surface finish.
The types of wood subfloor material include oriented strand board (OSB), plywood, high-performance OSB, particleboard, and solid board. OSB, plywood, and high-performance OSB are acceptable materials for wood floor installations. Particleboard is not an adequate subfloor material for wood floors. Solid board subflooring can be used with certain products. Check manufacturer recommendations to be sure.
When assessing a wood subfloor, you first must understand the relationship between the joist/truss spacing and the panel thickness requirements. In general, the wider the joist or truss spacing, the thicker the subfloor panels need to be. Once you have identified the appropriate wood panel thickness, you should ensure the panels have been installed properly.
Single-floor subfloor panels should be installed continuous over two or more spans, with the long panel dimension perpendicular to floor trusses/joists. The panel ends should land at the center of a floor joist/truss.
To minimize the potential for floor squeaks, most wood panel subfloor manufacturers recommend that the subfloor panels be glued and nailed to the floor framing using recommended fasteners and subfloor adhesives.
Before installing any subflooring material, the panels must be properly acclimated and dry, and the subfloor flatness must be evaluated. Use a 6-foot or 10-foot straight edge, laser level, or string line to check flatness. Wood subfloors must be flat to within 1/4” in 10’ or 3/16” in 6’. Correct any high or low spots before installing any wood flooring.
With new construction, it is the responsibility of the builder to control moisture during the building process. It is the flooring contractor’s responsibility to ensure the wood subfloor material is at an adequate moisture content (MC) level before installation of any wood flooring.
In hot and humid climates, or during humid seasons, the subflooring should not exceed 13 percent MC. The MC of the subfloor should be compared to the MC of the wood flooring. Wood flooring less than 3 inches wide should be within 4 percent MC of the subflooring. Wood flooring 3 inches and wider should be within 2 percent MC of the subflooring.
When subjected to water leaks, flooding, rain, or snow during construction, wood subfloor panels have the potential to absorb moisture, which may result in swelling and expansion in panel edges, as well as in length, width, and thickness.
For plywood, swelling, distortion, linear expansion, and delamination can occur when the material is exposed to high levels of moisture. Moisture tests should be conducted using insulated pin, hammer, probe-type meters on the surface, on the backing, and within the core of the material in several areas to properly assess the extent of the moisture damage. For both plywood and OSB, damaged material should be replaced with material that is within the acceptable MC ranges prior to wood flooring installation.
Subsequent drying of previously wetted panels also may cause the plywood or OSB to shrink. If wood flooring is installed before the subfloor is thoroughly dry, the eventual panel shrinkage can cause gaps along the flooring joints, between the flooring and subfloor, and result in noisy floors. Installing wood flooring prior to adequate drying also can result in the expansion of the finished flooring planks as they absorb moisture from the subfloor.
It is important to evaluate a subfloor carefully to ensure it demonstrates structural integrity. If there is movement, objectionable noises, delamination, or any other type of damage in the subflooring material or system so that it doesn’t meet minimum installation standards, it should be addressed appropriately prior to installation of new flooring.
The National Wood Flooring Association has detailed information about evaluating wood subfloors available through NWFA University, an online training platform that is convenient and affordable. More information is available at nwfa.org/nwfa-university.aspx.
