A remodeler in Houston called for guidance on fixing problems with an existing hardwood floor. The floor made popping sounds—and the occasional squeak—on several spots when stepped on.
The house was built on a slab foundation. Engineered 5-inch oak planks were glued to the concrete subfloor.
Typically, in a full glue-down installation, hollow spots between the subfloor and the boards are to blame for popping sounds and squeaks in hardwood flooring. In most cases, and almost always when the subfloor is concrete, these “voids” result from an uneven subfloor. The subfloor should not vary more than 3⁄16 inch within any 10-foot radius (or 1⁄8 inch in a 6-foot radius). Larger deviations can lead to hollow spots beneath the wood flooring, which cause a popping noise or squeak as the floor is walked on.
How to Fix the Floor
Hollow spots larger than 6 inches usually require board removal and replacement, but in this case, the hollows in this installation appeared to be small enough to allow use of an injection repair kit. Many adhesive manufacturers offer these kits, which enable contractors to inject adhesive through the board to fill in the hollows. Because the subfloor was concrete, filling the voids was the only viable option; the contractor could not have screwed down the subfloor from below.
The contractor first walks the floor to determine the locations of the hollow spots. For most adhesives, tapping the suspected spot can help locate the center of each hollow spot. A 2-inch piece of blue painter’s tape at the center of the hollow can be used to mark the spot and aid in cleanup. Next, the contractor drills a small hole through the tape and plank using the smallest drill bit possible. This hole enables the contractor to inject the repair adhesive under the plank to fill the troublesome hollow spot. It’s a good idea to drill the hole at an angle to prevent backflow of the adhesive during adhesive injection.
Adhesive applicators vary across repair kits, but most use a syringe or special tip that inserts into the hole, enabling the contractor to neatly inject adhesive into the hollow. You may want to experiment with different kits to find the adhesive that flows the most easily. Typically, when the adhesive begins to overflow the hole, the hollow is filled.
Next, the contractor needs to plug the drilled hole. An ordinary toothpick can help fill the void—the contractor simply inserts the toothpick into the hole after injecting the adhesive, waits five to ten minutes, then breaks the toothpick off flush with the floor. Colored putty or a crayon is ideal for filling any excess gap around the toothpick.
In the Future
Hardwood floors must be installed over a flat concrete subfloor. If the floor is not flat to recommended tolerances, it must be made so before installation begins. We recommend grinding concrete floors rather than using filling compounds to even out the floor. However, any filling or leveling compounds used should be Portland-based cementitious materials with a compressive strength equal to or greater than 3,000 psi when cured.
Subfloor flatness becomes all the more important when installing planks thicker than 1⁄2 inch and wider than 5 inches. Thicker, wider boards are less likely to conform to uneven spots in the subfloor. Another tip: Non-slumping wood flooring adhesives can help reduce hollow spots in new installations.
Mark Lamanno is technical market manager-flooring at Columbus, Ohio-based Franklin International.