Glue Down Wood Floor Sounds Hollow

Flooring retailers and installation contractors hear the complaint regularly concerning hollow sounds. Hollow means hollow, indicating a void under the wood flooring.
For purposes of this topic we must isolate the subject to glue down floors on a concrete sub-floor, a subject we’re most familiar with here in the south.

Hollow sounds do not always equal a bad installation or a problem to deal with in the future. Small areas of hollow sound reflection, with tapping of the floor, may be as simple as small area glue voids or slight imperfections in the concrete surface. These small areas will not take away from the performance of the wood flooring as they seldom allow movement. Movement, in the flooring industry, is referred to as deflection.

Up and down movement of the wood flooring (deflection) can be a problem. Movement in larger areas causes excessive pressure against the tongues of the flooring causing broken tongues which, in turn, will remove years of wood flooring performance.

Hollow sounds in wood flooring are directly related to the flatness of the sub-floor the flooring is adhered to. Should the installer not perform his duties to bring the slab surface to a specified flatness condition of the manufacturer, hollow sounds and movement of the wood flooring will result in poor performance of the new floor.

The industry associations require the sub-floors be flat, clean, and dry and the sub-floor flatness be within 3/16″ in 10′ radius and 1/8″ difference in 6′ radius. The purpose of this specification is to minimize undulations in the concrete surface that result in hollow sounds and deflection.

Concerning sub-floor flatness, always follow manufacturer specifications. The wider the plank, the more stringent the sub-floor flatness requirement becomes. The wider planks require more support and the wider the plank, the less likely the plank will conform to the undulations in the sub-floor surface.

Hollow sounds do not equate to a flooring failure. Deflection in the wood flooring may equate to a flooring failure, and requires attention and repairs. There are several ways to repair this problem without removing the floor. The most successful repairs are performed using the drill and Inject method. A 3/32″ hole is drilled; a syringe filled with specially formulated adhesive is then injected into the hole, filling the low spot in the sub-floor.

About the Authors: Certified Flooring Inspectors: Ray Darrah, Las Vegas, NV and Linda Lockwood, Barnegat, NJ