Getting To The Root Of The Problem -- An inspector uses an electronic moisture meter to see if vapor transmission through the concrete slab is the cause of this cupped wood floor.

I’ve come to the conclusion that I should address wood flooring problems in this article. Some of these problems I have written about before, while others I’ve not mentioned previously deserve their 15 minutes of infamy.

Cupping. I have previously discussed cupping of wood floors, which oftentimes is caused by an imbalance of moisture throughout the wood (see chart). Typically, this happens when the wood is wetter at the bottom and the top, when subjected to heat and low humidity, shrinks and thereby produces a cupping effect. This is the most common cause, usually due to moisture in the form of vapor traveling through a concrete slab on grade.

However, a less common and less understood cupping effect is the result of topical moisture (due to spills) or high humidity. When exposed to either or both of these moisture sources, the wood flooring can expand and compress with cupping then occurring.

Humidity-related cupping is what occurs from closing up a building for an extended time without air conditioning or ventilation to relieve the floors from the effects of high humidity. This is called the “greenhouse effect.” The greenhouse effect will also occur when floors are exposed to direct sunlight while moisture vapor rises through the slab beneath. The generated heat lowers the humidity on the face of the floor and the resulting moisture imbalance between the top and bottom of the flooring creates the cupping situation.

Under some circumstances, the cupping can be fixed. If caught early, you may allow the floor to stabilize, and then sand it flat and refinish. Of course, you must eliminate the moisture source first.

Disbonded or buckled floors. In wood flooring, this possibly is the greatest visual failure that stems from exposure to excessive moisture. I have seen floors that were raised inches above the slab. Even the presence of liberal expansion joints at the floor’s perimeter may not constitute enough of a “safety valve” to prevent this problem.

One might wonder why the adhesive wouldn’t prevent such a situation. Well, it’s possible there wasn’t adequate transfer of the adhesive between the wood flooring and the subfloor (transfer should be a minimum of 80 percent), possibly due to the use of a small or worn trowel. Or perhaps the adhesive had skinned over. Or even worse, the slab could be excessively contaminated by residues such as old cut-back adhesive.

A rule of thumb for perimeter expansion is to allow a space equal to the thickness of the wood or at least 1/4 inch.

Cured concrete. Most, if not all, wood floor manufacturers specify that a newly poured slab should be allowed to cure at least four weeks before the flooring is installed. It is known that concrete hydration and strength gain has a seven-day and a 28-day plateau. After 28 days, the concrete achieves somewhere between 75 and 85 percent of its final strength.

However, curing continues after that 28-day plateau. A concrete slab, as a general rule, requires one month per inch of thickness to fully cure. This means that a 21/2-inch-thick concrete slab requires 10 weeks to cure. Taking into account wood’s sensitivity to moisture, I believe no wood floor should be installed on grade any earlier than that time frame indicates.

Substrate not level. Have you wondered what causes a hollow-sounding wood floor? Most likely, it sounds that way because the substrate beneath the flooring does not meet the manufacturer’s requirements for levelness. Always check the manufacturers’ spec because they can vary between suppliers. Offhand, I can think of at least three different guidelines for levelness -- one manufacturer specifies no more than 1/8 inch variance within a 10-foot area, another says 1/8 inch variance in a 6-foot area is acceptable and yet another says no more than 1/8-inch variance in an 8-foot area.

A final note. I have been asked to call installers’ attention to the need for cleaning the face of wood flooring when urethane adhesive products are used and some of that adhesive somehow gets on the face of the floor. Do not use anything other than the specific adhesive removal product recommended by the wood floor manufacturer. One might be able to remove the adhesive with a solvent, but the floor finish also might be damaged by that solvent.

Wood floors are attacked from below the slab, from spills, from uncontrolled humidity, and from exterior moisture sources caused by poor landscape grading, excessive irrigation, and even ineffective doors or windows. As you can see, moisture remains our biggest problem when it comes to wood floor maladies.