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The Problem
Last fall I received a call from the worried owner of a hardwood flooring installation company. He said, "My foreman just called me and told me something was wrong with the wood glue." They installed 600 square feet of 5/16-inch solid wood flooring using a urethane wood adhesive, and when they returned the next day, they found the end joints were lifting and the entire perimeter of the floor sounded hollow.
The Procedure
The owner said that he told his foreman to use a urethane adhesive. Since he usually uses our moisture-cured urethane adhesive, he figured he'd give us a call to see if we had experienced similar problems.
The home was less than a year old, and the concrete slab was on grade. The installer said he had to remove the carpet and patch one small area of the floor. As we walked around the job, I immediately noticed the end-lifting and the hollow sounds around the perimeter of the room. I asked the installer a few more questions about the installation and the adhesive. He didn't recall the name of the adhesive, but he insisted it was a urethane and it was cream-colored.
We removed two floorboards from different areas of the room, and I sent them to the lab for analysis. When we pulled the floorboards, we noticed the adhesive on the bottom was tacky, which is not typical of moisture-cured urethane adhesive.
The Cause
I sent the floorboards to our analytical lab along with samples of our moisture-cured urethane adhesive and another company's water-based urethane adhesive. The adhesive on the floorboards matched the water-based urethane wood glue. The wood glue the installer used was a water-based urethane, not a moisture-cured urethane.
Water-based wood adhesives usually are recommended for engineered wood flooring and parquet. When used in the recommended job site conditions—relative humidity within the correct parameters and a dry substrate that hasn't been sealed—the water in the adhesive can wick into the concrete or evaporate. This reduces the amount of water absorbed by the wood. Wood flooring always will absorb some of the water present in the adhesive. Engineered wood flooring is expected to handle this small amount of water, but solid wood flooring usually can't.
RELATED: Sticky Questions: Q&As on Essential Adhesive Know-How
The adhesive used contained approximately 18 percent water. The available moisture in the adhesive quickly began to be absorbed by the thin solid wood, which caused expansion. This expansion caused stress on the wood, which resulted in edge- and end-lifting. The end-lifting was much more noticeable around the perimeter, because the force of the stress accumulates there.
How to Fix the Floor
Some adhesive manufacturers say the wood flooring in such cases eventually will dry out and go back to the original dimensions, but many say it won't. Regardless, the adhesive used was not recommended for this type of flooring, and in this case, the contractor decided to rip up the floor and re-install a new one with the recommended adhesive.
In the Future
Failures of glued-down hardwood on grade can almost always be traced to one of the following:
1. Using the wrong adhesive for the flooring being installed
2. Using the wrong trowel for the job, or using a worn trowel
3. Not checking the substrate for moisture content and pH level
4. Not making sure the slab is flat to the manufacturers' specifications (usually 1/8 inch in a 6-foot radius, or 3/16 inch in an 8- to 10-foot radius.)
5. Improper sub floor preparation with regard to removing paint, drywall compound, sealers, etc.
In this case, the job failed because of reason No. 1—which is also one of the easiest problems to avoid. Had he read the directions, the foreman would have known that he specifically needed the moisture-cured urethane adhesive.
No matter which type of product you're installing, the first step should always be to read the manufacturer's directions. Manufacturers' research and development teams spend substantial amounts of time and money experimenting with their products to discover what works and what doesn't. Doing their experiments on your job sites is always a gamble of your time, your money, and your reputation.