Why Is Our New Floor Buckling?
The concrete slab underneath seemed dry. What happened?
We replaced our carpeted floors with beautiful engineered hickory flooring, which a contractor glued to the concrete slab with a moisture-barrier glue. When the carpet was pulled up, both it and the slab were clean—no mold or wetness—but two months later, the new flooring started to buckle and ripple. Nobody seems to know what’s causing this. Any ideas? —Kirk Clouse, Baton Rouge, LA
Kevin O’Connor replies: I asked wood flooring contractor Pat Hunt, who has worked on many This Old House TV projects, to weigh in on your problem. Here’s what he had to say.
“In my experience, this sort of buckling can almost always be traced to moisture coming up through the concrete. Wood flooring swells when it absorbs that moisture, and if it can’t move laterally, it buckles. Engineered wood flooring is much more stable than solid wood, which is why it’s often used over concrete, but its glued veneers aren’t immune to swelling and shrinking as the humidity changes.
“Although you had no apparent signs of moisture on your concrete slab, that doesn’t mean it was dry two months later. Water vapor could have been coming through at a rate high enough to penetrate the adhesive and reach the wood. There are tests that can verify the moisture level of concrete before flooring is installed, but they only give you a snapshot of conditions at that time. If water migrates beneath the slab during periods of heavy rain, like the ones you’ve had recently in your area, moisture levels in the concrete can increase dramatically.
“That’s why I never glue wood floors directly to slab-on-grade or below-grade concrete, even with a moisture-resistant adhesive. It’s just too risky. In those locations, I’ll install a floating floor, in which the planks and strips are glued or locked edge to edge and not attached to a substrate. There’s less chance for them to buckle that way.
“To lay a floating floor, the existing floor will have to be pried up and the adhesive scraped off the slab. Then the concrete should be covered with a 6-mil plastic vapor barrier, taped at all seams, and followed by a thin foam underlayment, also taped to stop moisture from getting through.
“Also, make sure the new flooring is taken out of its boxes and given time to acclimatize to its surroundings. If you go with hickory again, be especially patient during this stage. Like other very dense woods, hickory is slow to lose or take up moisture, and moves unpredictably when it does.
“As the new flooring is installed, be sure there’s a gap of at least ½ inch between it and the walls around the room. Without that gap, the floor may find itself with nowhere to move as it expands, and it will likely buckle again.”
Shown: A pencil demonstrates how this glued-down engineered floor buckled when moisture penetrated the concrete slab.
Thanks to Patrick Hunt, Hunt Custom-Milled Wood Floors.