Why Are My Wood Floors Cupping?
Kevin O’Connor calls on the National Wood Flooring Association to assist in a floorboard ‘cupping’ problem
A couple of winters ago, a contractor installed new solid-oak flooring over ¾-inch plywood with a vapor barrier. The following summer, the floorboards began cupping, which is quite noticeable when you walk on the floor with bare feet, or when the light hits it at a low angle. How can I correct this unsightly washboard effect?
—Chester Frederick, Charlotte Hall, MD
When it comes to technical questions about wood flooring, there’s no better resource than the National Wood Flooring Association. Brett Miller, the NWFA’s vice president for education and certification, explains what’s causing this problem, and how to keep it under control.
“It’s important to understand this key fact about wood behavior: When the relative humidity goes up, as it often does in the summer, all wood absorbs water vapor and gradually swells, mostly across its width. And when there’s a roomful of wood strips in a humid environment all pressing hard against each others’ edges, it creates enormous stress. At the same time, there’s usually a moisture imbalance between the slightly moister bottom and slightly drier top face of each strip. As a result, the bottom swells more and those top edges inexorably curl up, despite the hundreds of nails holding the flooring down. (If wood strips ever get soaking wet, they can swell so much that they buckle and actually pull those nails out of the subfloor!)
“Cupping is not just an aesthetic problem. If the pressure gets high enough, it can crush the wood fibers along the strips’ edges. And then the strips won’t recover their orginal dimension when the humidity goes back down in the winter, and you’ll have permanent gaps between the strips.
“The way the wood is cut also affects how it behaves during these seasonal cycles. Plainsawn (a.k.a. flatsawn) strips, which have wood grain that runs parallel to the strip’s face, show the greatest amount of movement. Strips that are quartersawn, with the grain running at right angles to the faces, remain more stable when the humidity fluctuates.
“I suspect that your floor, like most wood floors, is made up primarily of plainsawn strips. But it’s a little too late for you to change the wood now. The only practical way for you to control cupping and gapping at this point is to keep the inside humidity level between 30 and 50 percent all year long by using air conditioners and dehumidifiers in the summer and humidifiers in the winter.
“But, please, don’t try to sand off those cupped edges. That would be about the worst thing you could do, because then the strips would become crowned—high in the middle—when the swelling went down in the winter.”