- Q. Recently we installed a new oak strip floor during renovation of an old unoccupied house, and now the floor has permanently cupped. The house is built over a fairly damp crawlspace and has diagonal board floor sheathing. The original yellow pine flooring was badly damaged by termites, so we laid new 3/4-inch plywood subflooring over the yellow pine, put down felt paper, and installed the floor. I’m assuming that the floor cupped because the oak was put down dry in a damp location, picked up moisture, and swelled to the point that it has cupped upward at every joint. Even when air-conditioning was added to the house to dry things out, the floor has remained cupped. Now that the flooring has been installed for several months and the house has a more constant climate, can I sand the floor level and refinish? Or will it just cup again?
A.Howard Brickman responds: You’re right — the cupping is a result of excessive moisture from below. You can sand a cupped floor as long as the cupping is actually permanent. Remember that most structures undergo a moisture cycle with the changing seasons. In the winter when outdoor temperatures are below freezing, buildings tend to dry out and wood flooring will shrink. In the summer, with higher temperatures and more humidity, the moisture content of the wood increases and cupping is more pronounced. If you sand a cupped floor that later flattens out as the relative humidity and the moisture content of the wood decrease, then the floor will crown (develop a convex profile).
The secret to sanding any cupped floor is to be absolutely certain the cupping is permanent. I would advise observing the floor for an entire heating season before sanding.
Howard Brickman is a wood flooring contractor and consultant in Norwell, Mass.