- 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.