Cupped solid hardwood floor

About the Authors: Ray Darrah and Linda Lockwood

Cupping is the condition of the edges being higher than the center of the boards, providing a wavy, non-flat, washboard appearance. The cupped plank, or strip, has a concave or dished appearance. Cupping is one of the two most common complaints in the hardwood flooring industry, yet least understood.

Cupping of wide boards, four inch to seven inch widths is characteristic to a degree. Boards are rectangle in shape and the tree is round, giving the board a natural tendency to be round, depending on the cut of the board. Wide width cupping is not normally seen in normal lighting but noticed in reflective light. Slight cupping of wide width planks is characteristic and not associated with this discussion.

The most common condition causing cupping is the sub-floor having higher moisture content than the wood itself. The moisture moves from the sub-floor into the drier wood flooring, leaving the bottom higher in moisture content than the top of the board, causing the bottom to swell more than the top of the board. The top is not swelling at the same rate as the bottom, leaving the top smaller and cupped.

An example of this condition would be a new construction home without heat or air conditioning when the floor is installed. The condition of new construction buildings are generally wet after with the sub-floor high in moisture content. The installers arrive to install wood that is drier than the sub-floor. The heat, or air conditioning, is turned on after the installation, drawing the moisture from the sub-floor into the drier hardwood flooring resulting in the cupped condition.

Homes with crawl spaces require placement of a vapor barrier on the ground to stop the capillary water movement from the soil into the buildings sub-floor.  After placement of the vapor barrier, the sub-floor must be given time to acclimate to the new conditions of the crawl space changed by placement of the vapor barrier. Failure to allow time for the building to acclimate to the new conditions may result in cupping due to the higher moisture content in the sub-floor compared to the wood flooring.

Professionals know the need for the crawl space vapor barrier, but novices are unaware, and do not as a general rule, take the time to investigate the site. The novice, or untrained installer, installs the wood flooring without acclimating to the conditions of the site, does not check sub-floor moisture content and installs the new flooring without a vapor barrier placed on the ground. Cupping could easily appear within days.

To prevent cupping, remember that the wood must contain the same, or nearly the same, moisture content of the sub-floor, the home must be acclimatized to “in use conditions” before and during the installation, and moisture barriers must be in place.