Have you ever seen a hardwood floor where the boards are either higher in the middle than at the edges, or the reverse? At the least, it looks odd, but when it’s severe, it can lead to buckling of the floor and require expensive repairs. What’s the cause of this problem? In a word, moisture.

Wood swells when it gets wet, and shrinks when it dries out. If a piece of hardwood flooring has more moisture in the wood on one side than the other, the side with more moisture will swell. The moist side expands, forming a crown, while the dry side cups.

One common cause of cupping in a new hardwood floor is that the hardwood is installed on a subfloor that contains too much moisture (as in a new home where the air ventilation system, dehumidifier, and/or central heating are not yet operating). When the subfloor begins to dry out, moisture moves into the underside of the drier hardwood floor above and causes cupping.

Other situations can cause cupping in a preexisting hardwood floor: basement flooding, moisture problems in a crawl space, or drying of the air above the floor. In some locations, these conditions are seasonal. Crowning, of course, involves the upper surface of the wood absorbing moisture, which is less common.

The good news is that cupping and crowning of hardwood floors frequently corrects itself if the moisture problem is resolved and the wood dries out. If the problem persists, it may be necessary to sand and refinish the floor.

The moisture content of a subfloor should always be checked before hardwood is installed to be sure it conforms to manufacturers specifications. Moisture problems should be corrected first.