The most common reason a wood floor cups is the presence of moisture beneath the floor. This may be because of the omission of a vapor barrier between the floor and the subfloor, or a leak in the plumbing beneath the floor. The bottom of the board expands as it absorbs the moisture, leaving the underside wider than the top. This forces the edges of the board upward. When excessive moisture is the cause of your floor cupping, it will affect several boards, and there are typically no gaps between the boards. Before attempting repair to a cupped floor caused by moisture, find and repair the source of the moisture. Allow the flooring to dry after the moisture is removed. Typically this requires at least one heating season. The boards will often flatten on their own once dry.
Wood floors may cup due to improper acclimation of the boards prior to installation. If the flooring is brought into an environment with a higher moisture content than the previous one, it requires time to adjust and expand. If this is not allowed, or if there isn’t enough space left at the edges of the floor to allow for expansion, it causes stress on the outer edges and forces the boards to cup. Cupping due to improper installation typically shows around the perimeter of the flooring only and is often not repairable. Wait a full heating season for the floor to adjust on its own. If the cupping hasn’t decreased during this time, then it is not likely to do so. Permanently cupped floors must be sanded flat and refinished.
When wood loses moisture rapidly, it shrinks or contracts. With hardwood flooring, the dryness affects the upper surface only, causing the bottom to be wider than the top, as with excessive moisture, and this pushes the edges upward. If cupping is caused by dryness, there will also be gaps between the boards. As it acclimates, it may flatten, but the gaps may remain. These can be filled and refinished if they are not severe, but often small gaps are usually temporary. As the floor acclimates, the wood will flatten and the gaps will close. However, in areas where there is constant heat, such as near appliances, the gaps may be permanent. If the gaps are minor, they can be filled and refinished once the flooring has had several months to fully acclimate.
What to Avoid
Do not sand a cupped floor before giving it time to naturally flatten. If you sand a cupped floor that has not yet balanced its moisture content, the floor will crown once it has dried. Crowning occurs when the surface of the wood is higher than the underside. This occurs as the bottom of a cupped floor flattens out after drying. The middle of the board lifts up, and the edges are forced down. The only repair for crowned floors is replacement.