Cupping, buckling, and crowning are three common problems encountered in wood flooring (flooring). These problems are caused by a combination of the normal material characteristics of the flooring and improper installation of the flooring.
Flooring expands when its moisture content increases and contracts when its moisture content decreases. Flooring also expands when its temperature increases and contracts when its temperature decreases. The amount of moisture-related expansion and contraction is always larger across the width of the floorboard (across the grain) than along the length.
The overall floor expands when the temperature and/or moisture content of the individual boards increases uniformly through their thickness. Expansion in the flooring is accommodated by installing the flooring with a gap between the perimeter walls of the room and the perimeter edges of the flooring. The required size of the gap varies depending on the size of the floor. The National Wood Flooring Association (NWFA) recommends, as a general rule, that a ¾-inch-wide gap be provided between the edge of the flooring and the abutting walls. The presence of the perimeter gap allows the flooring to expand without restraint.
Without a perimeter expansion gap between the walls and edges of the flooring, the walls will restrain the expanding flooring (Figure 1). The restraint of the flooring creates large internal forces in the flooring, which will cause the flooring to buckle upward (Figure 2). The upward buckling of the flooring, which is also known as “tenting,” commonly occurs in the middle of the floor.
Floorboards are commonly 2 ¼ inches in width and ¾ inch in thickness (Figure 3). Flooring will “cup” when the moisture content of the bottom surface of the floorboards is larger than the moisture content of the top surface of the floorboards (Figure 4). The side edges of “cupped” floorboards are distorted upward relative to the center, which creates a concave or “cupped” surface on the boards.
“Crowning” occurs when the moisture content of the top surface of the floorboards is larger than the moisture content of the bottom surface. Crowning is a condition in which the centers of the floorboards are distorted upward relative to the side edges. The top surface of a “crowned” floorboard has a convex profile (Figure 5).
Crowning can be inadvertently caused by improperly repairing cupped flooring. A commonly used improper repair method is to sand the cupped flooring surface to a flat surface while the moisture content in the flooring is elevated. The result is a crowned surface to the flooring once the flooring dries out to its normal moisture content. Sanding the flooring to remove crowning should not begin until the proper moisture content of the flooring has been restored.
Flooring problems are encountered when the flooring has not been properly acclimated to the room that it is to be installed in. Acclimating flooring prior to its installation minimizes problems resulting from differential temperature, moisture content, and relative humidity between the flooring, the air in the room, and the subfloor that the flooring is to be installed on.
The NWFA recommends that flooring be installed after the building has been enclosed, the heating/air conditioning system has been turned on, and the moisture content of other new building materials have attained their equilibrium temperatures and moisture contents. The NWFA also recommends that the moisture content of 40 floorboards per 1,000 square feet of flooring be checked before installation to verify that they have moisture contents that are within the flooring manufacturer’s recommended range.
The NFWA recommends that a vapor retarder be installed between a concrete slab-on-grade and its prepared subgrade, when flooring is to be installed on the concrete slab, or when it is to be installed on sleepers that bear on a concrete slab. The NFWA recommends that the vapor retarder be “impermeable,” with a permeability rating (perm) of 0.15 or less. One method to achieve this perm rating is with the use of a 6-milimeter-thick polyethylene film vapor retarder. Without the use of an impermeable vapor retarder, moisture will migrate from the subgrade soil, through the concrete slab, and into the flooring via vapor pressure transmission, and damage the flooring (Figure 6).
When installing flooring on a wood subfloor, the NFWA recommends the use of a semi-permeable vapor retarder, which has a perm rating between 0.7 and 50. The use of a semi-permeable vapor retarder between a wood subfloor and flooring reduces but does not eliminate the potential for moisture or vapor-related problems.
Flooring problems are also caused when the flooring and wood subflooring are installed above improperly ventilated crawlspaces. An improperly vented crawlspace allows humidity in the crawlspace to increase to levels that can damage the flooring. The water vapor contained in the crawlspace air migrates through the subfloor and flooring, increasing their moisture content, which can damage both. A similar condition is created by water ponding in the crawlspace (Figure 7).
When installing flooring on a wood subfloor, the NFWA cautions against installing an impermeable vapor retarder between the wood subfloor and the flooring because it may trap moisture in the subfloor, elevating its moisture content and damaging it.
 For more information about hardwood flooring, refer to Technical Article A100, “Water and Wood, How Moisture Affects Wood Flooring,” National Wood Flooring Association (NWFA), 1996.