Crowning & Cupping in Hardwood Floors

CROWNING - The center of the pieces of flooring appears to be higher than the edges.

Cause: While it is theoretically possible that excessive moisture could cause crowning,it is more likely that the floor cupped and then was sanded flat before it could dry and flatten on its own. When the floor boards did dry to a normal condition, their edges had been removed, making them lower than the center of the board. Gaps are generally formed as the flooring dries.
Cure: First, determine if the moisture content is normal and if all of the crowning from the original cupped condition has occurred. After the floor has stabilized, resand and finish.

CUPPING: Cupping occurs across the width of the individual pieces of flooring. The edges are high and the center is lower. It generally develops gradually.

Cause: A moisture differential within individual pieces of flooring, usually excessive moisture on the underside of the flooring. More subtle cupping can be caused by lack of proper acclimation (this is generally permanent cupping). Potential sources of excessive moisture include:

  • Building leaks
  • Poor drainage
  • Plumbing leaks or overflows
  • Leaks from dishwashers or refrigerator ice making units
  • Wet or damp basements/crawlspaces
  • Concrete subfloors that have not cured
  • Plywood subfloors with excessive moisture
  • Poor or no ventilatio
  • HVAC system not operating.

Flooring also may cup when a wood floor experiences conditions that cause rapid drying on the surface. This condition occurs with gaps as the flooring shrinks.

Cure: Never attempt to repair a cupped floor until all of the sources of excessive moisture have been located and eliminated. This can be verified only with a moisture meter that takes readings of the underlying subfloor. As long as the wood is not permanently deformed or damaged, the flooring will return to its original shape and size when the excessive moisture is removed. This may take weeks, months, or even an entire heating season.

Attempting to sand a cupped floor while it is still too wet may cause subsequent crowning when the floor dries. Flooring that does not return to its original shape, even after completing an entire heating season, probably is permanently deformed. (Taking moisture readings at different levels in the wood flooring also can help determine this-if there is a gradient of 1 percent or more between the top and bottom of the boards, they probably are not done drying.) If the boards are permanently deformed, the cupped edges may be sanded off.

For floors that have cupped due to drying, relative humidity should be increased. Relative humidity below 20 percent is considered very dry for wood flooring, and it is suggested that humidification be provided under such conditions.

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