Cupping and Crowning: Spotting Trouble in Your Wood Floors

Man Repairing Wood Flooring

Installing wood floors is a big decision. Wood lovers eagerly anticipate successful installation and they watch for problems. However, many homeowners and installers do not recognize that the wood moisture content (MC) of the wood flooring optimally must be measured and monitored before, during and after installation for the lifetime of the wood floor.

Seasons of Wither

Sun and RainThe key is to know potential wood flooring MC problems before they arise. After installing wood floors, it is necessary to recognize that wood always loses or gains moisture depending upon the relative humidity (RH) and temperature of the surrounding environment. Specifically, wood shrinks as it loses MC. When the cold of winter causes people to turn up the heat in their homes, the RH levels in the home plummet and the wood flooring boards can shrink, and spaces appear between the boards. Wood expands when it absorbs MC from the more humid air, such as when people turn off the heat in spring and moisture returns to the air. Ironic isn’t it? Seasonal transitions bring two potential seasons of wither to the spaces between wood flooring boards.

Now for the tricky part. Expansion and contraction of the wood flooring are as normal as the RH seasonal changes are. Therefore, owners can expect some gaps to form between their wood flooring in winter months; those gaps may naturally close in spring.

Achy Joints

In order to treat those achy wood joints, wood flooring owners should invest in a wood moisture meter. It is a tool designed to measure the MC levels in wood flooring usually up to 3/4” deep into the wood. Lumber producers and contractors use wood moisture meter products as trade tools when installing wood floors. Optimally, wood must reach its equilibrium moisture content (EMC), a point of balance between the MC of the wood and the RH of its ambient environment. Mills process lumber to try to reach its EMC target, but installers still need to confirm that the MC of the wood is stable for installation purposes after the manufacturing process.

Wood flooring owners should also monitor the MC of the wood flooring after installation to remediate any potential future moisture related problems.

Cupping, Crowning, and Buckling

Cupping Illustration

Cupping occurs when the edges of a board are higher than its center. Assuming the flooring has been properly installed, cupping can occur due to excessive moisture which causes the wood flooring to swell, crushing the boards together and deforming them at the edges. The board edges form a “cup” due to excessive MC in the flooring. The first step to remediation is to identify the moisture source: high indoor RH, a water spill, a leak from a dishwasher hose or a plumbing leak, for example. Once the cause of the moisture is identified and controlled, cupping can possibly be reversed. Wood flooring owners can use a wood moisture meter to identify MC changes on a regular basis before cupping occurs.

Crowning Illustration

Crowning is the opposite; the center of a board is higher than its edges. This can occur when the surface of the floor encounters moisture or is left in wet or humid conditions for an extended period of time. Crowning may also occur due to previous floor cupping problems. If the floor is cupped, the floor should be given ample time to dry. If the floor is sanded while the boards remain cupped and moisture is still present, the sanding process can sand off the top edges of the board and thus, the edges are lower than the rest of the board when it returns to a normal MC.

Buckling occurs when wood flooring actually pulls away from its sub-floor, lifting up to several inches  in one or more places. This is one of the most extreme reactions to moisture that can occur in wood flooring applications. Fortunately, this does not happen often. The most common reason that buckling occurs is after a floor has been flooded for a period of time. Other causes on nailed floors might be insufficient nailing, incorrect nails or incorrect subfloor construction. On glued floors, use of incorrect or insufficient mastics to an inadequate mastic transfer, a subfloor separation or a subfloor contamination can cause buckling.

Any MC imbalance creates possible problems in all wood. With a wood moisture meter, flooring owners may find MC problems in time to pre-empt flooring problems or failures.

Moisture Meter

The wood moisture meter has come a long way. Wagner Meters, a company formed in 1965, carries wood moisture meter products for lumber mills, building inspectors, installation contractors, wood hobbyists and wood flooring owners. Obviously, these are primary tools for the expert flooring installer and building inspector, but homeowners can benefit from their quick “scan” capabilities as part of their regular home maintenance. Wagner Meters pinless technology allows the user to accurately scan the wood without the time-consuming effort of driving pins into the wood and damaging the wood’s surface.

The Wagner Meters MMC220 Extended Range wood moisture meter measures the MC in wood flooring and woodworking applications. The MMC220 measures the MC between 5-30%. This Wagner Meters hand-held device also assesses hardwoods, softwoods, and tropical wood species.

For more general building materials, Wagner Meters BI2200 Building Inspection moisture meter measures wood, sheetrock, concrete and many more building materials for non-damaging moisture checks throughout the home.

Spotting MC problems in advance in building structures and in wood flooring and other wood products in your home or business is a preemptive act of successfully maintaining the value of your investments.

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Ron Smith

Ron is an application specialist for Wagner Meters, and has over 30 years of experience in instrumentation and measurement systems in different industries. In previous positions, he has served as a regional sales manager, product and projects manager, and sales manager with manufacturers involved in measurement instrumentation.


  1. bert says:

    If I set up tent over damp hard wood floors and put a humidifier hooked to it will it in reversing cupping its in a small area ?

  2. Kathryn A says:

    water spilled on my wooden floor and i have minor cupping as a result. can this be reversed? it is tongue and groove bamboo flooring.

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