By Jason Spangler

Wood’s moisture content (MC), the amount of moisture in wood, must be managed to achieve optimum flooring strength and durability. When relative humidity (RH) rises, wood absorbs moisture from the air. When RH falls, wood emits moisture back into the air. This natural process gives wood its strength and beauty, but excessive MC can cause many possible problems with your wood floors.

Visible Signs

Some moisture-related problems are visible to the discerning eye. Primarily, wood flooring problems can occur when the wood expands and contracts. This “wood movement” occurs naturally according to seasonal shifts or when ambient RH levels change in the home. These changes can also present a less-than-desirable change in the appearance of your wood flooring.

Cracks and Separations between Boards: When homes are heated in the winter, RH levels can plummet, boards can shrink, and spaces appear between the boards as the wood loses MC. Some of these variations are seasonal and will close up on their own when the weather (and related RH) changes and moisture returns back to the air. Monitoring the room’s temperature and RH level, then taking corrective action, can also minimize seasonal shifts in the wood floor. As an example, homeowners can add moisture to the air during dry winter months by installing a humidifier in the furnace.

Cupping: This is when the edges of a board are higher than its center, due to moisture which causes the wood to expand. This can occur after water spills onto the floor and is absorbed by the wood. However, usually high RH is the culprit. As the wood expands, compression can result as the boards are crushed together, deforming the boards at the edges. Cupping is caused by a moisture imbalance through the thickness of the wood. The wood is wetter on the bottom of the board than on the top surface, which dries quicker than the bottom. Cupping most often appears after the floor has been installed and may or may not be an installation issue.

Crowning: The opposite of cupping, crowning occurs when a board’s center is higher than its edges. A common cause is moisture exposure or imbalance. If the surface of the floor is left exposed to water or left in humid conditions for an extended period of time, the moisture can saturate the wood flooring and cause crowning. Another cause is that the floor was previously cupped, but was sanded before the MC returned back to its normal state. In this case the floor should be given time to dry before sanding. The sanding process can cause the raised edges of the board to flatten while lifting up the middle of the board if the floor is sanded while the boards remain cupped and moisture is still present.

Buckling: This is the most extreme reaction to moisture in a hardwood floor. This occurs when the wood flooring actually pulls up from the subfloor, lifting several inches in one or more places. Fortunately, this is not a common occurrence. Buckling happens most often after a floor has been flooded for an extended period of time. If caught early, spot repair and replacement may be possible. Once the standing water is removed, several boards can be taken up from the floor so that the air can circulate across and below the floor. Once the floor has dried to a more stable moisture level, repairs can usually be made.

Invisible Moisture Problems

Some moisture dynamics can occur beneath a flooring substrate with equally devastating possible consequences for wood flooring applications.

Wood Subfloors: Determining MC is an essential part of quality control within the flooring installation process. Flooring installers must know the MC of the subfloor as well as of the wood flooring. Test for moisture at several locations in the room – a minimum of 20 per 1,000 square feet – and average the results. You should make sure to measure all exterior and plumbing walls. In most regions, a dry subfloor that is ready to work has an MC of 12% or less. If you record high MC readings, do not proceed with the installation until the origin of the moisture is identified and all moisture related problems are remedied.

Concrete Subfloors: All types of flooring are susceptible to failure if moisture conditions are not properly monitored and maintained. When the subfloor is concrete, a moisture problem may begin long before the flooring is installed. If the slab has not been properly dried, moisture problems are almost guaranteed with wood flooring applications. Accurate moisture testing is critical for any concrete slab. ASTM International has provided several standards for testing moisture with two different test methods before installing flooring over a concrete slab: in situ probes per ASTM F2170 and calcium chloride testing per ASTM F1869. The best indicator is RH testing using in situ probes per ASTM F2170.

Leaks or Water Intrusion: Any moisture that comes from below a subfloor or gets betweenthe subfloor and the flooring can cause problems over time. Leaking appliances, ground water intrusion through a perforated vapor barrier, condensation, and even moisture from a concrete subfloor that had not completely dried can introduce moisture to your flooring equation. Also from the outdoors, it might be the terrain of the home’s lot with rain and runoff not moving away from the foundation that can cause moisture-related problems with the wood flooring over time.

The Antidote

The best cure is to accurately measure and assess the moisture in the subfloors previous to wood flooring installation and also measure the MC of the wood flooring before, during, and after the flooring installation.

A wood moisture meter helps wood flooring lovers to navigate naturally-occurring seasonal MC changes on a regular basis. Pinless moisture meters for wood can measure the MC below the wood surface without doing any damage to the wood flooring. Wagner Meters uses pinless technology to non-invasively measure the wood flooring’s MC, which can help accurately pinpoint possible problem areas for repair or remediation. Wagner Meters manufactures moisture meters for wood that can measure hardwoods, softwoods, and exotic wood species, and also manufactures specialty meters for wood finish applications. Wagner Meters also manufactures RH in situ probes for testing concrete slab subfloors.

When MC management makes all the difference, using a Wagner Meters wood moisture meter is an invaluable way to effectively resolve the warning signs on wood floors.


Jason has 20+ years’ experience in sales and sales management in a spectrum of industries and has successfully launched a variety of products to the market, including the original Rapid RH® concrete moisture tests. He currently works with Wagner Meters as our Rapid RH® product sales manager.

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