A floor installation is about far more than laying planks. A professional flooring contractor must account for every material that interacts with the floor system—before, during and after installation.
The first critical factor is the moisture content of the flooring material as delivered to the jobsite. This must be verified using a moisture meter. Industry standards typically call for a moisture content between 6% to 9%. If measurements fall outside this range, the manufacturer should be notified immediately. For humid or very dry climate zones the range for permissible moisture contents may differ from the standard 6-9%.
Flooring problems begin when the moisture content of the floor planks changes. Wood is a hygroscopic material—it absorbs and releases moisture depending on its environment. As moisture levels fluctuate, wood expands or contracts. This movement can lead to cupping, crowning, buckling or cracking. Even a perfectly installed floor can develop defects if environmental conditions are not properly controlled.
To prevent these issues, an equilibrium must be established between the flooring, the subfloor or concrete substrate and the surrounding air. The balance between wood moisture and relative humidity is known as the equilibrium moisture content (EMC).
Environmental conditions matter. Maintaining stable indoor conditions is essential. That means relative humidity should be 30%–50% at an ambient temperature ranging between 60°F–80°F. Within these ranges, wood typically stabilizes at an EMC of 6%–9%. Minor fluctuations within this range result in minimal expansion and contraction.
For example, at 40% relative humidity and 70°F, wood at 7.7% moisture content will remain stable. Or, in a warehouse at 60% relative humidity and 50°F, dry wood will absorb moisture over time until it reaches approximately 11.2%, regardless of species or starting moisture level.
There are factors that disrupt moisture equilibrium and disturb this balance and lead to flooring failure. These include:
Subfloor moisture. If the subfloor contains excessive moisture, it can transfer that moisture into the flooring. Manufacturers provide specific guidelines for acceptable moisture differences between subfloor and flooring materials. These must be followed carefully and verified with a moisture meter.
Concrete substrates. Concrete is a common source of hidden moisture. Industry standards now favor in-situ relative humidity (RH) probe testing over surface-level handheld meters. While handheld devices may measure up to about one inch deep, they often miss moisture deeper within the slab. This hidden moisture can migrate upward after installation and be absorbed by the wood.
To mitigate this risk, manufacturers often recommend vapor barriers to prevent moisture transmission from concrete subfloors.
Ambient air. Air conditions play a continuous role in maintaining stability. As noted, keeping humidity between 30% and 50% and temperature between 60°F and 80°F helps maintain a stable EMC and minimizes wood movement.
The floor, subfloor, concrete substrate, even walls and ceilings as well as in-door temperature and relative humidity form an interconnected system. Each component influences the others. For a successful and long-lasting installation, all elements must be in equilibrium from the start—and remain so over time.
Lastly, documenting moisture readings at the time of installation is a critical step. These records establish the conditions under which the floor was installed and can protect the contractor in the event of future claims. If problems arise later, this documentation provides a clear starting point for identifying the cause.
Grete Heimerdinger, vice president at Lignomat, has been heading the moisture meter division of the company since 1981, when the first pocket-size mini-Lignos were introduced. She graduated from the Technical University in Stuttgart, Germany, with a teaching degree in Mathematics and Physics. For more information, contact Lignomat at 800.227.2105 or email sales@lignomat.com.
