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Why Flooring Installers Need to Measure Moisture Content in Wood Subflooring

When flooring installers lay a wood floor on top of a wood subfloor, it is critical that they measure moisture in both the wood floor and the wood subfloor.

The reason for this is simple. Since both the flooring and subflooring are wood, both are subject to absorbing and releasing moisture from the environment in which they are being used.

Failure to measure both the flooring and subflooring can be disastrous. Just ask the National Wood Flooring Association (NWFA). The NWFA reports that the vast majority of calls it receives annually are due to moisture-related problems such as warping, cupping, cracking, and shrinking.

Why Test a Wood Subfloor? A wood subfloor is typically made of either plywood or Oriented Strand Board (OSB). Both are wood structural panels made by compressing and gluing pieces of wood together. While OSB and plywood are viewed to be the same and interchangeable, they are manufactured differently and have their own unique strengths and weaknesses.

There is one thing both these materials have in common. When either plywood or OSB absorbs excess moisture, it expands. When expansion occurs, the flooring will move, buckle, or cause joints to show through.

To illustrate, lay a dry paper towel across a damp sponge. Notice how the dry paper towel quickly picks up the excess moisture in the sponge and expands. This is what happens when wood flooring is installed on a wet subfloor.

To take our illustration one step further, consider what happens if wood flooring is installed over a wood subfloor during the hot, humid summer months – both the flooring and subflooring take on additional moisture and begin to swell. Later, during the winter months, the heat comes on. Although the flooring may have been installed properly with a proper fastener schedule, fit with adequate expansion space, and looks okay initially, things will change as the subfloor gradually begins to dry out.

As the subfloor dries, it shrinks, and the once good installation begins to change. Fasteners loosen, expansion space shifts, the wood floor/subfloor system begins to work against each other. The more the subfloor dries out, the greater the chance the wood flooring will make audible noises or show gaps.

How Many Subfloor Readings Are Needed? Wood subfloors are easy to check for moisture content (MC). According to NWFA guidelines, an installer should take 20 readings per 1,000 square feet for wood subfloors, then average and record the results.

Subfloor Moisture Variance When wood flooring is installed over a wood subfloor, what you have are two different types of wood that have to live together. But when one piece of wood either contracts or expands at a different rate than the other, problems are sure to follow.

For that reason, there needs to be a close variance in MC between the two different woods. The NWFA offers guidelines on this variance:

  • For solid strip flooring (less than 3” wide), there should be no more than 4 percent difference in MC between properly acclimated wood flooring and subflooring materials.

  • For wide-width solid flooring (3” or wider), there should be no more than 2 percent difference in MC between properly acclimated wood flooring and subflooring materials.

The best way to ensure that this MC variance is within NWFA guidelines is to make sure the wood subfloor and wood flooring acclimate to the environmental conditions in which the flooring will be installed.

In most regions, a dry wood subfloor that is ready to work on has an MC of 12 percent or less (as low as 4 percent in low relative humidity states).

During the winter, an overly moist subfloor can be dried out by running the heating system for a few weeks. During the summer, air conditioning will also remove moisture from the air, but in many cases, use of a dehumidifier may be necessary to get the subfloor down to acceptable levels for installation.

Ambient environmental conditions, therefore, will dictate what the MC of the subfloor and wood flooring is going to be. However, always follow the wood flooring manufacturer’s recommendation regarding MC for a proper subfloor.

Moisture Meter: A Necessity Flooring installers can easily determine the MC of the wood subflooring (and, of course, the wood flooring) by using a hand-held moisture meter. A moisture meter should be part of every flooring contractor’s toolbox as it can provide a good representative picture of the entire subfloor.

Final Thoughts Any successful wood floor installation requires proper moisture testing. Unfortunately, what many installers often fail to recognize is the MC of the subfloor.

When the subfloor is overlooked, an imbalance between the MC of the subfloor and the new wood flooring can cause unsightly and costly problems. To avoid these problems, it is always wise to use an accurate moisture meter to ensure the wood flooring materials and the subfloor are both ready for the installation.

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