How Much Moisture Can Your Floor Handle?

Do you want your floors to last for the long haul? Achieving a floor that will excel in key characteristics—such as resilience, strength, durability, and functionality—means paying attention to multiple factors, including one that’s not so readily apparent to the eye. Sometimes it’s what lies below the floor’s surface that matters most.

The (largely) hidden moisture factor

Take moisture for instance. The presence of moisture is not typically noticed until the damage is already rearing its ugly head. Yet moisture is always present and may be lurking below the surface in amounts that could cause catastrophic harm to your floor.

Do you know how much moisture your floor can handle? You should. Most flooring failures happen because of excessive moisture, and the wide-ranging problems: for example, buckling, crowning, warping, cracking, mold, and mildew.

Concrete subfloors: a common source of hidden moisture

The subfloor is one of the most common sources of excessive moisture, particularly if it’s made of concrete. While a slab may look and feel dry to the casual observer, no concrete slab is ever completely dry. In fact, water is one of concrete’s most essential ingredients, along with cement and aggregate. Without water, concrete won’t cure properly to provide the characteristic strength that makes it such a valuable material for construction.

Concrete can also be compared to a sponge with tiny pores that absorb, store, and release moisture according to changing ambient conditions. Concrete contains capillaries that facilitate the movement of water toward the surface where it will evaporate over time. As this drying process occurs, a moisture gradient forms with more moisture deeper down in the slab than at the surface.

long lasting hardwood floors

To ensure key characteristics of a beautiful floor, such as resilience, strength, and durability, last for the long haul, pay close attention to moisture.

Taken together, these things mean the moisture condition of a concrete slab is dynamic—constantly changing, assuming the slab has at least one surface open to the surrounding environment. However, this dynamic state is lost as soon as the slab gets sealed with any finished floor product that can act as a barrier to water passage. At this point, the slab’s moisture gradient gradually disappears, effectively creating a moisture equilibrium with more moisture near the surface than before.

Concrete’s rule of thumb

There’s a general rule of thumb that it takes a concrete slab about one month to dry for each inch of slab thickness. For a five-inch slab, this equates to allowing about five months of drying time before installing the finished floor. Keep in mind, however, that this rule of thumb is only an approximation, and the clock only starts when proper drying conditions are first achieved. The actual drying time can vary widely and will be affected by several important variables, such as ambient relative humidity (RH), temperature, and whether or not the structure is fully enclosed.

This means it is never safe to rely on the one-month-per-inch rule of thumb. Think of it merely as a general guide that can help with project planning. For peace of mind that a concrete floor is sufficiently dry, use a quantitative test to evaluate the actual moisture condition of the slab.

The best method of testing for concrete moisture

Several test methods are available, but only one test—the in-situ RH test—has been scientifically proven for accuracy and reliability. That’s because it is the only test that fully accounts for the moisture dynamics of concrete. Studies at Lund University in Sweden and elsewhere have demonstrated that measurements taken at 40% of the overall depth of the slab (for a slab drying from one side) accurately predict the amount of moisture the finished floor will “see” once the slab is sealed and the moisture inside the slab equilibrates.

The RH test is quick and easy to perform. It consists of drilling a hole into the concrete to the required depth, waiting 24 hours for the hole to equilibrate, then inserting a probe or sensor that measures the RH within the hole.

water in concrete

Water is one of concrete’s most essential ingredients, which makes it vitally important to test for excessive moisture before the floor installation.

How to know if the moisture level is acceptable

Once you obtain your test results, the question becomes this: How do you interpret them? How can you know if the RH readings indicate that the slab is dry enough for your flooring installation?

This is really quite straightforward. The F710 standard for RH testing states that your readings must be at or below 75% RH—unless the flooring product manufacturer has provided different guidance. In fact, most manufacturers have developed RH specifications for their products. In some cases, manufacturers’ guidelines indicate that their product can handle RH levels of 95% or even higher. Always consult the manufacturer to find out what RH value the product you are installing can handle.

Be a stickler for accuracy!

Of course, when you compare your moisture data with the manufacturer’s product specifications, you’re only going to protect your project from ruin IF you obtain accurate RH moisture measurements. So be sure to follow the ASTM guidelines for the in-situ RH test. And do not be fooled if anyone tells you that a surface-based test method, such as the anhydrous calcium chloride test, will do. It won’t!

concrete moisture testing with app

When assessing the moisture condition of a concrete subfloor, always aim for accurate, reliable results by using the in-situ RH test method.

Finally, look for an in-situ RH test kit that’s convenient, simple to use, and known for its testing accuracy, one such as the popular Rapid RH L6 system from Wagner Meters. If you opt for a test kit that you find to be cumbersome to use, you may be tempted to cut corners in your testing or use a different, less reliable test method.

So, how do you know how much moisture your floor can handle? It’s quite simple. Learn the manufacturer’s specification for the product you are using. And always consider the hidden moisture factor lurking in the concrete subfloor by using the one concrete moisture test that’s been scientifically proven for accuracy and reliability—the in-situ RH test.

For more information about the Rapid RH L6 system for testing concrete moisture, contact Wagner Meters at (800) 585-7619 or visit

Previously published in The Flooring Contractor 

Last updated on June 1st, 2022

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.