Getting it right: Wood flooring over a concrete subfloor

The in-situ relative humidity (RH) test method provides reliable and accurate concrete moisture assessment.
The in-situ relative humidity (RH) test method provides reliable and accurate concrete moisture assessment.

It is important to keep in mind this must be thought of as no more than a rule of thumb. It can help guide one’s expectations but should never be used for making an installation decision.

The key step to preventing moisture-related problems is always insisting on the performance of a moisture test prior to any flooring installation. Getting accurate, reliable test results provides the essential information needed for deciding when wood flooring over a concrete slab can be installed safely.

Two types of concrete moisture tests are often used in the United States. Historically, surface-based tests, such as the anhydrous calcium chloride test, were employed to evaluate the moisture condition of the slab. Today, this type of test is becoming less commonly used as more people in the industry learn about the advantages of using another type of moisture test known as the in-situ RH test.

An inherent problem with a surface-based test is it is unduly influenced by ambient conditions and can easily give false results. Another problem is this type of test is at best indicative of the moisture condition at or near the surface of the slab. It is based on the false premise this is the only information needed to make a proper decision about time of installation. The test offers no information about the level of moisture existing deeper within the slab.

This consideration is critically important. The moisture in a concrete slab exists in a gradient, with significantly less moisture at the surface than deeper down. However, once a slab is effectively sealed with a finished floor product, so moisture can no longer evaporate from the slab’s surface, the moisture inside the slab will tend to even out and the moisture gradient will disappear.

The net effect is the moisture at or near the slab’s surface, which is what the finished floor will now be in contact with, becomes higher than what a surface-based test would indicate. For this reason alone, one should not rely on this type of concrete moisture test. Results from the test may mislead and this could end with flooring failure.

In-situ RH test

On the other hand, scientific studies in recent decades have demonstrated a slab’s moisture condition (and how it affects installed flooring) can be best measured by looking at the RH deep down in the concrete using an RH probe set into the floor slab. Since 2002, the in-situ RH moisture test, as standardized in ASTM F2170, Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes, has become increasingly favored as the ‘gold standard’ for moisture testing.

The RH test is useful as is it is a depth-specific test that fully considers what happens to concrete moisture after a flooring installation seals off the slab’s surface. Studies at Sweden’s Lund University determined the placement of the RH probe at a depth of 40 percent of the overall depth of the slab (when drying from one side) will provide RH readings that accurately predict the moisture that the finished floor will ‘see’ once the flooring is installed.

In growing recognition of this and other major advantages of the RH test (less vulnerable to changing ambient conditions, faster and easier to perform with test results within 24 hours, and ability to easily track and record RH changes over time), a large number of flooring manufacturers now provide specifications for their products based on the RH test’s numeric results.

In the author’s experience, no other method of concrete moisture assessment provides the same level of reliability and accuracy as does the in-situ RH test. Therefore, no other method can give the same level of assurance for avoiding costly moisture-related flooring failures and the many types of damage (such as cupping, crowning, buckling, mold, or mildew) that could otherwise occur in wood floors.

What it all means for the project’s specifications

It is imperative a project’s specifications always identify and require the exact concrete moisture test to be performed. This is an easy addition to the project’s plans given the in-situ RH specification is available for free download.

It is important to recognize the in-situ RH test has been shown to give reliable results leading to consistent, successful project outcomes. Additionally, unless this specific test is spelled out for the general contractor (GC) and/or the flooring professional, some parties in the industry may unwittingly choose to employ another test that may place the project at risk. Therefore, it is crucial to be very specific about which concrete moisture test is desired.

Jason Spangler, Wagner Meters’ flooring division manager, has more than 25 years’ experience in sales and sales management across a broad spectrum of industries. He has successfully launched a variety of products to the construction market, including the original Rapid RH concrete moisture test. Spangler, who received an MBA from West Texas A&M University, has extensive industry involvement, including the National Wood Flooring Association (NWFA) and the International Certified Flooring Installers Association (CFI). Spangler is also vice-chairman of associations for the Flooring Contractors Association (FCICA). Spangler can be reached via e-mail at

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3 comments on “Getting it right: Wood flooring over a concrete subfloor”

  1. …the in-situ RH specification is available for free download.

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