When 20% Is the Right Depth for RH Testing

Moisture in concrete slabs can cause serious problems for building and flooring projects, leading to costly repairs and delays. That’s why concrete moisture testing has become an essential practice in the construction industry.

In this article, we’ll walk through the history of concrete moisture testing, from the calcium chloride (CaCl) test to relative humidity (RH) testing, and look at the strengths and limitations of each method.

• Concrete Moisture Testing: A Brief History
• CaCl Test
• The Hood Method
• Concrete Moisture Meters
• Problems with These Tests
• Relative Humidity (RH) Testing
• 40% Concrete Moisture Testing
• 20% Concrete Moisture Testing

While 40% of the slab depth is the right RH testing depth for most installations, there’s an important exception: slabs that dry from both the top and the bottom are tested at 20%.

Over the past few decades, concrete moisture testing has seen significant changes in both methods and technology, and those changes have improved the building and flooring industries’ understanding and best practices.

Concrete Moisture Testing: A Brief History

Concrete has been used in construction for centuries, and many structures built with it are still standing. However, there are also many cases of concrete flooring projects failing or having lasting flaws because of moisture problems. Measuring moisture levels in concrete has become a critical part of the process.

Over the years, several methods for measuring moisture in concrete have become common:

CaCl Test

The calcium chloride (CaCl) test, also known as the moisture vapor emission rate (MVER) test, uses a desiccant sealed under an impermeable cover and measures how much weight the crystals gain over a set amount of time.

That gain is treated as moisture emitted from the slab and as an indicator of the moisture remaining in it. The test began as a simple “dampness test” in the 1940s and became a standard test in the flooring and building industry, even though it only reflects conditions near the surface of the slab.

The CaCl test is covered by ASTM F1869, although it is not allowed for lightweight concrete.

The Hood Method

The hood method uses a moisture-resistant, insulated hood to cover a probe placed on top of the concrete surface.

The idea was that the test would recreate the conditions a floor covering would experience once installed on the slab and indicate whether the slab was ready for flooring. In the US, it was the test method in ASTM F2420, which was published in 2005 and withdrawn in 2014.

Concrete Moisture Meters

Concrete moisture meters work much like other moisture meters. An electrical signal is sent into the concrete, and the meter translates the response into a reading that indicates the relative moisture level near the surface of the slab.

ASTM F2659 covers using concrete moisture meters for a preliminary, comparative evaluation of slab moisture. They’re useful survey tools for finding wetter areas, but they aren’t a substitute for a test that determines whether a slab is ready for flooring.

Problems with These Tests

The problem with each of these three methods is that they only reflect conditions at or near the surface of the slab. (Moisture meter readings can also be affected by what’s in the concrete, like rebar or admixtures.)

In a drying slab, moisture forms a gradient, with moisture levels increasing toward the bottom of the slab. Ambient conditions can quickly change the surface without changing the moisture deeper in the slab.

A more accurate way to test moisture in concrete slabs was needed, and research showed that relative humidity (RH) testing is more reliable.

Relative Humidity (RH) Testing

RH testing as we know it today began in Sweden in the mid-1990s. Studies there concluded that the best way to determine a slab’s final moisture level under a floor covering was to place a sensor in the slab itself. That’s why this method is sometimes called in-situ RH testing. It’s covered by ASTM F2170.

There are two types of RH test probes. The first uses a cylindrical sleeve placed in a hole drilled in the slab, and a sensor is then inserted into the sleeve to take the reading. These probes are reusable, but their calibration must be checked within 30 days before each use, and they must be recalibrated at least once a year.

The second type, like the Rapid RH^® L6, combines the sensor and sleeve in one unit that goes into the slab, and readings are taken with a separate reader.

Doing the Math: 40% Concrete Moisture Testing

With either type of RH probe, research shows that readings taken at 40% of the slab depth match the RH the slab will reach once it has fully equilibrated under installed flooring or a sealant.

For a slab that dries from the top only, which covers most flooring installations, ASTM F2170 calls for placing the RH sensor at 40% of the slab depth. So for a 6-inch slab, the test hole should be drilled 2.4 inches deep.

But 40% isn’t the right depth for every installation.

Doing the Math: 20% Concrete Moisture Testing

Testing also showed that 40% doesn’t give accurate results for a slab that dries from both the top and the bottom. For those slabs, 20% of the slab depth is the right testing depth.

When moisture leaves the slab from both sides, it’s highest near the middle of the slab, rather than near the bottom as it is in a slab that only dries from the top.

So for a 6-inch slab drying from two sides, the test holes should be drilled 1.2 inches deep, or 20% of the slab depth.

To learn more about the science and history of RH testing, watch our free Rapid RH training videos.

Testing at the right depth for each slab helps you make informed decisions and avoid moisture-related flooring problems.

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Last updated on October 13th, 2026