Top 15 Concrete Moisture Testing Myths

Common misconceptions about concrete moisture testing can cause flooring professionals to make decisions that may increase the risk of moisture-related problems with their flooring installations. We’ve assembled the top 15 myths below with answers to help you avoid moisture-related flooring failures.

Before diving into the myths, let’s talk about how to test concrete for moisture.

How to Do a Moisture Test for Concrete Slabs

Testing concrete for moisture, ASTM F2170 stipulates the use of in situ probes to determine the moisture condition of a concrete slab at the time that it will be sealed or covered by flooring material. In situ probes provide relative humidity readings at 40% of the slab’s thickness when drying from one side or at 20% for slabs drying from two sides.

In situ measurement provides scientifically proven, reliable and accurate data to help you make the best flooring installation decisions possible. Using RH probes, you’ll need a minimum of 3 tests for the first 1,000 square feet and 1 additional test for each additional 1,000 square feet.

Concrete Moisture Testing Myths:

MYTH #1: If the concrete surface is dry, the slab is dry.

Not necessarily. Repeated scientific studies and demonstrations have proven that the surface of the slab cannot serve as an accurate indicator of overall moisture levels. Because there are so many variables that can impact the drying rate of a concrete slab (air movement, ambient temperature and relative humidity (RH), troweling techniques, and more), conditions at the surface do not accurately represent the presence of moisture within the slab. Any method that only tests the surface for moisture should be highly suspect.

In situ RH testing is the only way to accurately determine the moisture condition of a concrete slab. Placing a sensor inside the slab, below the surface, and at the correct depth, gives a clear picture of the final concrete moisture condition if the slab were to be sealed at that point in time.

Myth #2: All floor products have similar moisture tolerances.

At one time, flooring adhesives and flooring materials were more consistent in their moisture tolerances because most products had a petroleum-derived base in their compositions. As formulations began to change for different applications it became increasingly important that concrete slabs under the flooring installations were dry in order to prevent moisture-related problems down the road.

Can moisture come up through concreteThe industry has recently put a lot of focus on lowering VOCs (volatile organic compounds) in building products, different adhesives, and flooring products. As a result, the tolerances to prolonged and elevated moisture conditions can vary greatly from one adhesive product to the next.

Therefore, it is advised to always perform accurate RH testing in order to determine the current slab moisture condition, ensure that the specified product is compatible with the slab moisture levels, and choose the correct product to meet the current moisture conditions of the slab. For a quick link to a multitude of manufacturers that specify relative humidity (RH) tolerances for their products, visit

MYTH #3: The concrete is old so it has to be dry.

Even when a concrete slab has been in service for years, it’s still possible that moisture levels within the slab are high for many reasons including recent flooding, leaking pipes, and rising water table. This means that, unfortunately, it’s not unusual for an old flooring system to be removed, only to have the newly-installed flooring start to exhibit signs of moisture-related problems once it has been installed.

Another reality with older concrete and flooring systems is that the products used in decades past were inherently more moisture-resistant than many of the products on the market today. The move to lower environmental impact products (like products with lower VOCs) has resulted in the need for more awareness for moisture tolerances in flooring adhesives, sealants, and flooring products.

So, an older floor that has never shown signs of moisture problems may contain too much moisture for the RH sensitivities of the new flooring product.

An older concrete slab can also be facing moisture intrusion from an unidentified source (a compromised vapor barrier, an appliance or plumbing leak) that has raised the slab’s moisture but not yet reached failure levels for the older flooring.

The best insurance for older concrete slabs is accurate RH testing before installing any new flooring system. Fortunately, the unique design of the Rapid RH® L6 system seals a sensor in each test hole for immediate test results. It’s fast, accurate and affordable testing that saves you valuable time and keeps the ASTM-compliant testing process as simple as the touch of the Total Reader®.

MYTH #4: The slab is not on grade, so there’s no need to do moisture testing.

All concrete is affected by ambient RH and temperature conditions. Therefore, all concrete is susceptible to moisture-related complications and should be adequately tested before the next stages of flooring installation can begin.

There are two significant differences for concrete slabs not on grade:

  • Slabs on grade (or on pan decking) have only one surface that moisture can use to exit the slab. Slabs not on grade have two surfaces from which moisture can move out of the slab. This also means that concrete slabs not on grade have two surfaces that can also absorb moisture from a humid environment or external moisture source. The need to do accurate moisture testing has little to do with the format of the pour. The correct test method, however, has everything to do with the on-grade or not-on-grade status of the slab.
  • ASTM F2170 determines that, for accurately testing final moisture conditions within a slab, 40% is the correct test hole depth for slabs poured on grade or in pan decking (or with one surface from which the moisture will evacuate the slab). For slabs with two surfaces exposed to ambient conditions, the correct test hole depth is 20% of the slab’s thickness.

In situ RH testing for concrete is the only way to ensure against moisture-related problems over time. Only testing done at the appropriate depth will provide accurate and actionable results.

Free Download – 4 Reasons Why Your Concrete Is Taking Forever to Dry

MYTH #5: Surface testing concrete gives more accurate readings of the moisture level.

How to test moisture in concrete As was discussed above, any test method that reports on surface moisture conditions is at risk of producing a false reading. Several factors can skew the results of a surface test which has given cause for many authorities in concrete moisture testing to evaluate surface testing as unreliable.

There are two test methods that only provide results at a slab’s surface:

  • The calcium chloride (CaCl) test (or MVER) seals a desiccant material under an impermeable cover and then calculates the MVER rate by weighing the material after a set amount of time. The theory behind this test method is that the desiccant will absorb moisture emitting from the slab. That weight gain will express a ratio of the remaining moisture in the slab. Unfortunately, there is no scientific basis to the test and it has been specifically disallowed for some concrete types.
  • Concrete moisture meters operate by sending an electrical signal into the concrete and measuring its resistance as an indicator of the moisture level in the slab. While they can be a useful survey tool, they typically operate at about ¾” depth and can be susceptible to other elements within the slab such as rebar, certain types of aggregates and more. ASTM F2659 provides guidance to use concrete moisture meters as a means of comparative measurement only.

Ultimately, understanding the distribution of moisture in a drying slab makes it apparent that surface-based test methods are unreliable at best.

Moisture levels in a drying concrete slab tend to be higher at the bottom of the slab but will equilibrate (disperse evenly) through the slab once it has been sealed by a floor covering or other technique. Therefore, any test method that doesn’t measure below the surface into the slab cannot give an overall picture of what moisture levels will become once the flooring is installed.

MYTH #6: Drying time can be estimated accurately.

An industry rule-of-thumb for drying concrete is to allow one month of drying time for each inch of slab depth. However, an understanding of the process of moisture movement in a drying slab will show why any attempt to estimate a slab’s drying time can be a gamble. In order for concrete to dry, a large percentage of the excess moisture in the slab must be able to reach the surface and evaporate. Moisture within a concrete slab follows a number of small pathways, or capillaries, that form as it combines with the initial mix elements–cement, aggregate, and any other admixtures. If these pathways are blocked for any reason, or if they are not dispersed well through the slab, the amount of time it takes for the moisture to move out of the slab will be slowed significantly.

An overzealous troweling job, or sometimes even a curing compound applied to the surface, will prevent the moisture from escaping the slab, and that’s only the direct influence on the drying process. Ambient temperature and RH humidity conditions also have a very significant impact on concrete drying times, so any change in RH or air movement and temperature will ultimately affect the final drying schedule. Only ASTM F2170 in situ RH testing can monitor the actual moisture conditions inside a concrete slab for dependable results.

MYTH #7: All RH tests are the same.

Rapid RH L6 Insertion ToolAll RH test methods are the same in that they are based on research that demonstrates how internal concrete moisture measurement (measurement is taken below the slab surface) is proven to be more accurate when testing concrete moisture conditions. However, the way each RH testing product is designed can have significant impacts on testing time, accuracy and performance.

Most RH test methods use removable sensors that must be inserted into the test hole and then allowed time to acclimate to the hole for each and every reading. (Calibrations for each sensor must also be checked no more than 30 days before each use.) This type of removable sensor process can make RH testing a time-consuming and tedious process that often involves more waiting than testing. It also leads to “leapfrogging”: moving the sensors from one hole to the next, often with insufficient time allowed to let them equilibrate to each new test hole. Only the Rapid RH L6 features Smart Sensors that install directly into the slab for a one-time acclimation to the test hole and instantaneous readings taken with the Touch-n-Sense™ powered Total Reader. Calibration is always certified since each Smart Sensor comes with a NIST-traceable certificate of calibration.

MYTH #8: Test hole depth is approximate.


Click the image for a larger view

The target of in situ RH testing is to be able to determine the moisture condition of a concrete slab once the floor covering is applied. Why? Because, if the moisture condition is too high, it puts the entire floor system at risk. More than a billion dollars is spent on repairing moisture-related flooring failures each year.

It’s important to understand how moisture distributes in a drying concrete slab. Moisture levels inside the slab tend to be higher at the bottom of a slab that is unsealed but will equilibrate (disperse evenly) through the slab once it has been sealed by a floor covering or other technique. Until the slab is sealed, the depth of the test hole will definitely impact the accuracy of the RH reading.

That fact is why standards like ASTM F2170 are in place–to ensure that testing happens at the correct depth. For slabs drying from one side, like slabs poured on grade or pan decking, 40% has been proven to be the correct depth for RH in situ test holes. If a slab is drying from two sides, testing at 20% of the slab depth is necessary for accurate RH test results.

Only in situ RH test holes drilled to the correct depth will provide results that reflect true moisture conditions in the slab and allow the flooring installer to proceed with accurate information about the moisture conditions that will be in contact with the installed flooring once the job is complete.

MYTH #9: Concrete additives don’t impact drying time.

The irony of this concrete myth is that some concrete admixtures are MEANT to impact drying time. Superplasticizers, for example, like lignosulfonate or polymer compounds, are designed to reduce moisture in concrete slabs so (in theory, at least) drying times can be reduced.

Other additives cover a range of intended uses from reducing shrinkage or damp-proofing to extending workability to adding pigments for color applications. Each class of admixture chemically enhances or changes the properties of the concrete batch, and can subsequently impact the actual drying time of the slab. Different admixtures can also impact certain types of moisture test methods. Only RH testing provides the most accurate moisture testing results for any drying concrete slab.

MYTH #10: Dust on a job site doesn’t impact RH testing.

Dust and grit may seem inevitable on a job site involving concrete, but it really can impact the accuracy of RH testing if the test hole is filled with residue from the drilling process. By compromising the air volume around the sensor, excess dust or concrete residue limits the ability of the sensor to provide accurate readings. In order to assist in drilling the cleanest test hole possible, each Rapid RH L6 flooring kit comes with a wire brush to help dislodge any loose particles in the test hole that might remain after drilling. A vacuum attachment is also included to remove all loose particles all the way to the bottom of the hole.

Between readings, the provided green flush-mount protective cap not only helps identify each hole but also features a butyl rubber seal to prevent site contaminants from getting into the test hole as well. When planning to install any type of flooring or finishing product over a concrete floor, only RH testing like the Rapid RH L6 can provide accurate, affordable and industry-compliant concrete moisture testing. Don’t let myths about concrete moisture testing become costly mistakes for you.

Myth #11: You can accurately estimate how much time a concrete slab needs to dry.

A popular method for calculating how much time a concrete slab needs to dry before the flooring can be installed is to allow one month of drying time per inch of the slab. While this gives a rough indication of what to expect, it’s an approach that overlooks completely the many variables that can impact how much time a given concrete slab needs to dry.

We know that moisture has to move through the concrete to evaporate. How easily it can move through a slab has a significant impact on drying times. The moisture needs to flow through the capillaries (small pathways) in the concrete. The size and number of capillaries present depend on the unique mix of cement, aggregate and other admixtures used for that slab. The amount of water used in the mix also impacts how much moisture needs to evaporate.

These factors account for what’s happening inside the slab. However, outside factors directly impact drying time too. Aggressive troweling or applying certain curing compounds can trap moisture underneath the surface. The ambient conditions around the slab, such as temperature, humidity, and airflow, also affect the drying time.

Considering how all of these variables play into the drying time equation, why would anyone risk guessing, especially when there are cost-effective relative humidity (RH) tests available that accurately measure a slab’s moisture condition?

Myth #12: Old concrete is already dry.

This statement may seem to make sense. If a concrete slab has been around for years and has already had flooring installed over it, surely it must be dry enough for new flooring to be installed.

Unfortunately, this is wrong. Many older flooring systems used products that were more moisture-resistant but also had a greater environmental impact, than many of the products used today. This means that an old concrete slab may be holding on to moisture that couldn’t escape the older, more moisture-resistant flooring. That moisture could wreak havoc on the new flooring.

There could also be unidentified sources of external moisture (e.g., a plumbing leak or weak vapor barrier) that have been affecting the slab for years.

For these reasons, never assume an older slab is sufficiently dry for your flooring application.

Myth #13: The surface of the concrete slab is dry, so the flooring will hold.

Moisture Gradient SchematicScientific studies have shown that the dryness at the surface of a concrete slab won’t necessarily reflect the slab’s overall moisture condition. Ambient conditions discussed under myth #1 can cause the slab’s surface condition to be very different from what’s going on below, where it matters most.

In addition, as moisture moves through the slab and evaporates at the surface, a moisture gradient invariably forms, such that the moisture condition deeper within can be quite different than at the surface. Because of this moisture gradient, any concrete moisture test that only measures at the surface, such as the widely used calcium chloride test, can’t possibly give reliable results.

The moisture level that matters most is the slab’s point of equilibrium; that is, the moisture level that remains after the flooring installation and once the moisture gradient evens out. That’s the moisture condition that the installed flooring will “see” for the long haul. The only way to get an accurate, reliable assessment of that moisture condition is by taking measurements deeper within the slab, and RH tests are specifically designed to do just that.

Free Download – 4 Reasons Why Your Concrete Is Taking Forever to Dry

Myth #14: The exact depth of an RH testing hole doesn’t matter.

The depth of the RH testing hole matters a lot, which is why the ASTM F2170 standard for RH testing specifies the exact depth for conducting the test. Why is depth so important? We know that moisture levels within an unsealed slab can vary a lot as moisture moves up and out of the slab during the drying process. Taking measurements at various depths will yield significantly different results.

Rigorous scientific studies have determined the best depth for taking RH measurements, in order to get a true picture of the long-term moisture condition that will exist after the flooring is installed. This is the depth required by ASTM F2170. No guessing or estimating (see myth #1) allowed.

For slabs on grade and drying from one side, the precise RH testing depth is 40 percent of the slab’s thickness; for slabs that aren’t poured on grade and are drying on two sides, the exact depth required is 20 percent.

Myth #15: RH test hole depth is all that matters for an accurate reading.

The RH test provides the most reliable way of getting an accurate measurement of a concrete slab’s moisture condition, but it has to be done right. Placing the RH test probe at the correct depth is certainly critical (see myth #4); however, other issues can interfere with your test results if you don’t address them: dust and grit.

When you drill holes for placing the RH probes, you must clean them out properly. RH test kits, such as those from Wagner Meters, come with a wire brush and special vacuum attachment for cleaning out the hole before it’s used.

If you neglect this step, dust and grit can alter the air volume around the RH sensor, which in turn compromises the sensor’s ability to take an accurate RH reading. To ensure particulates don’t contaminate the hole between readings, make sure to plug the top of it with a protective cap.

We now know too much about how moisture moves through concrete and how to test accurately a slab’s moisture condition to remain slaves to myth. A properly administered RH in situ test, as set out in ASTM F2170, relies on the science instead. Myth or Science? Which do you want to rely on?

Bonus Question: Can Moisture Come Up Through Concrete?

Concrete is like a sponge absorbing moisture from under the concrete which causes the moisture to come up through the concrete capillaries. Typically, this will cause moisture-related flooring problems or cause mold and mildew growth.

Read our Tips for Concrete Moisture Testing in the Real World to learn more about concrete moisture testing.

Additional information on concrete moisture testing: Rapid RH Test

Last updated on August 31st, 2021