How to Evaluate and Interpret Concrete Moisture Readings to Avoid Flooring Failures

Moisture that hangs out in concrete can be impishly sneaky.

It doesn’t want to be caught, so it hides in annoying ways. It can simultaneously escape from the surface of a slab while also huddling in pockets below the surface—refusing to cooperate with flooring installers just trying to do their job properly.

And then, having no respect for the beautiful flooring installed over the slab it refused to leave earlier…it decides that it simply must go. Straight upward. No matter what stands in its way. Even if it has to be a damage-inducing squatter in the floorboards for a while before joining its free-flying evaporated brethren in the atmosphere.

So how are we to accurately detect these devious molecules of mischief when they won’t play nice to begin with? How can we truly know when a concrete slab is ready for flooring?

We have to meet that water vapor where it’s at—with moisture testing methods designed to measure at the depth of those hiding spots.

Only then do we get the accurate readings we need to decide how and when to move forward.

That’s why we’ll take a detailed look at:

But seriously folks, let’s start with some clarification about what’s really going on in that slab.

The Science Behind Moisture’s Behavior in Concrete

Concrete forms a moisture gradient as it dries, so while it may seem dry at the surface, there may be a significant amount of remaining moisture deeper within the slab.

Moisture is necessary at the beginning of concrete’s “life cycle” for mixing, pouring, and shaping. But after these steps are done and the concrete is cured, we want the excess moisture to dry out so we can get on with the job. And it does so by evaporating from the surface.

Sounds simple enough. But the tricky part is, concrete forms a moisture gradient as it dries. Moisture closer to the surface evaporates more quickly than the moisture at the bottom of a slab (or in the middle of a suspended slab), which naturally takes more time to move up and out.

What This Means For Flooring Installers

If there’s still excess moisture hiding in the slab after a floor covering is installed, the moisture gradient still continues the equalization process—the moisture from the wetter parts of the slab moves to the drier parts.

But even though it can no longer freely evaporate off the surface, nothing can stop this upward movement of excess moisture. So as more and more moisture settles at the surface of the slab, it will either pass into the flooring or collect under it (depending on the type of floor covering installed on top).

You can probably see where this is heading. Excess moisture in the flooring material leads to damage that can include:

  • Adhesive breakdown
  • Warping
  • Buckling
  • Cracking
  • Cupping
  • Crowning
  • Discoloration
  • Mold/mildew growth

And even if the flooring material itself is moisture-resistant, moisture can still gather under it, leading to:

  • Delamination
  • Mold/mildew growth
  • Adhesive breakdown
  • Peeling or bubbling
  • Discoloration

Since no installer, or any of their clients, want to deal with a moisture-related flooring failure, proper moisture testing is key. So when installing a floor over concrete, you need a testing method that can detect moisture levels far below the slab’s surface.

The Most Accurate Concrete Moisture Test: Relative Humidity In-situ Probes

Relative humidity (RH) in-situ probes measure moisture at 40% depth, giving a more accurate picture of how much excess moisture remains within and will eventually equilibrate throughout.

Relative humidity (RH) testing measures moisture levels deep in the slab, which lets you more accurately determine what the moisture level will be at the surface once the moisture in the concrete slab has equalized.

This is far more helpful than surface-level methods like calcium chloride testing, which requires significant prep work and takes 72 hours to obtain a reading. And that reading can only tell you how much moisture has been released through the surface of the concrete during the testing period. It does not indicate how much moisture is still deeper in the slab.

RH tests involve sensor probes that are inserted into the slab at a 40% depth and left to equilibrate for 24 hours. Then you’ll come back to a reliable reading of the amount of moisture left in the concrete.

Aside from its testing depth, another contributing factor to RH testing’s high accuracy is that each in-situ probe measures below the ambient conditions. The sensors are not affected by temperature or relative humidity above the slab, like calcium chloride tests are.

RH testing is also far easier to set up and gives you a more detailed result. Take the Wagner Meters’ Rapid RH® L6 test, for example. After inserting the sensors, you can easily extract the results via Bluetooth and view them in the corresponding mobile app.

And if scheduling is a concern, the Rapid RH® L6’s readings can get very close to the final number within a few hours. This can allow you to make some preliminary decisions about remaining dry time and when the installation can begin.

Now let’s break down the RH testing process into steps so you can be clear about what’s involved and how to prepare. We’ll use the Rapid RH L6 test for a working example.

The ABCs of RH Testing With the Rapid RH L6

Getting accurate readings from your RH tests depends on proper preparation and execution in compliance with the ASTM F2170 standard..

Setting Up Your Test

  1. Establish service conditions. A full 48 hours before testing, make sure that the indoor air reaches the temperature that is going to be used in the building.
  2. Determine where to place the in-situ probes. According to ASTM F2170, place three probes for the first 1,000 feet, and one more for every 1,000 feet after. (Optional tip: To help determine the best places to insert the probes, find moisture “hotspots” with a moisture meter like Wagner Meters’ C555.)
  3. Drill insertion holes at the locations you just determined. Use a rotary hammer drill and a ¾-inch drill bit (provided in the kit) and drill to a depth equal to 40% of the slab’s thickness. (Note that this is for standard concrete drying from one side. If the concrete is drying from both sides, drill the hole 20% deep.)
  4. Clean each insertion site with a wire brush (included in the kit) to scrape off any debris or loose bits of concrete.
  5. Vacuum out each hole after cleaning, using the attachment included in the kit. (Any dust or debris left behind could affect the accuracy of the reading.)
  6. Verify the diameter and uniformity of each hole with the insertion tool alone before inserting the probe.
  7. Verify the Smart Sensor by inserting the Total Reader into the sensor and taking a reading. It should display temperature and RH numbers for the air surrounding it.
  8. Place the Smart Sensors into each hole with the insertion tool, pressing firmly until the sensor fits snugly at the bottom.
    NOTE: If the slab is thicker than four inches, you’ll need one extension sleeve for every extra inch of concrete. To apply the sleeves, remove the protective cover on the sensor cap to expose the butyl rubber seal. Add the extension sleeves on the end.
  9. Close the testing holes. Put the sensor cap on the top of the hole, pressing firmly to ensure a proper seal.

These steps don’t take more than a few minutes to complete. Then, all you need to do is wait 24 hours for the sensor to equilibrate before taking readings.

Reading Your Results

To take readings after the 24 hours have passed, remove the latex cover from the center of the cap and insert the Total Reader (or the DataGrabber, if you have that instead). Here are directions for both:

  • Total Reader: The display screen will switch between the RH% reading and the temperature reading. You can either record the readings on the paper provided in the kit or send them to the DataMaster app via Bluetooth.
  • DataGrabber: Once the DataGrabber is inserted, open the DataMaster L6 app on your device and go to the “Devices” screen. Here, you’ll find a list of all the Rapid RH sensors within range. The sensor with the stronger signal will be at the top. To access data, just select the sensor you want.

If you need to take more readings at a future time, you can do so immediately—without waiting another 24 hours. Once the sensor has been equilibrated, it doesn’t need to do so again.

Now that you have your readings, let’s look at what the numbers mean and how they help you determine your next steps.

What Your RH Results Mean

Your measured RH levels (displayed as a percentage) will indicate the relative humidity levels in the slab so you can determine if it’s ready for flooring to be installed over it, or if more time is needed for drying.

According to the ASTM F2170 standard, the relative humidity levels should be 75% or below to safely install flooring. However, you should default to the RH determined by the manufacturer of the floor covering. So if your readings are at 83% and the flooring manufacturer requires 80% or less, you can go with 80 percent as your moisture target.

Every floor is different, and following ASTM standards in cooperation with the manufacturer’s specification is always the best choice to save time, money, and your good reputation.

Using a Concrete Moisture Meter

For some smaller jobs, a concrete moisture meter is the only practical way to measure the moisture in the slab, even though they can only detect moisture in the top layer of the concrete slab. But using moisture meters for comparative readings can reveal areas of higher moisture, or “hot spots.”

Wagner Meter’s C555 has been tested to be the most accurate meter for the job. Its readings return a number that ranges from 0.0% to 6.9%.

(If you’re using a concrete meter alone, be sure to check with the floor covering manufacturer to confirm that they will honor their warranty if a meter is the sole moisture testing method used.)

Meter readings are best utilized in helping you determine if further moisture testing with RH probes is needed.

If you’re looking at a reading you think is too low or uncharacteristically high, external factors may have affected the test.

The Effects of External Factors

There’s a good reason that the first preparation step before RH testing involves regulating the ambient conditions around the slab. ASTM F2170 states that the test needs to be performed when the room has reached occupancy conditions (doors and windows closed, HVAC running, etc).

Otherwise, you could risk a flooring failure when the concrete shifts in response to a change in ambient conditions.

Now that you have test results you’re confident in, how should you use this data to determine your next steps?

What’s Next? Making Data-driven Decisions

If your test results are well within the readings specified in the ASTM standards and/or the flooring manufacturer’s requirements, congratulations! You can safely install a floor covering now.

But let’s go over what to do if your moisture readings still aren’t what they need to be.

If you’d like to monitor the concrete until it reaches the proper moisture content, you can use True Remote Monitoring™, which will save you time and travel expenses to all the job sites to obtain readings. True Remote Monitoring uses Floorcloud technology that allows you to access these readings anywhere you are, as long as you have an internet connection.

Solutions for High Moisture Readings

If the concrete moisture readings are only slightly higher than the specified safe range, the slab may just need a little more drying time. Waiting a few more days can be inconvenient, but it’s far better than being called back after completing an installation because of a flooring failure.

If the reading is significantly high, you’ll want to investigate what could be prolonging the drying process.

Start by double-checking the following:

  • How long ago the slab was poured
  • Whether the proper vapor retarders were installed before pouring the slab
  • If service conditions have been established and maintained before and during the testing period

If the slab was poured without a vapor retarder, the best way to move forward is to seal the concrete to keep the moisture from interacting with the flooring. You can do this by coating the surface with a high-quality two-part epoxy. (This can also be an option if you’re not able to allow extra time for the concrete to finish drying.)

Just Don’t Skip the Test!

RH testing is a simple, quick method for ensuring the concrete is at an appropriate moisture level for flooring, and that those sneaky moisture gremlins won’t show up and mess with the flooring after it’s installed.

But when a project is behind schedule, pressure might come from multiple directions to rush the moisture testing, or even skip it altogether. Remember that this pressure isn’t likely coming from the one who gets called back to deal with a flooring failure!

As a flooring installer interested in protecting both the integrity of your flooring work and the integrity of your business, you will get the most useful results using RH testing to determine if the concrete is indeed ready for the floor covering.

It helps to remind everyone you work with that concrete can look and feel dry on the surface even when there’s a significant amount of moisture underneath. But proper RH testing procedure is the best way to safeguard a freshly-installed floor from moisture-related damage.

Protect the quality of your work with the best RH testing kits around. Check out Wagner Meters’ Rapid RH test kit and the C555 concrete moisture meter to get started, and you can always contact our team with your moisture testing questions!

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