What to Do When You Find an Old Wet Concrete Slab

old wet concrete slabSome flooring contractors assume that an old slab is a dry slab and they don’t take the time to test for concrete moisture in remodeling projects. This assumption can be costly.

Excess moisture in the concrete slab can cause serious flooring problems, such as adhesive failure, discolored tile or grout, bubbles in sheet vinyl, and buckling of wood floors. An old slab under existing flooring can easily contain enough moisture to cause these issues for the new flooring.

It is easy to understand how the normal wear and tear of building use could cause a moisture problem. Leaky plumbing, appliances, or roofing can introduce water that is then absorbed by the concrete slab. Changes in property grading or failed drainage systems can allow outside water to drain toward the building and into the slab. Also, the under-slab vapor retarder can deteriorate over time and begin allowing groundwater into the slab. In fact, in older buildings, an under-slab vapor retarder may never have been installed in the first place.

But it surprises some contractors to learn that another possible source of excess moisture in an old concrete slab is the water that was used to pour and cure the slab during initial construction. You can’t assume that the slab was given adequate time to dry and in the right conditions before flooring was installed. You can’t even assume that the concrete slab was accurately tested for excess moisture prior to flooring installation.

Even if the current flooring isn’t showing signs of excess concrete moisture, that doesn’t mean the new flooring won’t fail. It might be only a matter of time before the excess moisture starts causing visible problems. Also, older flooring materials often had higher moisture tolerances. So, even if the slab was dry enough for the old flooring materials, it may not be dry enough for the flooring materials you are about to install.

To protect your customer and your reputation, never assume an old slab is a dry slab.

Check Early for Possible Moisture Problems

 vapor migrating through the sla

Efflorescence is caused by vapor migrating through the slab bringing soluble salts to the surface of the concrete. As it is a crystalline deposit of salts, very intricate patterns can develop over time.

Only accurate testing will tell you whether there is a concrete moisture issue. However, a quick visual inspection of the existing flooring may help you alert your customer to possible moisture problems earlier in the project.

Check for buckling or shrinkage in wood flooring, discoloration in tiles and grout, and bubbles in vinyl sheets. Look for adhesive that is bleeding at joints or loose floor coverings. If possible, do spot checks underneath the existing floor covering. Is the adhesive dry? Is there a good bond between the concrete and floor covering? Do you see or smell mold or mildew?

Depending on what you find during your visual inspection, the customer may need to call in other experts, such as building and health inspectors.

Test for Success

Before installing flooring, always test concrete moisture to ensure the relative humidity (RH) levels are within the manufacturer’s specified limits.

Know the Moisture Tolerances of the Flooring Products

Flooring manufacturers specify requirements for concrete moisture levels. This is true for all components of a flooring system, from subflooring up to the floor covering itself. Installing a product in conditions that exceed the specified limit can result in a flooring failure and a voided warranty. The only way to be sure that the slab is dry enough to install the flooring is through accurate testing.

Use an RH Probe Test Method that Complies with ASTM F2170

For many years, the moisture vapor emission rate (MVER) method was commonly used in the United States. This method, also called the “calcium chloride” or “CaCl” test, is often considered less reliable than scientifically-backed tests that industry experts choose.

Also, testing with a concrete moisture meter alone isn’t enough. Although a concrete surface meter may help you find problem areas, it’s the moisture level within the concrete that really matters. Before a concrete slab is covered, moisture is constantly traveling from deep in the concrete up to the surface, where the moisture evaporates. If nonpermeable flooring is installed (flooring that does not allow moisture to escape), the moisture from the concrete gets trapped between the slab surface and the flooring. If the moisture is excessive, the flooring can fail.

That is why many flooring manufacturers now recommend, or even require, concrete moisture testing that complies with the American Society for Testing and Materials (ASTM) F2170 standard. This testing method has been proven to accurately predict what moisture levels will be at the slab surface once a nonpermeable covering is installed.

The ASTM F2170 standard specifies the testing method for finding the RH in concrete floor slabs using in situ probes. To purchase a copy of the ASTM F2170 standard, go to the ASTM website. Some of the key points of the ASTM F2170 standard are summarized here.

Conditioning: For at least two days before testing begins and during the 24 hours testing period, the temperature and RH levels in the room need to be at the conditions that will exist once the room is in use (“service conditions”).

Test locations (probe installation sites): Perform three tests for the first 1000 feet and at least one additional test for each additional 1000 feet.

Probe installation: Install each probe at 40% of the slab’s thickness for a slab drying from one side and at 20% for a slab drying from two sides. Thoroughly clean each drilled hole to remove dust that could cause false readings.

Probe equilibration: Allow 24 hours for the probes to reach moisture equilibrium with the concrete.

Ambient conditions at time of test: Read and document the ambient RH and temperature at the time of concrete RH testing.

Reporting: Record the RH and temperature readings from each probe site and the ambient RH and temperature readings, along with details about the testing location.

Calibration: Calibrate probes at least annually and within 30 days before use.

The Rapid RH® L6 Makes ASTM F2170-Compliant Testing Fast and Easy

The Rapid RH® L6 concrete moisture testing system was specifically designed to make it easy to take fast and accurate readings that follow the ASTM F2170 standard.

For simplified probe installation: Rapid RH L6 Smart Sensor probes are 1.6” in length (40% of a 4” thick slab). Each Smart Sensor pack includes extensions for thicker slabs so that you can easily install probes at the correct depth. Rapid RH L6 testing kits include an insertion tool, vacuum attachment, and other accessories to further simplify installation. For details, see Rapid RH L6 and 5.0 Smart Sensor Installation.

Datamaster App

For fast and easy ASTM F2170 compliance, use the Rapid RH L6 concrete moisture testing system.

For fast and accurate readings: Taking a concrete moisture reading is easy with the Rapid RH Total Reader®. You simply insert it in the Smart Sensor, wait for the first reading to appear (about three seconds), and then remove it. The display toggles between the RH and temperature values. Once the Total Reader is removed from the Smart Sensor, the readings from that Smart Sensor will continue to display for about 5 minutes or until the Total Reader is inserted into another Smart Sensor.

Within an hour after installation, the Rapid RH generally gives readings within 3-5% RH of the readings you will see after your official readings at the ASTM-required 24 hours. This lets you start making business decisions sooner, and having any conversations you need to have without holding up the schedule. Because Rapid RH L6 Smart Sensors stay in place, you also do not lose time waiting for the probes to equilibrate between readings.

For monitoring of ambient room conditions: The Smart Logger™ tracks ambient temperature and humidity, even when you’re away from the job site. The Smart Logger monitor works in tandem with the Smart Logger mobile app.

For easy reporting: Forms provided with the Rapid RH L6 make it easy to record the information required by ASTM F2170. Or, you can download the DataMaster™ L6 app to your mobile device to greatly simplify ASTM F2170 reporting.

Calibration is never required: Rapid RH L6 Smart Sensors are calibrated at the factory and ready to use right out of the box. Also, Wagner Meters provides a NIST certificate for each sensor, which meets the ASTM F2170 requirements for calibration documentation. Because Rapid RH L6 probes are used only once, recalibration is not required.

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

When You Discover an Old Wet Slab …

If the Moisture Source Has Been Removed

If the moisture problem was caused by a water source that can be removed, such as leaky plumbing or faulty drainage, then flooring installation should not begin until the problem has been fixed.

Once the moisture source has been removed, then the situation is a lot like a new construction project. The concrete needs enough time to dry and in the right conditions. Be sure doors, windows, and the heating and cooling systems are working. If repairs required new paint and plaster, allow them to thoroughly dry. Ensure the room is at the temperature and humidity it will be once the room is in use. Once drying conditions are right, give the concrete slab enough time to dry.

Before installing the flooring, repeat the RH probe testing to verify the slab is dry enough for the flooring materials you are about to install.

If Moisture Control Is Required

If the concrete moisture source can’t be removed, as in a vapor retarder that is no longer intact or was never installed, then you will likely need to find a moisture control solution.

Use only products that are compatible with the finished product you are installing, per the manufacturer. When in doubt, contact product technical representatives for guidance. Provide them with the concrete moisture testing results so that they can give you project-specific recommendations.

Moisture control systems generally fall into three categories.

Moisture-Tolerant Adhesives

Some adhesives are more moisture-tolerant than others. When this is an option, it is generally the least expensive solution. Make sure to discuss your specific application with the appropriate technical representatives, especially in a slab that doesn’t have an intact vapor retarder.

Moisture Membranes

A moisture membrane is a physical layer that is installed between the concrete slab and the floor covering. Membranes are designed to lessen the level of moisture that reaches the floor covering. These membranes come in rolls or sheets. They may be referred to as “vapor barriers” or “vapor retarders.”

Because wood flooring is particularly susceptible to absorbing moisture from its surroundings, the National Wood Flooring Association (NWFA) recommends always installing a membrane between the concrete slab and wood flooring. For specific recommendations, see Subfloor Focus: Minimizing Moisture – Part 2: Use of Vapor Retarders over Concrete Subfloors.

Moisture Control Coatings

Moisture control coatings are designed to minimize the amount of moisture that is in the slab from interacting with the flooring system on the slab. A broad range of products fall into this category.

The installation process can be as simple as a single roll-on coating or as complex as concrete resurfacing followed by application of two-part mixes. Some even require additional installation steps, such as the application of primers and sand broadcast layers. Installation and replacement costs for these systems can be high, which makes close collaboration with the coating manufacturer especially important.

Datamaster app Rapid RH L6

Don’t Let an Old Wet Concrete Slab Cause Your Flooring Project to Fail

Never assume an old slab is a dry slab. No matter how old the slab, some of the water used to pour and cure the concrete could still be present in the slab. No matter how good the existing flooring looks, the concrete slab moisture could be too high for today’s newer, moisture-sensitive products.

ALWAYS test concrete moisture before installing flooring. For accurate testing, use an RH probe test method that follows the ASTM F2170 standard. For fast and easy ASTM F2170 compliance, use the Rapid RH L6 concrete moisture testing system.

If you find a wet slab under the existing flooring, do not start installation until the problem is resolved. When using moisture control systems of any type, protect your customer and your reputation by following the manufacturer’s guidelines.

Last updated on June 1st, 2021


  1. Sandra says:

    Hi Jason,
    My mom lives in a patio that was built on a slab in 2000. She had vinyl tiles laid 3 years ago. At the time of installation they told here there was on the floor and laid it anyways. A few weeks later she notice bubbles. They kept getting worse. Then a little over a year ago glue starting oozing out of tile seams. We pulled up a tile today and it was wet under the tile.
    She called a water basement controller he came out and told her she would have to tear out the floor dig a trench under concrete slab and put in a sump pump to elevate the wet slab issue. She has no water in her floor ducting, pipes have been tested for leaks. None of the other owners around her have water issues that they know of. Since you have experience with this, I thought maybe you could givev us some suggestions. She really does not want to have her floor dig up to the tune of $7000.00. Please contact me with any help .My 90 year old mother is fed up and doesn’t know who to believe.

    • Jason Spangler says:


      Sorry your mother is having these troubles. Before I did anything, I would look to a geotechnical testing company like http://www.terracon.com to test to see if there is an intact vapor retarder under the slab (guessing there isn’t) and then with those results, get with a reputable flooring installer to determine options for the type of flooring and installation technique that would be best. Good luck.


    Hello Jason
    I am a mold contractor in Mexico, I have an issue with a home that has cork under the granite floor, in the 4th floor of a building. The house was flooded due to a faulty pipe, looots of water run for hours over the floor. After many months, now the house smells like wet cork. The option now is to remove all the floor and replace the cork insulating barrier. this will be very expensive. Do you have any recommendations to avoid replacing 100s of square foot of floor. Thank you for your time, regards.

    • Jason Spangler says:

      Unfortunately, if there is trapped moisture below the finished flooring as resting on the cork there is no real way to dry out the cork without removing the floor covering.

  3. Sarah Paulk says:

    We are dealing with a home that we purchased and renovated in 2019. The home itself was built in the 80’s and is built into the side of a hill. The flooring we removed was an engineered wood that showed no issues with moisture. The contractors trenched parts of the existing slab to install new plumbing and poured new concrete to cover. We then installed true hardwoods that started cupping and buckling within just a few weeks of installation. The contractor said the glue used to attach the wood was rated for 99% moisture. In just over a year, we have pulled and replaced areas of damaged wood four times, each time with the contractor not addressing the real issues. The areas we have replaced were not situated over the new poured slab, but instead along the exterior walls of the ground level. We have spent thousands running french drains outside of the home to direct any water away, as well as sealing the patio along the exterior walls. Why would the old slab suddenly have so many problems? The moisture readings were 99%, 96%, 83% and 75%.

    In the long run, it seems the solution will be to pull up all flooring and start over, with a correct barrier in place. What product would you recommend be installed and what type of flooring would you use?

    • Jason Spangler says:


      Thanks for the question. Since you provided the RH% information, which gives me more of a base to start from, you allow me to ask questions which are best-answered voice to voice. When you have time, call me at 800-634-9961 X235. Thanks.

  4. Javier Rodriguez says:

    Hello Jason,
    I am working on two buildings where the roofing system is really bad and is going to be replaced. There is a lightweight concrete slab over the structural roof slab. We have tested the slabs for moisture content by two different testing companies and the results from both is about a 50% moisture content mainly on the topping slab throughout but there is also high moisture content in many areas of the structural slab. We have found a roofing company that will install and warrant the roofing system however, I am afraid that if the new roofing membrane is installed over these “wet” slabs, the moisture in the topping slab is going to be trapped as it has no way to evaporate, and eventually cause damages to the structural slab by corroding the steel reinforcement and subsequent concrete spalling. Is there a feasible way to remove the moisture from the topping slab before installing the new roofing system? the alternative would be to remove and replace the topping slab which would cost $1.5 million.

    • Jason Spangler says:


      Thanks for the questions. I guess my first question would be, how did they get the moisture content %, oven drying a core sample? Second, for this application and roofing material you are going to use, is 50% MC too high? What are the manufacturer recommendations for substrate moisture. I can tell you that on interior slabs, structural or otherwise, the flooring industry used moisture encapsulators on the slab surfaces all the time without any issues. Thirdly, depending on the ambient conditions on the bottom side of this structural slab ie warm consistent temperature and humidity, the moisture could escape there. Hope this helps. Good luck.

  5. Kate Munson says:

    Hi there. My mom’s house, built in the ‘80’s slab on grade, likely without a vapor barrier or if it exists, it’s likely compromised.

    We haven’t noticed any water damage- current flooring is vinyl and older marmoleum.

    We need to redo our bathroom and the tile floors are always freezing, but we want to make sure we don’t have any moisture issues before replacing the floor. We haven’t had testing done yet but are planning on it.

    We are concerned about mold. If we put a vapor barrier down- and we have a moisture issue, mold will grow under that barrier.

    Any recommendations for either sealing or a new coat that isn’t to toxic- super sensitive to chemicals.


    • Jason Spangler says:


      Thanks for the question. The first thing to determine is how big of a problem, if any, you have. At this point, there may be multiple options. You have more moisture-resistant adhesives that may work. You have products such as https://gcpat.com/en/solutions/flooring-solutions-every-type-building. There are also epoxy products that can be applied to the surface of the concrete to minimize the moisture movement. You can look at Uzin, Mapei, Ardex, and Koster to name a few. Good luck.

  6. Bob and Kathy wainright says:

    Our home was built in1976. It is one a hill in San Diego. The lot is surrounded on three sides by a canyon. We installed a hardwood floor which failed after three years, due to moisture damage. The floor was removed, a moisture barrier was put down, and a new wood floor installed. Now after three plus years this floor is having the same problems. Leak detection both times found no leaks. The lot is flat. We are at the top of the canyon. We have no plants near the exterior walls and we have had artificial turf for ten years. When tested, the moisture levels in many areas were as high as 32. Is there a way to solve this problem?
    Before the second floor was installed, an epoxy moisture barrier was applied. According to the installer, he used the best on the market.

    • Jason Spangler says:

      B and K:

      Thanks for the questions and I am sorry for the problems. First, a “32” on concrete doesn’t really make any sense. Neither of the two acceptable methods for testing moisture in concrete (in the flooring industry) has numbers that would correspond to this. So, in order to have an opinion, I would really need to know what the moisture test results were prior to installation. Either way, I would recommend you visit http://www.nwfa.org and find a certified inspector in your area to evaluate the issue and see if they can determine a potential cause. Good luck.

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