What to Do When You Find an Old Wet Concrete Slab

Some 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

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.

old wet concrete slab

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 – 7 Things You May Not Know about Concrete Slabs

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 April 4th, 2022

20 Comments

  1. Debbie Bailey says:

    HI Jason:

    About 2 years ago I had LVP flooring (w cork underlayment and no moisture barrier) installed in my 42-year old house, on concrete slab. about 6 months in, I noticed it started warping. I pulled up the floor and found areas of black mold, and random staining on the cork underlayment. The worst mold was over two areas of the slab where the plumbing supply lines run underneath. The leak detection company found no leaks and neither did the plumber. The calcium chloride tests came back w/ 11, 14 & 16 lbs with the worst being in the immediate vicinity of the underground water supply lines. An experienced installed said that with a moisture barrier (6 mil visqueen) the new LVP will be fine.
    Not sure what is causing the moisture and what my next step should be….would appreciate any suggestions you have.

    thanks!

    • Jason Spangler says:

      Debbie:

      Thanks for the questions, and sorry for the problem. The problem COULD be that there is no vapor retarder under the slab, so moisture from the soil is making its way through the slab. It seems you have taken steps to ensure there are no leaks. I would ensure your drainage on the home’s perimeter is sloped away, and the gutters are working correctly. Now, for the floor, the 6mil could very well work, but whatever LVP product you use, I would make sure to read the installation instructions to ensure the specific manufacturer approves this installation and whether they require complete moisture testing. Good luck.

  2. Andrea says:

    Hi Jason,

    First of all thank you for your time.

    I live in Miami, FL about a year ago we removed our tile floor and installed vinyl laminates over the concrete without any vapor barrier.
    The flooring installer insisted that such barrier was not need it because the vinyl laminates had a barrier already.
    Fast forward 15 months and we had water coming up through the seams of the floor. To make a long story short,
    we had Sleuth Leak Detection come out to the house and their testing showed that our concrete slab has 100% humidity making
    tile and carpet the best option for a replacement. The technician advised NOT to seal or waterproof the floor as this will prevent the
    concrete from breathing. He also explain that the floor will breathe through the grout.

    I would love to know what your opinion is on his advice.

    With much appreciation,

    • Jason Spangler says:

      Andrea:

      First thing, thanks for the email. Second, you can install any floor you want on that concrete, as long as you find a competent installer. Third, there are millions of sq ft of concrete “sealed or waterproofed” every year in even the most stringent environments. The first thing in this situation is to try and determine why the humidity is so high. Was this new construction that the concrete wasn’t allowed to try? Was the slab built without an intact vapor retarder below it? (you can have a geotechnical engineer come in and do a core sample of the concrete to determine this) Is there a plumbing and/or drainage issue causing a problem? Once you have gone through this, look for a viable product to use to mitigate the surface of the concrete. Not all are created equal. Stay with epoxies, they are proven. Look at companies like Uzin, Ardex, Mapei, Schonox and research their installation instructions so you have a basic understanding of what is necessary. Then, find an installer that is competent in using these products and let them do their job. It won’t be cheap and it will be a bit messy but done correctly, it should solve your issue.

  3. Rick Kercheval says:

    #comment 187904
    Paula – likely your home has been modified with any of the new efficiencies such as AC upgrade, insulation renovation, tighter window/door upgrades, etc. since the original build out. This improvement or reno will reduce the vapor pressure (relationship between temperature and humidity expressed in psi) in the home and will hold conditions desired much more successfully. A regulated temperature and humidity will usually provide a LOWER vapor pressure inside the building envelope than below the slab and surrounds. The porosity/permeability of the concrete when first poured 40 yrs ago is static(unchanging) based on water added and curing practice. However, any upgrades to the home that improves efficiency with temp and humidity will produce a new vapor pressure dynamic on the interior air in the home. Another way to say this is when you have a 73o F temp inside with a target of 40-45% humidity(summer) and resulting much lower humidity in winter from heating, you will have roughly a vapor pressure quotient of about 0.145 psi of pressure in the home compared with the humidity under the home that generally approaches 100% with temperatures between 65-70o F all year depending on how far north you are. That under the slab value in vapor pressure is about 0.33 psi. So, there is almost a 237% greater vapor pressure under the house compared tot he interior. Since the concrete substrate is acting as a barrier between the two different air masses with different vapor pressures, air ladened with moisture will begin to move toward the lower pressure because of leaking through the permeability of the concrete . Thus, moisture in the form of humidity is constantly moving to the interface line of the substrate and floor. Additionally, calcium hydroxide, potassium hydroxide, and other minerals that are fairly high base materials on pH scale are adding to the erosion of any adhesives holding down the floor. Albeit the pressure are barely noticeable even at peaks, it will be significant over time delivering moisture to the floor system if the floor cannot pass the air and moisture as fast as the salt is passing it. Thus, consistent problems will exist until the air volume is controlled to a point in which the floor will not be stressed to failure. My email is ottrick@me.com if you want more explanation. I fixed hospitals and residences for years.

  4. Phil Norman says:

    Hi Jason,

    I own a property in NW Indiana. The vinyl floor strips will not stay glued to the concrete. Three borings show that the moisture readings are at 100%. I am investigating to determine the source. What can be done remediate the concrete so that the glue will stick?

  5. Paula Young says:

    Hi Jason,
    I have a 40 year old house with a concrete slab foundation. We had previously had carpeting in the house. Last year we had the carpeting removed and had floating luxury vinyl planking installed. The installer did not test the slab’s moisture content. Now a year layer the vinyl is cupping, rising up where the planks meet. The installer came back and said he never had this happen before. He removed some of the planks and then tested the moisture in the slab. He used a gauge that test the moisture in wood and it registered 15.8. He said it should be between 4 – 7. I then had a leak detection company come and they did a pressure test of our plumbing. They did not find a leak. They also measured the moisture in the slab which showed 100%. I am now stumped and don’t know what to do or who to call to determine the problem and a solution. With Your experience and knowledge I was hoping you could provide some assistance in how I should proceed.
    Thank you so much.
    Paula

    • Jason Spangler says:

      Paula:

      Thanks for the question and sorry you are having problems. With very limited information, the first place I would suspect is under the concrete slab. I would guess that if there was a “plastic” vapor retarder under the slab originally, it’s not there after 40 years. This allows moisture to interact and penetrate the bottom of the slab and work its way to the surface. I am not familiar with the scale the flooring company is referencing, so I can’t comment about that. It would have been nice had the moisture testing been done prior to installation because it may have shown problems prior to the installation. I would contact a geotechnical engineer to do a core sample of the concrete to either prove or disprove if there is an intact vapor retarder. If there isn’t one, then whoever is installing the floor needs to have a solution to put a vapor retarder-like product on the surface of the concrete, prior to reinstalling the floor. 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.

  7. 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.

    Thanks

    • Jason Spangler says:

      Kate:

      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.

  8. 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:

      Javier:

      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.

  9. 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:

      Sarah:

      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.

  10. JOSE LIMA GONZALEZ says:

    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:

      Jose,
      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.

  11. 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:

      Sandra:

      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.

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