Managing Moisture Between the Flooring and the Concrete Slab

Laying Ceramic Tiles

Water is an inherent part of the hydration process of concrete. However, allowing excess moisture to leave the slab after it’s poured is crucial to a successful flooring installation.

Once the slab is poured, the excess moisture must leave the slab in order to strengthen the concrete bond. The slab must also dry to a specified level of moisture before flooring materials can be installed on top of it. Moisture-related damage to the flooring materials is possible.

3 Common Flooring Materials Run The Risk of Moisture-related Problems:

  1. Adhesives
    Flooring AdhesiveMoisture-related adhesive failures are a problematic reality in the flooring business. Recent trends towards restricting volatile organic compounds (VOCs) in flooring adhesives have increased the number of moisture-sensitive adhesives used. If the adhesive used to install the flooring does not have the correct moisture tolerance for the concrete subfloor, the entire installation can be at risk.
  2. Floating Floors
    Floating floor systems are attractive because they don’t need to be attached directly to the subfloor. Instead, the floor pieces “lock” together to become a cohesive unit that’s not as vulnerable to seasonal shifts, dimensional challenges, or other moisture-related issues. In fact, floating floors are often recommended on projects where moisture risks are high with standard attached floor systems. For floating floors, manufacturers often recommend installing a moisture barrier between the subfloor and the floating floor to prevent moisture intrusion. The difficulty, of course, is that if the moisture barrier is compromised in any way, moisture from the slab beneath can still damage the flooring or finish.
  3. Grout or Cementitious Bonds
    Excess moisture issues in a grouted tile or mosaic floor often appear as efflorescence, a whitish residue on the surface of the grout. It’s the result of water-soluble minerals getting transported to the surface of the grout with the moisture as it evaporates away. Since minerals don’t evaporate, they’re left behind on the floor surface as a visible residue. The more porous the concrete or grout, the more likely efflorescence will appear. In the majority of cases, these minerals are actually part of the concrete slab mixture. Although they can possibly be in the ground beneath the slab, and seep into the concrete if no moisture barrier was installed. If the slab was not dried to the required specs before the tile was installed, the natural moisture migration of the drying concrete will impact the grout. Remediation steps will be necessary to correct the problem. In extreme cases, excess moisture can lead to flaking or chipping of the grout which results in a complete grout or thin-set failure.

Are you seeing a theme yet? The real risk to a successful floor lies with the moisture that can accumulate within the layer between the concrete slab and the flooring itself.

Adhesives, floating floors, and grout or cementitious bonds are 3 common flooring materials that can run the risk of causing moisture-related problems. Moisture control is often one of the most crucial, yet most overlooked, elements of any floor’s success over time. Responsible moisture control (having accurate moisture measurements) starts with the concrete slab.

Make sure to use the most accurate concrete rh test from Wagner Meters.

Shop Rapid RH L6

Moisture in a Concrete Subfloor

For moisture to accumulate between the concrete slab and the flooring, it needs to find its way to that middle layer. In this section, we breakdown the main ways water can get into your concrete, which causes a buildup of excess moisture, and list effective methods to prevent moisture problems from occurring.

Sources of Moisture in Concrete

The primary source of moisture in a concrete slab is the proportion of water mixed with the cement. No water source has a greater impact on the time it will take concrete to dry.

Yet you have other water sources to worry about. A variety of potential external water sources at a worksite can affect slab drying and curing.

  • Rain, snow, and sprinkler systems are culprits on a work site open to the elements. These water sources increase in danger if the grade of the grounds around the slab slope towards it. Not only is the concrete absorbing the water from above, but it’s also taking on the runoff from the areas around it.
  • The concrete slab can also absorb the groundwater below and around it. Thus, the amount of natural groundwater has a huge impact on concrete moisture conditions.
  • Unnatural water sources can also leak water. Any poor plumbing installation at the worksite creates a high risk of excess moisture. Old plumbing that’s degraded and has leaks presents the same risk.
  • Ambient conditions can also increase the concrete slab’s water content. Condensation develops on a slab that has a lower temperature and moisture level than the air’s dew point. The dew point is the temperature at which the air can hold no more moisture. You know, when dew (or condensation) starts to form. The slab will absorb some of the condensation.

    The slab will also absorb moisture from its environment when its relative humidity (RH) is below the air’s RH. Moisture wants to level out. If the air holds more moisture than the slab, as evidenced by its RH, that moisture will move to the concrete.

Rain, snow, groundwater, leaks, increasing water content, and sprinkler systems are all potential sources of free water. That is, water the concrete doesn’t need to cure. Any moisture the slab doesn’t need is moisture that can undermine your flooring installation.

What Are Causes of Excess Moisture in a Concrete Slab

Inadequate drainage around the slab exacerbates the risk of any source of moisture. In fact, the existing water source itself may not be the problem. A little rainfall or a bit of groundwater can drain away with well-designed drainage. Even minimal sources of water can pool on the concrete without adequate water lines and drains.

Excessive water can also intrude in a structure due to poor subfloor protection. Groundwater will move into the concrete if no vapor retarder sits between the ground and the subfloor.

The more likely cause for poor subfloor protection is using the wrong vapor retarder. Certain ASTM standards allow a vapor retarder to have a perm rating of 0.3 perms, which could allow up to “approximately 18 gallons of water per week in a 50,000 square foot area.” A vapor retarder with too low a perm rating won’t do the job it needs to do.

In other cases, the vapor retarder may have been sitting on the ground. It’s helpful to have a separation barrier between the ground and vapor retarder. Contractors should install the vapor retarder over granular fill to create extra separation from the groundwater.

A torn vapor retarder is another potential hazard to subfloor protection. Torn vapor retarders can occur in a reckless worksite. Hurried construction schedules create all sorts of moisture (among other) threats.

Fast-paced project plans often mean that concrete slabs don’t get the time they need to dry. For example, slabs may get power troweled to speed up preparations for flooring. The compression caused by the troweling closes off the evaporation outlets in the slab. The result is that over troweling will extend the drying time. If the schedule doesn’t allow for that time, then adhesives or surface membranes get installed on concrete with too much moisture. A moisture-related flooring failure is practically guaranteed under such circumstances.

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

How to Avoid Excess Moisture

The best of intentions to avoid excess moisture doesn’t matter if you don’t have accurate concrete moisture testing. There are two main ways to suffer inaccurate moisture testing. The first is to choose the wrong concrete moisture test. Only the in situ RH test measures moisture below the slab’s surface. Any test measuring only surface moisture is necessarily providing inaccurate results.

The other way to get inaccurate moisture test results is to perform the in situ RH test wrong. If you don’t place enough sensors throughout the floor, you won’t get an accurate picture of the space.

ASTM F2170 requires three sensors for the first 1000 square feet and another sensor for each additional 1000 square feet. Other testing mistakes include not inserting the sensor to the proper depth.

Serious errors can even occur from simply miswriting readings on your chart. The Rapid RH® L6 sensors contain integrated data storage that automates results reporting. When the meeting takes place to decide when to install flooring, no one has to rely on paper notes.

Make sure to use the most accurate concrete rh test from Wagner Meters.

Shop Rapid RH L6

How to Prevent Moisture Problems on Concrete Floor

Moisture is an inevitable part of concrete construction. Moisture-related problems are not.

  • Keep a low water to cement ratio. The more water in the mix, the greater the chance the slab won’t get all the time it needs to dry. Try to avoid adding water to concrete that’s already mixed. That water is a new variable that makes managing timelines and moisture issues difficult.
  • Take all necessary actions if pouring concrete below grade or at a wet building site. Whether that means installing more draining lines, using pumps to dewater the site, or any other method – do it. Make sure that your water displacement methods aren’t generating run-off in the wrong directions.
  • The layers above and below the concrete slab to prevent water seepage into the slab. Start with a vapor retarder with a perm rating that reflects the needs of the space. Install it over a layer of fill. Inspect it before pouring the concrete and fix any tears it may have. If necessary, use a proper underlayment between the concrete slab and the flooring. This is especially true when using wood flooring. Installing a plywood underlayment can add extra protection, but must also be tested to ensure it’s not bringing in new moisture.
  • Give the concrete slab the time it needs to dry and cure. Review the project plan and schedule. Is enough time set aside for concrete floors to dry? No reason to start out behind the eight ball. Exert as much control as possible over the ambient conditions to accelerate the timeline. Protect the space from outside elements. If the season isn’t ideal, can you take steps to reduce swings in air temperature? Will a dehumidifier help the air absorb more moisture from the concrete? Use fans to increase airflow, which speeds up drying time.

All these methods point towards a single goal: not installing any flooring too soon. Preparatory materials like adhesives or plywood will seal up the concrete. Sealed concrete will stop releasing moisture. At that point, the slab has the moisture it will hold for the long term. If there’s excess moisture trapped in the concrete, it will eventually reveal itself in ugly and possibly dangerous ways.

Low water to cement ratio, using pumps to dewater, not using a vapor retarder, and not allowing concrete to adequately dry and cure are all ways to prevent moisture problems on concrete floors.

How to Tell If There Is Excessive Moisture in My Floor

A floor may already be showing some external signs of containing excess moisture. A floor with a white or greyish powdery stain (also called “efflorescence”) likely has excess moisture. Due to moisture moving up through the slab and then evaporating from the surface. The whitish stain is the salt left behind by evaporating water. Or you may see that the flooring installed over the concrete slab is blistering or peeling away. If a wood flooring has been installed on top of the slab, the wood may be cracking or warping. These types of flooring failures occur due to excess moisture trapped between the flooring and the concrete.

No one wants to wait until the ugly signs of excessive moisture make themselves visible. You want to know if your floor is holding too much moisture well before that.

The calcium chloride test is an older method for measuring the moisture level of concrete flooring. It’s also called the moisture vapor emission rate (MVER) test. It’s standardized as the ASTM F1869 (Standard Test Method for Measuring Moisture Vapor Emission Rate of Concrete Subfloor Using Anhydrous Calcium Chloride).

The MVER test uses the weight differential over a 72-hour period of calcium chloride salt placed on the slab’s surface. The calcium chloride, sitting under a sealed dish, absorbs the moisture evaporating from the slab. You calculate the rate of evaporation based on the weight differential.

Unfortunately, ambient conditions often corrupt MVER test results. F1869 doesn’t even allow its use on lightweight concrete. Of greater concern is what the MVER test measures. It measures moisture only at the surface of the concrete slab. It’s not testing the moisture condition that matters over the long haul. You need to know the moisture condition within the concrete.

Only the in situ RH tests for moisture below the slab’s surface. Sensors inserted into the slab measure the RH within the concrete.

And these aren’t random depths. ASTM F2170 (Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes) specifies the depth based on whether the concrete is poured on grade and whether vapor retarders are used.

Rigorous scientific testing conducted at universities and laboratories has determined and validated the proper depths. At the proper depth, the RH sensor accurately reflects what the moisture condition of the slab will be once the flooring is installed.

The Rapid RH L6 returns the scientifically reliable readings you need to complete a successful flooring project. Readings that the MVER test can’t provide. Even better, the RH test can be completed in 24 hours. That’s one-third the waiting time required to conduct an MVER test.

Excess moisture is caused by a number of ways a few being: the hardwood floor is cupping or cracking, you see efflorescence or a white residue on the concrete floor, or the floor over the concrete is blistering or peeling away.

Moisture Management Requires Accurate Moisture Readings

Total Reader and Test Results FormAccurate concrete moisture measurement is only achieved with RH testing. Unlike surface-based tests like calcium chloride tests, RH testing determines the accurate moisture condition within the slab by placing probes at a strategic and proven depth. Moisture often rises through a slab from the bottom to the top in the drying process. Only testing performed at the correct depth can let you determine if the final moisture condition of the slab will be compatible with the flooring and the products used to install it.

Wagner Meters has been assisting flooring professionals for over 50 years. During these decades, we have designed some of the most accurate, innovative, and easiest RH testing sensors on the market today. The Rapid RH L6 is the newest iteration, taking advantage of 21st-century technology to simplify reporting.

All our Rapid RH sensors and test kits are based on decades of scientific research and technological advances to help each builder and flooring specialist accurately determine the correct concrete RH level for a project’s chosen flooring materials. Our innovative Total Reader® and factory-calibrated Smart Sensor design delivers quick, reliable results. The Rapid RH line of products is affordable and conforms to ASTM F2170 requirements for easy recording and reporting.

We also understand that sometimes a building project schedule means making alternate choices in adhesives or even flooring products. The Rapid RH sensors help you make informed decisions in real-time. Along with accurate, actionable testing, we’ve also compiled a one-stop list of manufacturers that provide an RH tolerance specification for their flooring products at

The truest way to protect a floor system is to ensure that all components are safe from excess moisture intrusion from any source. The Rapid RH family helps you prevent your concrete slab from being the source of a moisture-related flooring adhesive or grout failure. Don’t let moisture problems come between you and a successful flooring installation.

Last updated on June 23rd, 2021


  1. Peter veltman says:

    Hi Jason, thank you for you’re time. It is a floating LVP floor from the Home Depot.I removed an existing floating Laminate floor and found that adhered to the slab was a layer of the old rubber pad from a previous install. I did not want to scrape it up and painted a layer of Kilz antibacterial paint over it. I did not do a moisture test as I believed that with the combination of the old rubber pad and the Kilz it would act as a moisture barrier. I bought a dehumidifier on the 27th and have it running continuously @40. I am emptying the 25 pint collection bucket every 12 to 16 hours. I am in the process of removing the flooring and saving what I can. I will have someone come in and scrape the floor to bare concrete and then do a moisture test. I obviously need to figure out if I do reinstall the LVP this problem of water collecting under the flooring. I hope you or someone with a similar problem can help out.
    Best Regards, Peter.

    • Jason Spangler says:


      Thanks for the reply. I would say that once you get the moisture testing results, this will help you better understand what needs to be done to install the floor. Good luck.

  2. Peter Veltman says:

    Hello Jason, I bought a 1100 sq.ft. Villa in N Ft Myers Fl. In January of this year and had a new air conditioning unit installed with a Remy Halo air purifier on March 5th. The owner/installer suggested we run the unit full time even without air conditioning because it would help with my COPD. I installed vinyl plank flooring from HD on March 13th in the 400 sq ft living room. There are two sliding glass doors that go out to the lanai and an entry door to the kitchen. For 6 weeks I worked on the place frequently going in and out to saw in the lanai. About 2 weeks ago I noticed dirt collecting in the seams of the flooring which I cleaned up with a wet rag. We moved in last week and the seams started to collect dirt in more places than before and as I was cleaning a seam I pressed down and liquid seeped up and onto the flooring. I subsequently removed a couple of planks and there was and is standing water on the concrete floor which I had sealed w Kilz prior to the install. So my question after reading you’re other answers, is it possible for this moisture collecting under the flooring to be caused by running the ac unit continuously and if so how can that be mitigated.
    Thank you for listening.
    Regards, Peter Veltman

    • Jason Spangler says:


      Thanks for the question. You offered up a lot of good details, but I still have a few questions:

      1) Is this a floating floor or adhered floor?
      2) Did you do moisture testing on the concrete slab prior to installing the floor?
      a. If so, what were the results? Are these in line with the manufacturer’s recommendations?
      b. If not, did the manufacturer recommend doing so?
      3) Were there any recommendations in the manufacturer’s installation instructions regarding the placement of a vapor retarder on top of the concrete?

      To simply answer your question, the constant running of the AC COULD be causing condensation on the top of the slab, depending on how cool the surface of the concrete is compared to the dew point of the air, but this may not be any part of, or the entire cause of, the problem.

  3. Kenneth Pitzer says:

    Dear Jason,

    We have a similar moisture issue as the previous post from Justin Lee on December 5, 2017 at 7:01 pm.
    We request your recommended tile installation procedure for moist slabs.
    The backstory is, we live in Orlando, Florida and know we have a high moisture area under the concrete slab and we’ve tried to find out the source. I’m wondering if we have a spring or a brook under our home. We live with a pond behind our home, but the moisture problem is near the front of our house about 10 feet away from the outside wall. The pond is approximately 40 feet behind our house with the water level about 3 feet below our slab. Our home was built in 1991 and we are the original owners. We became aware of the moisture issue in 2016 a year after we had new engineered wood floors installed to replace carpet and tile. However, the area with the wood floor problem used to have porcelain with no issue. We began having issues with the wood surface bubbling and discoloring. That led to us also discovering moisture damage under the carpet in the bedroom adjacent to the wood floor damage and damage to the nearby bathroom cabinet which was installed directly on the slab. We have ruled out any water leaks. Prior to the remodel we had the whole house re-piped with piping in the attic. After discovering the wood floor issue, we hired a leak detector who said there was no leak and it could be moisture leaking through the slab as we did have one side of the house where the ground was continually wet. Even then, we were not convinced that was the issue because the moisture issue was not around the edges of the rooms/house, so we hired two separate companies who scoped the drain pipes under the house to check for damage/leaks. Again, no leaks. We first added gutters and last spring had french drains installed on both sides of our home. We now have a dry yard, however, we do still have high moisture in the same areas inside. We also have many efflorescence areas forming on top of the slab in our garage.
    We have an easement on what was the “dry side” of the house that has an underground drain pipe running from the pond between our home and our neighbor’s home and empties into the woods behind the homes across the street. The county said there is no possibility of leaky pipes/drains on their end.

    All this to say, we want to replace the floor with no worries of future moisture issues. We plan on replacing the wood with tile, however, have read of many water issues with tile also. Our biggest question now is, do we seal the concrete slab in the home before installing tile? We’ve heard two different opinions on this… most installers say to seal the floor before installing a new floor. A certified wood flooring inspector said to replace with ceramic tile (instead of porcelain) and use hybrid epoxy grout (as wide as the tile manufacturer recommends) so the floor will breathe. So far, we haven’t had any standing water, just high moisture. We are thinking of sealing the floor first, however, could we be dealing with a hydrostatic pressure issue under the house in the future? We would be most grateful for your help and advice. There was no solution posted to Justin Lee’s post on December 2017.

    • Jason Spangler says:


      Thanks for the question and detail. Based on your explanation, it sounds as if you may not have an intact vapor retarder so moisture is able to get from the soil to the bottom of the slab. Since you have taken many positive steps with regards to water drainage and ruling out water leaks, I would say your best plan of attack for this would be some type of moisture mitigation product applied to the surface of the concrete prior to installing the finish. If you are going back with tile, you may also want to investigate some type of uncoupling membrane. As far as hydrostatic pressure, this is pretty uncommon and will usually only be found in basement-like settings where the slab is at a level below the actual grade and water table. Good luck.

  4. Mohsin Kabir says:

    I am really shocked to see you, Jason. You guys are given a reply to each and every comment. I really appreciate your time and thoughts on this.

    • Jason Spangler says:

      Thanks, Mohsin.

      We TRY to answer them all. Our philosophy is if someone spends their time submitting a question, we should do our best to respond.

      Take care.

  5. Luciane Maimone says:

    Dear Jason,
    Thank you for such a detailed explanation about the concrete part. I have a question about best practices installing a tongue and groove flooring over a cement patio. We have paid a builder to do that, but he cannot figure out a way to avoid the wood to bow up. The slab was done several months ago, so I’m assuming it had time to dry. Before doing something more drastic I’m trying to find information about what the proper techniques are. Can the wood be in contact with the cement or should we use raisers? What kind of flooring is adequate in this case (thickness or wood we should avoid). Should we use a polyethylene barrier or something similar? Should there be a scape route (that’s as technical as I can be, lol), for example to let the water that condensates evaporate? It’s been really hard to find information. I can mostly find instructions for indoor flooring or the regular deck floor (not tongue and groove). I appreciate your help!

    • Jason Spangler says:


      Thank you for the questions. I believe your best bet would be to reach out to the National Wood Flooring Association at They have resources specific to your needs and the ability to give you some guidance. Good luck.

  6. johnbosco says:

    Great work

  7. William Ian Wightman says:


    I own a ground floor condo. directly below our unit is the underground parking lot.

    Our floor is concrete and the concrete ceiling in the parking directly below us is insulated with spray on insulation that looks like dirty cotton wool. The garage is not heated and the large access door is gate.

    we recently installed vinyl flooring that locks together with tongue and groove.

    The vinyl is now cupping and creating a trip hazard.

    The flooring company has returned and lifted some of the flooring and discovered moisture on the concrete. once the flooring has been lifted off the moisture content drops to an acceptable level. I guess it just evaporated.

    any suggestions in relation to a fix.


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