Moisture Problems between the Flooring and the Slab

Laying Ceramic Tiles

It is no surprise to say that water is an integral part of the hydration process of concrete. However, allowing excess moisture to leave the slab once it is poured is just as crucial to a flooring installation.

Once the slab is poured, the excess moisture must leave the slab in order to strengthen the concrete bond and to permit successful flooring installations. Moisture-related damage to flooring materials is, of course, possible, but the real risk to a successful floor lies between the concrete slab and the flooring itself.

Three common floor materials run the risk of moisture-related problems.

  • Adhesives
    Flooring AdhesiveMoisture-related adhesive failures have always been a problematic reality in the flooring business. In addition, with recent trends towards restricting volatile organic compounds (VOCs) in flooring adhesives, there has also been an increase in the number of moisture-sensitive adhesives in use. If the adhesive being used to install the flooring does not have the correct moisture tolerance for the concrete subfloor, the entire installation can be at risk.
  • Floating Floors
    The attraction of floating floor systems is that they do not need to be attached directly to a subfloor. Instead, the floor pieces “lock” together to become a cohesive unit that is not as subjected to seasonal shifts, dimensional challenges or other moisture-related issues. In fact, floating floors are often the recommended product of choice for floors where high moisture situations may present issues with standard attached floor systems. For these floors, manufacturers often recommend the installation of a moisture barrier between the subfloor and the floating floor to prevent moisture intrusion. The difficulty, of course, is that if that barrier is compromised in any way, moisture from the slab beneath can still damage the flooring or finish.
  • Grout or Cementitious Bonds
    Excess moisture issues in a grouted tile or mosaic floor will often appear as efflorescence, a whitish residue on the surface of the grout. It’s the result of water-soluble minerals being dissolved by the moisture within the slab and being transported to the surface of the grout where the moisture evaporates away, leaving the minerals as a visible residue. The more porous the concrete or the grout, the more likely efflorescence will appear. In the majority of cases, these minerals are actually part of the concrete slab mixture (or possibly in the ground beneath the slab, if no moisture barrier was installed). If the slab was not effectively dried before the tile was installed, the natural moisture migration of the drying concrete will impact the grout, and remediation steps will be necessary to correct the problem. In extreme cases, excess moisture can lead to flaking or chipping of the grout and a complete grout or thin-set failure.

Moisture in a Concrete Subfloor

Are you seeing a theme yet?

Moisture control is often one of the most crucial, yet most overlooked, elements of any floor’s success over time, and that means accurate moisture measurement, beginning at the slab.

RH Testing in the FloorAccurate concrete moisture measurement is best achieved with relative humidity (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, so 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.

Because we at Wagner Meters have been assisting flooring professionals for over 40 years, we have also designed some of the most accurate and innovative testing methods on the market today.

Our Rapid RH® 4.0 is based on decades of scientific research and technological advances to help each builder and flooring specialist accurately determine the correct concrete RH level for his or her chosen flooring and products. Our innovative Easy Reader™ and factory-calibrated Smart Sensor design mean that results are quick, simple and reliable. 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, and the Rapid RH® can help you make informed decisions. Along with accurate, actionable testing, we’ve also compiled a one-stop list of manufacturers that provide an RH tolerance for their flooring products at

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

The following two tabs change content below.

Jason Spangler

Jason has 20+ years' experience in sales and sales management in a spectrum of industries and has successfully launched a variety of products to the market, including the original Rapid RH® concrete moisture tests. He currently works with Wagner Meters as our Rapid RH® product sales manager.


  1. JILL COX says:


    • Joe Dabbs says:


      I am pretty sure that this old home does not have a vapor barrier. If I had a hunch you probably see more “wet” concrete during the wet months when compared to the dry months. I say this because your concrete slab is like a sponge and it will soak up moisture when it is present. One of the ways to outsmart this problem is to create a “vapor” barrier on the surface of the slab. Epoxy Resins are applied to the surface to keep the moisture from escaping from the surface. If done correctly, the epoxy resin should create a barrier strong enough to defend “moisture”.

      Here is a link that explains this process in more depth:

      Let me know if you would like some more information or you can give us a call at 800-634-9961.

      Thanks Jill,
      Wagner Meters

  2. Gena Jenkins says:

    I am having an issue with my hardwood floors. The floor is turning black although the house. They are not rotten, just turning a dark color. The areas are in my closet, under my rug in the dining from, as well as in my kitchenew

    Would this be a foundation issue or the floor not being put down correctly?

    • Jason Spangler says:


      Thanks for the comment. It would be best to have someone look at your issue because it could be multiple things. I would contact and get one of their qualified inspectors to come out and take a look.


  3. dawn pecunies says:

    good morning, I just got up this morning to find that my ceramic (white)tiles in my kitchen are turning “gray”it looked like
    it was a shadow but with closer inspection the tiles are turning a dark grey. WHAT would cause this to happen???

    • Jason Spangler says:

      Dawn, overnight like that, being honest, I don’t know. I would contact a local contractor whom specializes in tile and/or contact National Tile Contractors Association ( and see if possibly they have any recommendations of someone who can take a look at it.

  4. Sam says:

    Today we had a home inspection and during the inspection of this home we are interested in purchasing was built in 2010. The garage show efflorescence on the bottom concrete blocks around the garage foundation slab on some of the cement whitish residue on the surface. The home was built on a raise slab. We are not sure if we should purchase home ? If the would be a problem in the future ? What type of repaired is needed and the cost if all. Currently considering purchase but not sure if this is a major problem or could be a problem in unseen surfaces of the home.
    Please help or advise .

    • Jason Spangler says:


      Thanks for the comment. I would recommend contacting a waterproofing contractor and see if it is a big issue. In general, this is a fairly common occurrence, but you should verify your specific situation.



  5. Martine Cameau says:

    Hello, we’ve been dealing with a moisture problems since we installed porcelain tiles in five rooms in our home built in 1989 (the rest of the house has ceramic tile that has been in place for 15 years). We had a drainage problem on one side of the house that resulted in wet walls (interior, not exterior) as well as major efflorescence in the new tile installation. We are currently in the process of installing well pointing and a sump pump to drain water away from the house. We are located in FL, and it’s raining every day here, and in on the rooms you can actually see water beading through the grout lines, but only in the middle of the room. We’re hoping draining water away from the house will ultimately dry out the slab. After reading the posts above, it seems the tile installer should have put down an epoxy resin before setting the tile. Is there any way we can do something to the EXISTING installation, and not have to resort to removing all the tile and starting over? Could we regrout and add something to the grout mix to help with the waterproofing?

  6. Jeffrey W says:

    My wife and I purchased engineered floors for our home, which is on a concrete slab. They were initially installed floating with a 3mm underlayment. Unfortunately, the slab was not leveled properly, which led to a number of dead spots and possibly buckling. So, the floors are being redone and the slab is being properly leveled. My questions are… I am concerned with moisture as the home has previously had moisture and mold issues. What is the best glue to provide a solid moisture barrier between the slab and the wood and/or do I still need an additional underlayment between the slab and wood in addition the glue (my contractor says no)? Thank you.

    • Jason Spangler says:


      Thanks for the comment. Your best place to start for best installation practices for wood is the NWFA at They have resources and phone numbers you can call to get expert advice. Regarding adhesives, you need to have appropriate moisture testing of the concrete to know what needs to be done to remedy the problem. I would start there and then based on the moisture testing results, you can shop for appropriate products to go on top of the slab.



  7. Taj Ahmad says:

    We have fully renovated our house this year. One of the major jobs was to put marble (Crema Marfel) on the floor. The floor was laid in February 2017. Two months later, we noticed powder like substance appear on the surface. We pointed this out to the contractor, who arranged for a chemical to be applied and buffing carried out. After the buffing, the the floor appeared clean and shiny. Two months later the problem appeared again. The contractor has arranged the application of chemical and buffing three times but each time the problem reappears after 3-4 weeks. We are now noticing dark stain marks on the joints. We moved into our house in March 2017, but have not been able to settle down because of the floor problem.

    Can you help with identifying the cause of the problem and proposing a solution.

  8. Martine Cameau says:

    Hello Jason, any thoughts regarding my post dated June 16? We hired a geo Engineer who determined that we don’t have a high water table, however there may be have been a drainage issue, as the soil samples indicates that one side of the house is about a foot wet. Likely soils holding water. Somehow it’s seeping into the area of dirt BETWEEN the slab and footer and has been wetting the soils under our floor. And we contacted the tile manufacturer; it seems the porcelain tile is creating condensation and it’s finding its way through the grout and our walls. We’re trying to figure out if there’s a way to dry the slab without having to rip out all of the tile.

    • Jason Spangler says:


      First, I apologize for missing the original question. I am not aware of being able to “fix” this problem without starting over. That being said, it may be beneficial to have a tile certified flooring inspector come out and evaluate and give some advice. I would start here ( and see what this organization can do to help.



  9. I had a bamboo wood floor laid that did not adhere to the glue, the home flooded over a year and a half ago , is there anything I can do to save my floor.

    • Jason Spangler says:


      Thanks for the comment. I would recommend having either a qualified wood flooring installer or a wood flooring inspectors come out and evaluate the floor to determine a course of action.



  10. Rosalie Atkinson says:

    I have lived in my apartment for about 10 years and recently an accidental water problem in the apartment upstairs caused some damage to my ceiling and kitchen floor. Management said my carpet was not damaged so did not need replacing. However, when I walk barefooted across the living and dining area which are carpeted the bottom of my feet feel dampness. Is there some way to determine if there is still moisture under my carpet?

    • Jason Spangler says:


      Thanks for the comment. There really isn’t a way, that I am aware, to test moisture in the carpet itself. One thing you may want to try though would be a visual test. Take some newspapers or paper towels and put them in a few areas, placing something on top of them to weight them down. Leave them for short period of time and see if the paper product shows any discoloration. Obviously, this doesn’t prove where the moisture came from, but it does show moisture.



  11. Justin Lee says:


    I live in north Tampa. I have battled water intrusion up through my grout lines in my newly installed porcelain tiles for over a year now. It occurs almost daily, pooling puddles, and a couple of times actually squirted through an opening when I scraped the efflorescence away. My home was built in 1980.

    When we first noticed the issue, end of 2015/early 2016, we saw water stain under our laminate floors. We filed an insurance claim and their investigation (sub-par at best) was negative for leaks and determined to be a ground water/seepage issue, which is not covered. It was denied. I remodeled my home and during the process of removing the damaged laminate, there was thick mold found underneath every sq inch. My kitchen cabinets base and my two bathrooms show water damage and were installed directly onto the slab, NOT the tile. There were 14×14 inch ceramic tile with a ¼ inch grout lines in the rest of the home, which never showed water or efflorescence. Even the un-remodeled bathrooms to this day do not show water intrusion. I installed 36 x 8 inch porcelain tiles with 1/8-1/16 inch grout lines. Well, I am positive that it is NOT a ground water issue through my own long term investigation by many different means, i.e. new gutters, surface ground drains, multiple drilled holes all the way through my slab with no signs of moisture, 5 holes- 4 ft deep, surrounding my slab on the affecting side of the house with NO ground water layers, new a/c return vent, correcting a negative air pressure issue within the home, and more. Im even having a couple tiles de-bonding. I also have a thinner, more powdery textured efflorescence showing on my garage floor and the paint has been flaking off. Over time, the leak locations have been slowly migrating further and further. The water is only on the surface of my slab. I haven’t noticed the slab insides being moist when drilling, or even in the dirt under the slab. I will add that I am on a pond, but it is approximately 40-50 feet behind my house with a major slope, approximately 3 feet lower elevation than my home.

    I have ruled out just about everything after 3 negative leak detections (1 included a drain inspection), 2 denied insurance claims, 3 geotechnical engineering firms blew me off (couldn’t figure it out and gave up), a building scientist unable to figure it out after almost 8 months trying, ruled out ground water, and more. This problem was consistent even during the 3 month drought we recently had. During this drought, the pond behind my home dropped about 12 inches in water level, but my water instruction continued just the same. The county water department and utilities departments both deny any possibilities of leaky pipes/drains on their end. Heavy rains do not seem to affect the frequency, nor does an extended time without rain. It’s major hydrostatic pressure and NO ONE is able to determine or even guess what the source is. I find it very hard to believe it would be condensation being as there was a couple instances where it showed pressure when coming through the grout. I occur on two opposite ends of my home, but not in the center area. I have even drilled holes through my slab, down into the earth and NO water showing in three different locations drilled. I cannot accept that NO ONE can determine a source for this water. None of my neighbors have noticed a similar problem, however, in the neighborhood due north of me, 3 or 4 homes have seen similar issues, just not nearly as severe. I have held off on installing major, overkill French drains around the perimeter of my foundation until I can define the source. Is it possible that it’s a newly exposed underground spring????

    I am willing to bet this would be your most challenging investigation yet, if you are able to help me out. Please text or email me anytime. The sooner the better. I will not stop trying until I solve this.

  12. Misty says:

    Good morning, we have lived in our house almost 7 years. We have stained concrete floors and have a huge problem with efflorescence. We have had the moisture test done where they drilled holes in the floor and we were told it is the highest they have seen. We are wanting to cover our floors but we don’t know where to even begin. Do you need to grind the stained concrete floors and what type of flooring would work with this moisture problem? Help

    • Jason Spangler says:


      Thanks for the comment. I would enlist a reputable flooring contractor to give you options. If the levels of moisture are very high in the concrete, there are various products on the market that can be applied to the surface (moisture mitigation product), to minimize how much comes out and can affect the flooring finish you wish to apply. Keep in mind, the high levels of moisture don’t bother the concrete, they bother the finish trying to be applied. I hope this helps.

  13. C says:

    We have a luxury vinyl plank flooring and just months later the flooring is bubbled and glue squirts out anytime you step near a seam. These were professionally installed and were glued directly to a concrete subfloor. The flooring company insists I have a leak and won’t repair, but they also never completed a moisture test prior to installing. Do they have any obligation to repair?

    • Jason Spangler says:


      Thanks for the comment and I am sorry for your issues. Most contractors have some kind of warranty on workmanship, but in the end, they have the ability to decide whether this falls under their warranty. Depending on where you purchased the material, you might get some help from them, or possibly even the manufacturer of the product. Good Luck.


  14. Jim says:

    We finished our basement two years ago and chose a vinyl tile with grout for the bathroom. Less than a year after, some of the tiles buckled up at the grout line and were replaced by the flooring people, the thought was there was not enough glue. Into the second year more tiles did the same and the flooring people said there is a moisture problem. Not all tiles are buckling, so upon recommendation we removed the toilet thinking it may have a leak. The seal was good, so that was not the problem. The contractor did need to break the slab to rough in the drain for the toilet and shower. We have not removed the flooring to see if there is an issue with that. The floor outside the bathroom walls is either carpet or bare concrete, neither have moisture. Another part of the basement we have a vinyl floor product that interlocks without grout, not sure if it was glued, with no issues. The basement has always been dry before and we have always run a dehumidifier. The current thought is to remove the flooring and let it dry to see where the moisture is now. I’d like to know what we need too look for, and correct as a next step.

    • Jason Spangler says:


      Thanks for the comment. First, I would want to test the concrete to see how high the relative humidity is in the affected area, Second, the areas that you state don’t have a moisture issue, may. Carpet is very breathable and, obviously, so is bare concrete. The moisture we are speaking of here is moisture vapor, not physical water. So it may be coming up through the concrete and once it gets to the surface, it evaporates and is processed through the dehumidifier. One of the big issues with basement renovations is you never know if there is an intact vapor retarder under the slab and in most older homes there isn’t. Without that, moisture from the soil is always available to the concrete. They make products that can be applied to the surface of the concrete, prior to flooring installation, that help minimize the amount of moisture that can come out of the slab. The first thing is to determine how bad the problem is.



Leave a Reply

Your email address will not be published. Required fields are marked *