6 Outside Sources for Concrete Moisture

Leaking Drain Pipe

Concrete, like the majority of building materials, constantly interacts with the conditions around it. Temperature, humidity, and other factors all have an impact on the internal moisture levels in concrete – even when it has been deemed “dry” or has been successfully finished with a flooring or sealant.

Problems can occur when external sources of moisture cause a rise in the internal moisture conditions of a concrete slab. Unfortunately, these problems are not always visible until they have made themselves known through a significant moisture-related problem like staining, blistering, flooring failure, mildew or more. Knowing internal relative humidity (RH) levels is crucial, particularly when it’s being affected by an external source.

Outside Sources of Concrete Moisture

  1. Leaking Water SourcesSprinkler Near House
    Sprinklers, appliances, plumbing…anywhere that water is directed through or near a slab can be a prime source of excess moisture in a concrete slab if a break or damage to the pipes occurs. Undetected leaks from these sources can cause significant moisture-related damage. If localized water intrusion combines with extreme temperature changes (i.e. seasonal variance freeze/thaw cycles, or an uncontrolled building environment), it can aggravate cracking, flaking, scaling or other cosmetic and structural flaws.
  2. Groundwater 
    In any building structure, groundwater, either because of a high water table or the natural moisture levels of the surrounding grade, can become a source of slab moisture, particularly if no vapor retarder was installed. Anywhere that the ground is in contact with the concrete can become a point of possible moisture intrusion. It’s also possible for the moisture from external sources like groundwater to migrate through the slab, causing adhesive failure, mold or mildew growth, or other moisture-related problems on the opposite slab surface.
  3. Inadequate Grade 
    Natural moisture runoff from rain, snow, sprinklers, and more can be exacerbated if the grade next to the structure slopes towards the building instead of directing that moisture away from the building. Even suspended concrete systems can be affected if moisture is able to wick up through the materials in contact with the moisture at grade level. Extra measures like low-permeable vapor retarders, external sealants, drainage systems, sump pumps or landscaping should be considered if the grade is contributing to moisture problems.

Foundation Drain Pipe

  1. Poor Drainage
    Directly connected to ground water and grade, adequate drainage is vital to removing excess water from contact with any concrete system. If drainage is not designed to handle the levels of moisture that surround a building, it can result in pooling or overflow that will ultimately be detrimental to the concrete and the building materials in contact with it. Obviously, this is best addressed during design and construction, but sometimes settling can occur, causing the drainage system to fail.
  2. Condensation
    If the difference between the temperature of a concrete slab and its surrounding air is significant, and the temperature and moisture level of the slab are below the dew point of its surroundings, moisture will collect on the concrete surface and be slowly absorbed. If this compounds with chemical interaction with the concrete’s mix components, it can also lead to scaling or micro-cracks in the slab surface.
  3. High Ambient RH
    Even the internal conditions of a building space are really external to the slab and can become a source of extra moisture in it. When the air within a building space carries more moisture (humidity) than the level within the concrete, the slab will absorb moisture from the surrounding air until it reaches equilibrium (in this case, an RH balance) with its surroundings. If conditions in the building enclosure must operate at high humidity levels, then a water-resistant sealant or flooring finish should be applied to the concrete to minimize the moisture movement into and out of the concrete.

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

Internal RH

Regular moisture testing can help to pinpoint possible sources of external moisture. Surfaced-based tools like concrete moisture meters or building inspection meters can be a vital part of any concrete maintenance routine but are not capable of addressing the degree of water intrusion from external sources.

Only RH testing can provide a true picture of the internal moisture conditions of a concrete slab. If an external moisture source is suspected, RH testing can indicate the level of water intrusion by accurately measuring the internal moisture level of the slab. RH testing can also confirm when repairs have successfully corrected a water intrusion problem from an external source. Because RH testing reads from within the slab, elevated moisture levels can be identified and localized for better problem solving.

Being aware of potential external sources of concrete moisture can improve maintenance over the life of a concrete slab. RH testing can confirm what may not be visible with the casual glance.

Are you looking for a concrete calculator to estimate how many cubic feet and cubic yards of concrete you will need to fill your space? Try out our concrete calculator.

Last updated on April 19th, 2024


  1. Teresa Bourland says:

    Hi Jason,
    There is a wet spot on my concrete floor under a covered patio about 2ft outside my bedroom door. It has been damp for several weeks but isn’t growing or drying. I thought this entire time my dog pottied on my door mat because it was raining and cold so didn’t think much about it. It has been sunny for over a week in AZ and the sun hits this spot and it is still wet. Any suggestions?
    Thanks Teresa

    • Jason Spangler says:


      Thanks for the question. Probably the first thing I would do is make sure all of the water is off in your house and go out and look at the water meter to see if it is moving. Be patient, it will probably be moving slowly if there is a small leak. If you see nothing here and it is really a concern for you, you can call a leak detection company and have them come out and investigate. Good luck.

  2. John Moore says:

    Most Floor inspectors simply determine if the slab is wet.
    Many slabs are dry at the time of floor installation.
    The presumption is the floor installer did not test slab.

    I have had many cases in one area that foudation leaks go unchecked.
    I have found tree roots growing under slabs along with hidden sprinkliers
    That pop up and flood the side of house , or drive water across the slab.
    Most condo hoas will repair the problem, but will not take liability for damage inside home.
    We can seal a slab with the best sealars but water will still cascade across the concrete and flood the floor.

  3. Darlene says:

    We had a concrete sidewalk laid poorly in our backyard toward the side of the house years ago. Our problem is when it rains it pools with water and the water is seeping into the foundation of the house where there are cracks around the perimeter of the house . Would concrete sealant keep water from seeping thru? Any suggestions would be appreciated. Thank you!

  4. ALAN R GRIGGS says:

    This is in response to your reply of February 23 and my initial question dated February 17.
    Yes, we did consider a dehumidifier. We already had a fairly new heat pump, heating and AC system. But, we tried using two 70 pint portable dehumidifiers with pump drains. We ran both dehumidifiers continuously 24 hours a day for about a month. The only difference we noticed was that they made the rooms a little warmer. They made no difference in the amount of mold on the floor. Note that we have never seen mold on walls, furnace ducts, or anywhere else. So we continue to vacuum and mop the floor almost weekly to pick up the mold pieces. We are ready to try a true French drain around the base of footers and run any water to street.

  5. Alan Robert Griggs says:

    We have a home built in 2005, on a concrete slab, with a plastic vapor barrier used. We have small particles of mold on top of our laminate and tile flooring, throughout the house This was identified by two different testing labs and determined to be Chrysosporium, mucor, and aspergillus mold. This started appearing after a very heavy rain in 2014 and was more noticeable after another heavy rain in 2021. We live about 5 miles north of the Gulf of Mexico and our indoor and outdoor humidity ranges from about 15% to 75% depending on the winds being from the South or North. The mold appearance is constant since 2014 and varies depending on humidity. I’m sure others in our community have the same range. We have a fairly new heat pump system with central heating and air. We installed a whole house gutter system and installed a French drain to eliminate any pooling of water in back yard. House was treated twice by a mold service company, and it still comes back. At this point we are thinking of installing a French drain type of system at the base of footers at back and both sides of house. Our house is on a gradual slope, with back being higher than front. And let the slope of lot drain to the street which is the lowest elevation. Then possibly connect gutters to a shallow French drain to run straight to the street, unless the drain at the footers solves our problem. Would connecting gutter drainage to the footer drain be too much for it to handle and maybe blockage from gutter leaves be a problem? Should we try the footer drains just on one side first to see if we notice any improvement, before we complete three sides and find no improvement and money spent. Would you suggest any other approach?

    • Jason Spangler says:


      Thanks for the questions. Regarding the proper drainage and plumbing requirements/restrictions, that is out of my field of expertise. As far as other suggestions, you seem to have done work to divert water away from the house, but I would question if the issue is more of a dew point problem in the house. You say the mold varies with humidity and I would wager it varies as in when there is high humidity in the house, mold appears. Is there a reason you don’t possibly look at trying to control the humidity in the house with a dehumidifier (or bigger dehumidifier) tied into the central system? Just a thought.

  6. Sheela says:

    Hi ,

    My house is in Florida. I have 2 to 3 inches wide wet bands/spots in the concrete in the front porch(covered) and back lanai (covered). Both these areas are immediately adjoining the wall. The home inspector measured the moisture in these spots and it was higher than the rest of the adjoining concrete. What could be the possible explanation?

  7. rich says:

    Hello- my concrete on one side of my pool always starts to build up some moisture NOV-FEB. No leaks and all my drains work. Why is this happening ? I live in Clayton Ca

    • Jason Spangler says:


      Thanks for the comment. It’s really hard to say and it could be a combination of multiple things. Slabs of this nature typically don’t have a vapor retarder underneath so moisture from the soil could be making its way up from the bottom. Also, during these months, with the proximity to the water and lower temperatures, you could be getting a condensing effect on the surface of the concrete in that area. Why it would be happening just on that side? I can’t answer. Good luck.

  8. Kathy says:

    I live in first floor condo built on a slab. The land grading around perimeter of condo is poor. Rain and snowmelt pools around and against slab foundation. Condo is carpeted and there are damp spots around entire condo. I have one damp rid containers in each of five closets which collects water. I have one arm and hammer moisture absorber bag hanging in three closets. Each fill up within three weeks. If I don’t stay on top of this my clothes and condo begins to smell. Engineer said humidity throughout condo I higher than normal. Walls are dry. No evidence of mold or water damage. HOA has been giving me the run around for over six years. They scratch their heads but have not performed any invasive testing. Why is this happening?

    • Jason Spangler says:

      Thanks for the comments/questions Kathy. You ask why this is happening, yet it sounds like you have identified potential culprits in the grade around the home. You say the engineer said the humidity way “high”. It is high compared to what? Other units? Did he offer any explanation? Do you all have the same types of HVAC units? Do any other units have this type of issue? I would start comparing your information to your neighbors and start looking for similarities and differences in your situations. This could be a dehumidification issue in your HVAC system, drainage, insulation or window variances, to name a few. Good luck.

  9. Lori Burroughs says:

    Hi Jason,
    I have a very unusual problem. A specific area in my engineered hardwood floors began turning dark.There was no cupping only a dark stain on the floor butt which grew as well as sppread to darken around a floor seam. I had the floor in that area taken up and the concrete was light in most spaces but a dark black color where the floor was turning dark. Additionally, when the floor was taken up, there was white powder pretty much in the entire area. The flooring guy didn’t take up all the glue yet to replace the floor but where the dark areas are, the glue came up easily and the places where there is dark is showing appear to be directly on the concrete but it’s not moist to the touch. I have a moisture detector which went beyond the reading in that specific area and along the line where the specific wood board was laid down. However, the area all around did not register too much moisture. I’ve had two plumbers confirm it isn’t a pipe leak and two moisture detection guys out who determined – while the floor was still on – that there was a very, very high level of moisture. That said, I can see absolutely no crack in the slab. I have no idea who to contact to help me fix whatever is happening. Any advice or explanation would be most appreciated!

    • Jason Spangler says:


      Thank you for the question. First, it may be beneficial for you to have a geotechnical engineer come out and do a core sample in that area. You may find that there isn’t a vapor retarder below the slab and this may be where the high moisture has decided to show itself. Depending on the outcome of this test, you may also need to quantify the moisture in the slab prior to installing a new floor. Determining how much moisture allows you to be able to figure out a proper solution. I would think the flooring installer should be able to help with this once all of the information is knows. Good luck.

  10. David Donohoe says:

    We have just screened in , an elevated patio
    It has a concrete floor

    First rain did not seem to come thru the screen and get the furniture wet or damp – but the floor appears to be soaked

    There is open space under the concrete floor — seeping up ? Some type of condensation effect ?

    It’s there a coating we could possible use ?

    Please advise
    Thanks in advance

    • Jason Spangler says:


      Thanks for the question. If I am understanding correctly, I would say it is a condensation effect. I would say that having the slab open underneath promotes keeping the slab cooler that if it were on the ground and this may be triggering the condensation on the surface of the concrete and not on the furniture. Even with a coating I would think condensation would develop on the surface. Heating things up, air movement, and dehumidification of the air may help. Good luck.

  11. Mildred Logue says:

    I live in a city, row townhouses; my cement yard has been wet for several days now; the yards on both sides have grass; could it be condensation from that or is it another problem; there is no plumbing, except for a hose that is turned off, and there are no water pipes underneath ? Thanks for any help.

    • Jason Spangler says:


      Thanks for the question. There could be various possibilities, but I would lean towards what you are thinking that the combination of moisture being retained in the grass and air in the vicinity and cool temperatures on the concrete surface are causing condensation.

  12. Willie says:

    What could be the leading factors of lifting/peaking boards? No cupping, no delamination- concrete subfloors, below grade, engineered 9/16”, glue down. Dry when installed, acclimated wood floor per manufactures specs, adequate expansion around perimeter. Adequate adhesive transfer on subfloor and wood flooring. I’m lost?

    • Jason Spangler says:


      Thanks for the details on this job. Some questions that would help to potentially identify the culprit(s)

      1) What were the air temperature and relative humidity in the house during acclimation and installation? Where these conditions different when you read them during the inspection?
      2) What was the measured moisture content of the wood prior to installation?
      3) What was the measured moisture condition of the concrete slab when you installed it?
      Hopefully, you have all of this documented because it not only helps put the pieces of the puzzle together, it can help prove that you did, in fact, install to the manufacturers guidelines.

  13. Jason: Our home in Minnesota is two years old with a small tuck under garage to store lawn mowers and gardening equipment. It is below a four season sun room and shares one common interior wall with our home. After heavy rainfalls, water pools in the center of the concrete (not sealed) garage floor. We worry about mold entering the home since some mold is appearing around the garage door although the water seems to pool away from the door. A higher stainless steel curb was recently installed at the base of the door in hopes of solving the problem. It hasn’t worked. There are two downspouts running from the roof above the sun room over the garage. Water runs under the ground through tubing to the outside. The problem appeared last summer while work on the house was ongoing and the problem was evident at that time. One of the builders told us that the problem would probably disappear after the final grading and retaining walls were installed. That work was done late this spring and early this summer. The problem persists. Our builder seems puzzled as to how/why this is happening. The plan for the house shows there was to be drain tile but no one knows if it was ever installed. Vapor barrier? Not sure! We’d appreciate any recommendations for products or processes to resolve this problem before the snow flies! Thank you.

    • Jason Spangler says:

      Thanks for the question, unfortunately, I am having a hard time visualizing the layout and drainage you are explaining. If you would like, send some pictures to my email with descriptions and I will see what I can do Jspangler@wagnermeters.com

  14. Nancy says:

    I purchased a condo with new Lauzon “next step” Ambiance Emira series, Hickory installed in May/2019 after a glycol leak. Floors were flat when I purchased. Approx 1 month later they cupped, gaped, and cracked. The installer came out and recorded 32.8% RH and determined dry cupping. (Lauzon requires 35% RH). My insurance rep recorded 35% and could not determine an environmental factor that caused the cupping. I should say floors are not level as well. Then, my insurance company suggested a floor inspector. To our shock, he recorded 22-25% RH!! When I asked him to look at other factors of the installation he stated that he was only there to determine dry cupping-a pre conceived conclusion!! The cupping is worse in one large room. In 4 other separate rooms (all with the same exposure and HVAC system) the cupping is too a lesser degree similar to each other with no cracking/gaping. If low RH is the culprit, wouldn’t the damage be similar throughout the unit and not worse where the glycol leak was (and, dirty room where the wet/hazardous materials were kept). I have over 2700 sq ft of damaged floors. What should I do?

    • Jason Spangler says:


      Thanks for the questions. To one of your questions, yes, if the environmental conditions are consistent throughout the house, I would expect to see the dry cupping throughout. That being said, the glycol and environmental conditions together may be amplifying the problem. I am going to assume that the inspector came out at the expense of the installer based on the fact that they were only looking at the dry cupping. I would go to NWFA.org and search for a certified wood flooring inspector and pay for them out of your own pocket. This way, you are now the commissioning party. I would have them evaluate the floor, from scratch, to give an unbiased opinion of the cause of the problems. Good luck.

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