How can I install Porcelain Tile over a concrete slab with moisture?

QUESTION

I want to lay porcelain tile on a concrete slab at grade. The slap has various areas of high moisture from, I assume water under the slab. I can't dry the slab with dehumidifiers. What can I do to allow me to lay this tile. The floor has had leveler applied throughout. The wet spots appear as dark areas. Some small areas blistered after leveler application.

ANSWER

ANSWER - I assume that the concrete slab is an interior application.  Generally speaking you don't what to install tile over a concrete slab that has a hydrostatic water problem unless it is remediated first.

Hydrostatic water pressure is when water from a source higher than the slab is subjecting the slab to water pressure that results in water literally migrating to the surface of the concrete slab.  e.g. being next to a mountain side and the slope of the yard adjacent to the house is sloped towards the house.   If you have a hydrostatic condition then you need to install perimeter french drains around your house to redirect the water away from the house.

Assuming you don't have a hydrostatic condition, you can test the concrete slab for its moisture content per ASTM C2170 Relative Humidity (RH) test.  If you have 80% or less RH then you should be able to adhere to the slab.   You can also apply a waterproof membrane over the slab to keep moisture from migrate up through the grout joints that can develop efflorescence (white mineral precipitate).

Keep in mind that you might have intermittent high moisture conditions.  If your concrete slab doesn't have a vapor retarder under the slab that comes up the side of the slab to keep moisture from migrating laterally through the concrete slab, then during rain events or when you water the landscaping you might get purges of high moisture migration that goes beyond what is allowable.

If the RH is over 80% then you  have to use an epoxy coated system with a cementitious self-leveling underlayment to seal the concrete.  Then install the tile over it.  Check with the manufacturer of the epoxy moisture barrier to make sure it will be suitable for your situation. e.g. Koster; Dependable; Mapei; Laticrete

Be sure to properly prepare the existing substrate to make sure it is structurally stable for membrane and tile installation.

 

38 thoughts on “How can I install Porcelain Tile over a concrete slab with moisture?

  1. Doug says:

    We bought an older home that we think has no vapor barrier or gravel under the slab downstairs in the basement(drilled a hole in the slab for radon testing and didn’t see any signs of a vapor barrier). Is it a problem to tile directly over the slab which has no signs of moisture or cracks? Thanks

  2. Donato Pompo says:

    It is possible there was a vapor barrier and it deteriorated over time. Either way, if you are going to install a moisture sensitive tile over a slab without a vapor barrier there is a risk that the floor has high moisture content either continuously or after rain events.

    You can perform the ASTM F2170 Relative Humidity (RH) test to see if it currently has excessive moisture. Ideally you want a RH of about 75% to 80%. With ceramic tile you might be able to get away with up to 90% depending on the tile and installation products you are using.

    There are epoxy coating systems for applying a vapor retardant over the concrete slab surface before you tile over it. It mitigates extreme moisture conditions. There are also liquid applied waterproof/crack isolation membranes that might suffice. You have to do the RH test and consult with the manufacturer of the tile and installation products to determine what to use.

  3. Douglas says:

    My slab had moisture spots on it. I noticed after it rained quite hard, while I was finishing up installing the tile in the living room and kitchen area. This concerned me greatly because I did not seal the slab in that area. On the flip side we are install 12 x 24″ tiles with 1/2″ of Versabond LFT under the tiles AND the old tile floor that was there took me 4 days to jackhammer up and another 2 days to get the old thinset removed, so I just figured it’s not enough moisture to bother the new installation. I mean, the old tile was there for 20 years or more. Should I seal the remain rooms, just in case?

    • Donato Pompo says:

      Ceramic tile, or porcelain tile which is a type of ceramic tile, in general is resistant to moisture. If you are getting moisture from the slab then it could result in getting efflorescence staining in the grout assuming the tile is a glazed or low absorbing tile. Assuming it is not hydrostatic moisture where water actually comes through the slab, then you could have applied a liquid applied waterproof/crack isolation membrane that would have created a barrier for moisture.

      Although if you only waterproof a portion of the floor and if there is moisture migration occurring then the moisture will be redirected to where there isn’t a membrane. Might be better to not use a membrane so the moisture can uniformly breath.

  4. Carolyn Badura says:

    Thank you for the information! I have been waiting on the installers to lay my tile floor for almost 2 months now. “Too much Moisture…can’t do it”. I’m sure testing for moisture while it’s raining or just rained doesn’t help the test results. It’s been so frustrating….

    • Cynthia says:

      Hello, I am experiencing the same situation. The contractor will not complete my installation since I have moisture in my concrete slab inside the house.
      What did you end up doing, I’m curious?
      Thank you

  5. Minh says:

    Hi, I have effervescent coming up only one side of my house through the grout of a proclean tiles, and at times we find small puddles of water. I checked for leaked And it came back negative. The water spots can be in the middle of the hallway and not always at the edge of the house. Do you think I need to replace the floors and add in an epoxy barrier under or what are my possible options to explore? Thank you

    • Donato Pompo says:

      The water has to be coming from somewhere, but finding the source can be difficult. If you know that there isn’t a plumbing leak somewhere in the house, then the water must be coming from the outside. The efflorescence is a symptom of moisture that has traveled from beneath the tile and evaporated from that spot.

      If the water is coming from the outside you could dig a test hole on the outside of the house nearest to where you see the water. Maybe installing perimeter trench drains would solve the problem. You could check with the planning department and see if a geological soils report is available for your lot to see if there is a high water table. If so then the only choice maybe to replace the tile and install an appropriate epoxy moisture mitigation system over the concrete slab before you install the new tile over it.

  6. Maureen Mead says:

    Hello Mr. Pompo. I am looking for a sealant recommendation for my garage floor. The garage is now an interior room and had tile at one time that literally popped off the concrete, presumably because of moisture. Thank you!

    • Donato Pompo says:

      Chances are the reason the tile came loose from the concrete slab is that it was contaminated. It should have been scarified and verified it would readily absorb water as an indication that there isn’t a contaminate on it and that you can get a good bond to the concrete with the proper thin-set mortar adhesive.

      If you are going to put tile on the concrete again DO NOT USE a sealer as it will act as a bond breaker. To determine if you do have excessive moisture in the concrete you should have a Relative Humidity test performed. If it is excessive there are tile moisture mitigation epoxy based products that can be applied over the concrete to seal it and you can then bond the tile to it as approved by its manufacturer.

  7. Debbie from Austin says:

    Hi Mr. Pompo! I appreciate you sharing your time and expertise for questions. My question is this: I have to replace the manufactured wood flooring in my house because it has been damaged by elevated moisture levels from the slab. I also have carpeting and tile on this same slab that is fine – so the moisture is not extremely high. I see that you recommend putting down a sealant before laying tile in order to reduce the moisture issues. If the slab is treated with some type of product that “blocks” the moisture, won’t that cause the moisture to go into my walls instead?

    • Donato Pompo says:

      First of all, it is best to refer to a waterproof or moisture barrier membrane rather than a sealant that could be misconstrued. You don’t want to apply any type of a sealer to concrete before adhering tile to it as some types will act as a bond breaker.

      In terms of applying a moisture or vapor barrier membrane over the concrete, whether you do that or not, the concrete beneath the interior cavity of the wall is still subjected to moisture migration in that area. The moisture or vapor transmission shouldn’t increase in that area unless that was a low pressure zone driving evaporation, which isn’t likely.

      Plus the interior base of a framed wall has a treated wood sill plate over the concrete that would greatly block and reduce any moisture migration.

  8. Diane says:

    I was planning on installing a vinyl tile flooring. And removing the old flooring in an entryway in a townhouse. But when the.flooring installer showed up, he tested the moisture on the floor and said it was too high and the vinyl tile would not adhere, even with underlay. He said only recommendation he could suggest was carpet. But I don’t want carpet in the entryway. Any suggestions? I am looking at another plan also that may eleviate the moisture problem. But I still want to update the flooring there. Could I just paint the floor with that speckled epoxy they use to finish garage floors with?

    • Donato Pompo says:

      If you have high moisture I wouldn’t put down carpet. An impervious porcelain tile would be more resistant to moisture.

      It depends on how high of moisture you are getting as to what you need to do. If you see a wetness you probably need to apply an epoxy moisture mediation system, if it isn’t a hydrostatic condition, and then bond your tile to it.

      If it is moderately high you can apply a liquid applied or sheet waterproof membrane over the clean and properly prepared substrate and bond your tile to it.

      If it is low moisture you could install the tile to the properly clean concrete slab and grout with a cementitious grout. The moisture will migrate through the grout and evaporate. If it is too much moisture you could get some white efflorescence staining.

      To help you decide how much moisture you might have in the concrete follow ASTM D4263 and take an 18×18 inch piece of 4 mil transparent polyethylene sheet and tape it down tightly along all edges with a good tape to a clean concrete slab in several spots. Then after 24 hours see how much condensation develops on the bottom of the plastic.

      Epoxy floors normally don’t do well with underlying moisture.

  9. Julia says:

    I was told before that effervescence is appearing on one side of my daylight basement floor and needed a French drain to fix it. I then had someone come tell me it’s because the waterproof barrier was worn off and I needed to fill the cracks in that you can see in from the outside and because that’s how the water from the rain was causing the effervescence on the floor. My home is on land that is slanted back for the daylights basement. I want to get ceramic floor installed but the effervescence has caused the stain that I put on the floor to chip and peel up. My questions are do you think filling in the cracks with a waterproof barrier will fix the issue of the effervescence and also can I install the ceramic tile over the floor that has chipped and peeled from the effervescence? Thank you so much for your expertise!!!

    • Donato Pompo says:

      Efflorescence is a calcium material that produces a whitish colored staining. It is normally easy to remove with an appropriate diluted acidic cleaner.

      Efflorescence staining is normal to have to some light degree, but can be a symptom of an excessive moisture problem if there is an abundance of the white staining. It is the result of water getting below the concrete and migrating up through the concrete and picking up minerals. As the water reaches the surface and evaporates it precipitates the minerals which is efflorescence staining.

      If the water is coming from the outside of the basement then a French drain might be a solution to divert the water away. The water can be coming up from under the concrete from the soil if there is a high water table or if water is draining towards the house.

      If you have cracks in the concrete floor that you want to tile and the cracks have a lot of efflorescence staining then you might need to install a waterproof membrane first or a waterproof mitigation system. First you have to fix the cracks.

      If the cracks are structural cracks where one side of a crack is higher or lower than the other side then you need to have a structural engineer to prescribe how to fix which is normally installing metal dowels with epoxy. If the cracks are shrinkage cracks, then you can fill them with the thin-set mortar adhesive and then waterproof over that after the floor has been properly prepared.

  10. Kelly Ryan says:

    I’m putting a Pergo waterproof laminate wood floor down in my basement with their Gold underlayment, moisture barrier. There is an approximately 18″ x 5′ section that I’m tiling to make a hearth area for an electric fireplace. Can I put the tile over the underlayment? Do I need a piece of backerboard first. I have trim for the transition, just not sure how to apply the tile. Thanks for your help!

    • Donato Pompo says:

      First of all the laminate may be waterproof and the Gold Underlayment may be a moisture barrier, but once installed you will not have a waterproof floor. Laminate and LVT manufactures mislead the public in the way they use the term waterproof.

      It may or may not be possible to bond a tile to the Gold Underlayment. If the underlayment company or the tile adhesive company doesn’t recommend it then you shouldn’t do it. Even if they do recommend bonding tile to the underlayment, the tile installation will only be as good as the performance of the Gold Underlayment. It would be better to bond the tile directly to the concrete substrate that has been properly prepared.

  11. Catherine says:

    Hello, I installed porcelain tile in my home last 22 yr old slab home March. It was damaged during installation and the tile shop came back in August to rip out and reinstall the entire job. When the tile was removed from one of the rooms there was a thin layer of water under the tile the length of the room extending about 1/3 of the room from exterior wall. We assumed it was a drainage problem and had a contractor dig out the slab to the footer and water proof it and also install French drains and regrade a portion of the yard on that side of the house. The slab is still very moist. The flooring guy put rh meters in the floor that are reading 99% in the problem area. We have had a contractor look at the property and there is not water coming from the roof, walls, windows, fresh water plumbing or waste water sewer pipe. ( the only thing we have not done is take out the master shower in the adjoining room and I don’t know how to assess if it is the cause of the problem) The tile shop is recommending that we have the floor sealed and then reinstall the porcelain tile. Is sealing the floor and reinstalling the tile a viable option, or are we likely to have moisture migration to the walls and other rooms?

    • Donato Pompo says:

      The key to remediating a moisture problem is to first determine the source of the moisture. Of course the source of the moisture can be a long distance from where the moisture is noticed.

      If the source of the moisture is from the ground beneath the concrete slab in spite of the fact that you installed French drains around the perimeter of the house, then sealing the concrete slab can work even at 99% RH. There are epoxy moisture mitigation systems sold by the manufacturers of tile installation products that can be applied and you can then attach the tile assembly to it.

      If there is hydrostatic moisture, which would mean that water from a higher location is traveling down under the concrete slab and up causing a pressure then the moisture barrier system would not work. But properly installed French drains could prevent that; unless there is a high water table that is creating the pressure.

      If the shower is linking, you should be able to use an infrared camera to trace a trail of moisture that would be traveling from the shower to the area where you see the wetness.

      So it is an investigation where through a process of elimination you can target the source of the moisture then determine how best to mitigate it.

  12. Cassandre says:

    Hello, my house was built in 1933. The previous owner painted the basement floor prior to our purchase, and over the past 4 years I have noticed many efflorescence patches showing up scattered all over.
    I want to lay brick veneer tiles over the basement floor – if I don’t mind the appearance of efflorescence coming through, can I just go ahead? Are there any potential issues I would encounter down the road with tiles popping up, etc?
    Or can you recommend if I should lay down some kind of sealant on the basement floor before laying brick veneer? Or should I just lay brick veneer and the seal that surface?

    Thank you!

  13. Kyle Turner says:

    Want to put porcelain down in mood room in Michigan and I’m on a slab and was wondering what to bond porcelain to as a moisture barrier and to help with the cold during winter

    • Donato Pompo says:

      Well it depends. You should do a Relative Humidity test on the slab to see how much moisture it is retaining or subjected to. A simple test is to tape about a 24″ x 24″ sheet of clear polyethylene plastic to the concrete slab that has been scarified and cleaned. Tape it tight around the edges of the plastic. Wait at least 24 hours and see of condensation collects under the plastic.

      If you do have condensation that you might have excessive moisture in the slab either due to a high water table or that water is migrating to it from somewhere like planters. You are suppose to have a vapor barrier under the concrete slab but often it is left off or it has degrade after many years.

      If you don’t have excessive moisture you can use one of the liquid applied waterproof membranes or a peal and stick membrane over the properly cleaned slab. Mapei, Laticrete, Custom Building Products and others have these products. Just make sure you follow their recommendations.

  14. Omar says:

    Mr. Pompo, I have a unique situation that requires a solution. A client of mine had a concrete slab constructed at the rear of his house as an outdoor pergola area in 2017. Over time the pergola was converted to an indoor habitable space and the surroundings were enclosed to create a dining area.

    The contractor that poured the slab is no longer around and it is difficult to ascertain if a Vapour Membrane was applied to the area prior to the pouring of the slab.

    As it is not known if a moisture barrier was installed and the area is now tiled, what is the best method to apply moisture resistance?

    This request is to satisfy the local Council that a moisture barrier is in place as part of approving the original works.

    Thanks

    • Donato Pompo says:

      There are ways to determine if you have a vapor retarder under the concrete slab. You can drill a core hole through the tile floor. Or you can dig outside along the perimeter of the concrete slab to dig below the slab to look for the vapor retarder.

      If you don’t have a vapor retarder you could cut out the grout joints and install an appropriate ASTM C920 sealant. This in theory seals the grout joints. If the tile is an impervious tile then in theory it is water tight. This technically does not make the floor waterproof and if you do have a high moisture condition under the concrete slab it could cause other issues.

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  16. David Brennan says:

    Hi the concrete in my bathroom sweats when covered. When I had lino on it mould developed under it. Can I put down tiles on this concrete ?
    Thanks

    • Donato Pompo says:

      If your concrete bathroom floor sweats (condensation) when covered then you have a high level of moisture passing through the concrete into the bathroom.

      Since you don’t have actual water coming up that would suggest there was hydrostatic water pressure coming through, there are waterproof membranes and moisture vapor retarder membranes that can be installed over the concrete to keep the moisture from passing through. The Vapor retarders are for very high moisture in the concrete and is an epoxy membrane that is applied. If the moisture meets the requirements of the liquid applied membranes or any of the other waterproof/crack isolation membranes you can use those as a moisture barrier and surface that you can adhered the tile to.

    • Donato Pompo says:

      Porcelain tile is a ceramic tile. Porcelain tile is most common floor tile sold today and it is technically impervious and very durable. Some tile products have a lot of variation in color and others have less. You will have to qualify that with the supplier of your tile.

      When tile is produced it may come out of the kiln with different shades of the same color depending on when it was produced. So always make sure that the tile you buy that all of the cartons have the same shade and calibration (sizing consistency).

  17. Katie Schaefer says:

    Hello! Our home is 40 years old on a concrete slab. We are not sure if there was a moisture barrier placed below it. We have now pulled up all of our prior tiles and want to place new porcelain. We were told by a foundation expert that our concrete slab is whicking some moisture and is slightly wet- he didn’t do an official test so it confused me. I have concerns something will happen to our new porcelain floors after they are placed!

    Should we seal it with epoxy and then also add a moisture barrier prior to laying the tile? There’s no signs of hydrostatic moisture or leaks under our slab. I think our slab is just old. Thanks!

    • Donato Pompo says:

      You don’t need both an epoxy vapor barrier and a waterproof moisture barrier.

      If you have 100% relative humidity in the concrete slab, per ASTM F2170 test protocol, and don’t have a hydrostatic condition, then you can use an epoxy vapor barrier that goes over the concrete and you can bond the tile to it. Laticrete has their VaporBan epoxy membrane and Custom Bldg Products has their TechMVC Vapor barrier.

      If you only have high moisture dampness you might be able to just use one of the ANSI A118.10 waterproof membranes where you can bond your tile directly to it. These are less expensive and easier to apply. Such as Laticrete Hydro Ban and Custom Bldg Product’s Redgard.

      You can hire a local concrete slab moisture tester to test your concrete slab for relative humidity per ASTM F2170. Depending on the size of the slab it should cost less than $500 for a residential slab.

      You do need to use an ANSI A118.15 thinset mortar for bonding the tile to it.

      Porcelain ceramic tiles are technically impervious at 0.5% absorption or less. So they are very resistant to moisture.

  18. Trish says:

    In my 1954 slab home I have 9X9 vinyl tile covered by carpet, but in one closet the floor is bare where the previous owner removed the flooring. That closet floor had beaded moisture last summer under a plastic tote. It’s been sitting bare since because I don’t know what step to take next. At times the carpet in some areas feels damp underfoot.

    I’d guess that hydrostatic moisture could be an issue based on the water table, but have not tested the moisture. Would ceramic tile work in this situation without remediation? If not then, what other inexpensive solutions would work?

    I’d say that the water table is very high here with no drains around the home. I had planned to install tile or even VCT, but unsure about it.

    • Donato Pompo says:

      VCT doesn’t work over a damp concrete slab unless you install an epoxy moisture mitigation system first. Ceramic tile is much more resistant to moisture. The grout joints does allow it to break to some degree if you use a cementitious grout.

      Hydrostatic water pressure is where water is flowing up through the slab, so you likely do not have that condition or water would be prevalent in the closet uncovered slab.

      If you want more moisture mitigation, you can install a liquid applied membrane over the concrete slab first, if the relative humidity or moisture vapor transmission isn’t excessive. Then you can bond the ceramic tile directly to the membrane.

  19. Peter dems says:

    Hi again Donato and thanks for all your feedback and suggestions.

    So on the same terrace im now ready to install glass balustrade which has U channels bottom rails and 84 holes need to be drilled through the porcelain and concrete using 1/2 ” chemical anchors.
    Is diamond tip the only way to go to drill holes and then add resin and stainless steel bolts to attach bottom rails?

    • Donato Pompo says:

      Yes you do need to use a diamond tip drill bit to drill through porcelain tile. You need to use was water to spray on the drill during drilling so it doesn’t over heat.

  20. Tim Burman says:

    I would like to install porcelain tile on a concrete patio that is undercover. Can I use the Brock 2×3paver pads? This is a condo and I may eventually have to remove the ceramic tile thus wanting to put it on a barrier and not directly to the concrete

    • Donato Pompo says:

      I don’t see how this Brock PaverBase panels can work well. On their website they don’t tell you what they are made of or what they will do. I assume they are like a drainage mat, but that is not a substitute to 4″ of gravel base that has been compacted. They say compact the dirt with a slope of 1/4″ per foot, add landscape fabric, add 1/2″ sand, and then put down their panel and the pavers over it. So where does the water go from rain or washing down? The dirt and sand isn’t likely to absorb it.

      Of course you are installing the porcelain tile over concrete, which is the best method of installation if you adhere the tiles to the concrete with a polymer modified thinset mortar.

      If you go with the 3/4″ thick porcelain pavers you could just set them over sand on the concrete in theory. I would never do it, but that thick and heavy of tile will likely be stable as long as you have a constraining edge around the perimeter of the tile installation so they can’t slip out.

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