How do you waterproof a tile floor that is already installed?


How do you waterproof a tile floor that is already installed? - trying to waterproof 18 x18 inch ceramic tiles on bathroom floors. spills are passing through to the floor below.


ANSWER - If the ceramic tile is already installed there is no way to fully waterproof the floor after the fact, unless you replace the tile and install a legitimate waterproof membrane during the replacement process.

Some people will try to put a sealer on the grout joints, but that doesn't waterproof it. It doesn't work. It only makes it more water resistant.

The only thing you could try on, on an experimental basis and if you deem it practical, is to remove the grout joints by cutting them out with a dustless diamond cutting wheel on a grinder.  Clean the joints out thoroughly and fill them with an ASTM C920 sealant (caulking).  This type of caulking is either a full strength silicone or a urethane sealant.  Be sure to also caulk the perimeter of the tile where it runs into walls and tubs with this same sealant.  If the tile is impervious and all the joints are filled with this type of sealant, you might end up with it being waterproof, depending on how good of a job you do.  Good luck.

38 thoughts on “How do you waterproof a tile floor that is already installed?

  1. Marcus Coons says:

    Thanks a lot for mentioning how using a sealant in your bathroom does not make it waterproof. It is important to remember that understanding how to properly waterproof surfaces can help you find the best way to protect them. We need to find a good waterproofing company that can help us waterproof our bathroom, so I’m glad I found your page.

    • Donato Pompo says:

      There are grout sealer sprays, but they do not waterproof the grout or the tile installation. The only way to properly waterproof a tile installation is to use a waterproof membrane installed under the tile during the tile installation process.

      A method that can theoretically waterproof a glazed tile installation after it is installed, is to remove the cementitious grout and replace it with an ASTM C920 sealant such as a 100% silicone caulking. This method is not an approved method by the industry or the product manufacturers.

  2. Hameed Manikkoth says:

    We are using silane Penetratives after cleaning the tile joints of bathroom floor and sealing joints by cementitious and Styrene Butadine latex.

    • Donato Pompo says:

      There are silicate chemicals that can treat concrete to make it waterproof or water resistant, but using it in cementitious grout joints will not make the tile installation waterproof. Nor does adding a Styrene-butadiene latex to your grout mixture make the grout or the tile installation waterproof. It does make the grout stronger, more flexible and water resistant.

    • Donato Pompo says:

      Using an epoxy grout in the grout joints in theory can work, but you can’t use epoxy at the transition joints as they need to be filled with an ASTM C920 sealant made of silicone or urethane as they need to be resilient joints. Technically it doesn’t make the tile installation waterproof, but in effect it might or might not do the job.

  3. Shafique says:

    house with exterior tile cladded walls gives moisture on internal surfaces and has spoiled the paint internally, the is no roof projection. How can this be rectified/reparred,?

  4. Donato Pompo says:

    The tile installation was obviously not installed correctly to prevent moisture migration into the interior walls. You could have microbial growth in the wall cavities as moisture with organic material propagates microbial growth. The only sure way to remediate is to remove the tile on the walls and re-install correctly.

    Although it is possible under certain conditions to remediate without replacing all of the tile. To prevent moisture from penetrating through the wall the tile has to be impervious. The grout joints are typically a porous cementitious material, which you can replace with an ASTM C920 sealant such as a 100% silicone sealant. If there are any openings on the wall around fixtures or on top of parapets or whatever you need to seal those up.

    • Donato Pompo says:

      That is a polyurethane coating material. In theory if all of the tile and grout is coated it might be waterproof. The problem is if there is high moisture content under the tile it can trap in moisture that can cause a whitish cloudy affect or peeling. To repair a tile or this coating you have to use a stripper that can be problematic. Plus it does not meet industry standards for a waterproof membrane.

  5. Peter Williams says:

    Hello! Around one part of our outside pool, where there is a cellar/storeroom below, we have water seepage. Our original builders did not seal the surface before they tiled. To that they are not grouted tiles. Is there a liquid (sealant) which we could pour over this tiled are that would find its way through the butted tiles, to under them, and would seal so that water no longer seeps to this are underneath?
    Obviously the best solution is to re-tile, after applying proper sealant and cement, but that would mean an area of 80 m2. Rather an expensive option.
    Any ideas? Thanks in advance.

    • Donato Pompo says:

      As you said, the only way to properly fix the problem is to remove the tile and properly waterproof the area. You are assuming the water is coming from the tiled deck. If the room where the leak is ending up is below the deck that would mean that the walls are adjacent to the earth and if that outside wall isn’t properly waterproofed water can seep in from there too.

      Whenever you don’t follow proper remediation procedures you take a risk that it won’t work. Assuming you have a impervious tile on the outside deck and you said they were butted together, you could cut open those joints to at lease 1/8″ or wider and then fill those joints with an ASTM C920 sealant caulking that is either a silicone or a polyurethane. The sealant will seal the grout joints and if the tile is impervious then theoretically it should prevent water from passing through. Make sure the floor to wall transition is also waterproofed/caulked to prevent water going through that joint.


    We have a 13 year old house. We are looking for bathroom renovations. 13 years ago while building we have done waterproofing. NowWe want to install tiles directly on old tiles. But can we do waterproofing in between the tiles?

    • Donato Pompo says:

      Installing tile over tile is an acceptable installation method. You have to make sure that the original tile is bonded well because the new installation can’t be any better than the original installation in terms of stability. You have to make sure that the added height won’t cause any problems. You have to make sure the original tile is properly cleaned and scarified in preparation for the new tile installation.

      You can waterproof the new installation. After preparing the old tile use the high strength polymer modified thin-set mortar and apply a skim coat of mortar over the existing tile to get a flat surface. Then apply a liquid applied waterproof membrane over the cure skim coat. Then bond the new tile to the cured waterproof membrane. Make sure that the floor to wall transitions are waterproofed. Best to go up the wall at least 3 inches. All transition joints should be caulked with an ASTM C920 sealant.

  7. Gert Rasmussen says:

    We have a saltwater swimming pool that has travertine natural stone pavers surrounding it, including pavers as the coping over the pool edge. Sealed yearly with high grade penetrative and film natural stone sealer. Coping pavers are showing obvious deterioration with dust and excessive pitting and wearing away. Similar on a small minority of non coping pavers.
    Suspect the salt content may be the culprit as it has been excessive for at least 2 years. Impractical to empty pool (70000 litres) and refill given water restrictions.
    Any ideas on maybe other methods of sealing particularly the underside of the coping tiles?

    • Donato Pompo says:

      Often people like to blame stone spalling (surface deterioration) on salt water pools. I have investigated many such cases by taking water samples and performing petrographic analysis on the damaged stones and we find that the percent of salt is very minimal in the water. After all, the salt is converted through electrolysis into hydrogen gas and hypochlorous acid making chlorine.

      The salt water might be a contributing factor, but generally speaking the spalling is due to improper waterproofing the stone during the installation to prevent it from being subjected to excessive moisture. Minerals in the stone, in the underlying mortar and concrete, and the water to which it is subjected to dissolve in the water (like salt does) and the water migrates to the stone surface where the moisture evaporates and precipitates the minerals that expand and cause the spalling.

      There are certain types of sealers that use silicate based materials that claim they consolidate and stabilize deteriorated stone. It allegedly penetrates into the pores of the stone and grows a crystallized matrix in the voids to block further moisture. One product is called PROSOCO Conservare® H100. You can find it at

  8. Bhanupriya Pathak says:

    We live in a rainy and hilly region where it snows every winter. We used grout in order to fix tiles on the roof top of our house. At first it was working alright but once it started to snow, all the grout came out and now the roof is leaking. Please suggest what the best we can do to fix it. Can using epoxy or White cement fix it?

    • Donato Pompo says:

      If you have traditional interlocking barrel roof tiles, the waterproofing is done under the tiles. They use two layers of a roof asphalt paper weather lapped and staggered over each layer. So I assume you don’t have that type if you are using a grout to fill in.

      If you have a flat roof tile that is adhered to the roof it too should have a waterproof membrane under it. Although lets assume you have flat roof tiles that are adequately attached to the roof but you have a leak. The joints between the tiles had a cementitious grout that came loose because the grout is porous and in freeze-thaw weather when it becomes saturated in water then freezes it causes the grout to break up.

      You could add a latex liquid in lieu of water to a cementitious grout and fill those joints so the grout is not porous and freeze thaw stable. Or you can use an exterior polyurethane sealant (caulking) to fill those joints that in effect will waterproof the joint. You have to make sure that the joints are clean and dry before applying the sealant.

      • Pushpesh Pathak says:

        Thank you very much for the information sir. But if under the tiles waterproofing is not done, will the caulking work??

        • Donato Pompo says:

          Using an ASTM C920 sealant (caulking) won’t in itself be a substitute for the lack of a proper waterproof membrane under the roof. Although in theory, if the proper sealant is properly applied to a properly prepared tile joint, the joint will be waterproof as the sealant itself is impervious.

          In theory the only way the roof becomes waterproof in this non-legitimate approach is if the tile is in fact impervious and that there are no other openings or adjacent areas that will allow water to migrate under the tiles.

  9. BobbyA says:

    We have a walk-in shower (vintage 2003) with 2″ square tile white ceramic tiles (& white grout) on the floor. There is a lot of grout with this floor, the grout grows mold or mildew constantly. We live in central Florida, so the house has A/C running most of the year. As you might expect the growth is worse closer to the drain which dries last. We end up scrubbing with bleach about every two weeks when we can’t stand the staining anymore. There had been some kind of sealing paint over the grout lines that was flaking off. I tried pressure washing to remove the rest and get the grout as clean as possible. I applied a similar paint like sealer (slightly elastometric ) product which lasted 5 or 6 months, then began to flake off too. Which of course restarted the scrubbing, that removes even more. Rinse & repeat. We have tried squeegee the walls, and floor to get it to dry faster, and the walkin door is always left open after use. I would love to find a coating that could be applied to the grout, or even a pour over the entire floor (providing it’s not slippery when wet) coating. I’m starting to think a sand filled epoxy might be worth trying. Any suggestions ? Thank you. Best regards, Bobby

    • Donato Pompo says:

      What you are looking for is to treat the symptom of a problem rather than the problem itself.

      A properly installed shower should not develop mold. Chances are the waterproof membrane and drain were not installed correctly and water that gets below the tile surface isn’t able to drain. There should be a waterproof membrane over a sloped surface that is tied into the drain where there are weep holes that are not plugged that allow water to escape.

      So if you don’t want to fix the problem that would require at least removing the tile around the drain if not the entire floor, you could cut out the grout and fill it with an impervious epoxy grout. You don’t use sand. The epoxy grouts do come with an aggregate that mixes into a part A and B liquids.

      This may or may not solve you problem. It might help prevent water to get below the tile if the tile is impervious and thus prevent the mold. Applying a coating is only a temporary solution.

  10. Camille Fenn says:

    Hello Donato,
    I have a new construction all concrete block but my upstairs outdoor veranda floors are plywood.
    It has been tiles with a ceramic tile and the water is seeping in and damaging the ceilings below.
    Under the tile they laid a roofing membrane up to 3” around the edges (corners were weirdly done). They then unfortunately drilled durok concrete boards and applied some Redgard in the joints and edges. The pitch was badly done so a lot of water can sit there after a South Florida torrential rain. I do have the “room” to lay the same tile floor over this one and get it waterproofed and pitched properly . I can send pictures if you’d like but was wondering how you recommend we go about it ? Also if you have a good tile layer to recommend ? We are in Fort Lauderdale

    • Donato Pompo says:

      Balconies over occupied space should be properly pitched/slopped to a drain or to a perimeter gutter. The slope starts at the subfloor. It should have a primary waterproof membrane that is either a hot mop application of tar or a sheet material properly applied over the slopped subfloor. Then a non-bonded wire reinforced mortar bed should be installed. Then a secondary waterproof membrane should be installed over the properly slopped mortar bed and then the tile should be bonded to it. All perimeters should be caulked with an ASTM C920 sealant e.g. silicone or polyurethane.

      You can tile over tile, if the existing tile is structurally sound. If so you could scarify the existing tile and then apply a polymer modified patching mortar over a slurry bond coat of a polymer modified thin-set mortar over the existing tile to fill and slope the surface. The use a liquid applied tile waterproof/crack isolation membrane over the mortar bed surface. Then adhere your tile to the membrane and grout. Make sure you install the movement joints at the perimeters.

  11. Ryan says:

    Hi Donto
    I have a bathroom on the first floor directly above a room on ground floor. I purchased this house about 5 years ago and recently came to know that no waterproofing was done in this house. The bathroom tiles were applied directly above cement and while cement grout is applied between tiles. From last few months I am observing damp walls and water leakage in the room below. What would you recommend should be done in this case.


    • Donato Pompo says:

      The correct thing to do is remove the tile and re-install it properly with a properly installed waterproof membrane.

      The only other thing you could try, on an experimental basis, if practical, is to remove the grout joints by cutting them out with a dustless diamond cutting wheel on a grinder or use a hand grout saw. Clean the joints out thoroughly and fill them with an ASTM C920 sealant (caulking). This type of caulking is either a full strength silicone or a urethane sealant. Be sure to also caulk the perimeter of the tile where it runs into walls and tubs with this same sealant.

      If the tile is impervious and all the joints are filled with this type of sealant, you might end up with it being waterproof, depending on how good of a job you do.

  12. Robert Kelson says:

    Hi Donato.
    I have recently installed a mosaic tile floor in my shower. All installed over a correctly laid and waterproofed shower tray with weep holes around the drain.
    I was about to use an epoxy grout,but am considering using a polyurethane sealant/adhesive instead now.I feel I can get a good finish and penetration through a caulking gun,and any excess could be wiped off with turpentine.Do you think this would give a complete and lasting job as far as wear and mould growth is concerned.

    • Donato Pompo says:

      Installing a polyurethane sealant isn’t the easiest product to work with, but neither is epoxy grouts.

      Using an ASTM C920 sealant whether it is a 100% silicone or a polyurethane will provide some mitigation for movement/stress that the tile will be subjected to. If installed correctly it does give you a water tight joint.

      You need to make sure it is anti-bacterial. Some sealants can stain from a microbial growth.

      Sealants aren’t easy to keep clean. You need to tool the joint to a slight concave surface so feet do not come in contact with the sealant to embed dirt.

      When you install a sealant put painters masking tape on both sides of the joint and then fill the joint and tool it. Then immediately pull the tape. If you wait too long the tape will pull out the sealant. This will make it very easy to clean up and it will give a good looking joint if you do it correctly.

  13. Shravan Mehra says:

    We have put Kotah stone on the roof with a 8 mm gap. The architect had put in a chips mixture in between the stone. However the chips mixture is showing cracks and seepage is now appearing in the roof. We have been advised
    1 Epoxy grout
    2. PU in the the gaps
    3.PU layer over the entire floor
    4. Relaying the roof with bricks

    What do u suggest ?

    • Donato Pompo says:

      I assume you are saying that the grout joint between the stones have cracks and that water is seeping through. Apparently there isn’t a waterproof membrane under the stone or if there is a waterproof membrane it wasn’t installed correctly.

      The correct thing to do is remove the stone and re-install it properly with a properly installed waterproof membrane.

      The only other thing you could try, on an experimental basis, if practical, is to remove the grout joints by cutting them out with a dustless diamond cutting wheel on a grinder or use a hand grout saw. Clean the joints out thoroughly and fill them with an ASTM C920 sealant (caulking). This type of caulking is either a full strength silicone or a urethane sealant. Be sure to also caulk the perimeter of the tile where it runs into walls and tubs with this same sealant.

      If the tile is impervious and all the joints are filled with this type of sealant, you might end up with it being waterproof, depending on how good of a job you do.

  14. robert says:

    Hi Donato,
    I have a newly laid porcelain mosaic tile floor in a shower.
    The base was a preformed tile over shower tray and membraned over
    to a leak control flange.I am aware that you are supposed to notch the
    adhesive channels towards the drain to allow for any sub surface drainage,but
    in this case,the mosaics would have flattened out any such channeling.
    I was going to use an epoxy grout,but reading above, you seem to think an adhesive sealant
    would be ok.Is this long lasting and what are the chances it may mould up?.I think I could
    get good penetration with the nozzle of a caulking gun and smearing wood be minimal.
    Interested in your thoughts.

    • Donato Pompo says:

      First of all, in a shower installation you have to achieve 95% contact with the thin-set adhesive between the back of the tile and the substrate. When you install a tile the trowel ridges are to be collapsed into the valleys between the thin-set ridges in order to get maximum contact. So intentionally creating notched adhesive channels is not a proper installation.

      Using an epoxy grout made for tile is the proper way to grout. You do need to use the ASTM C920 sealant caulking in the joints at the transition areas of floor to wall and from wall to wall .

  15. Bo says:

    Hello Mr. Donato Pompo
    I am trying to DIY to put a waterproof coat on my porch floor- outdoor old tile. Can you recommend a waterproof coat with anti slip? My porch tile floor is 55 SF. Thank in advance for your quick advices.

    • Donato Pompo says:

      We wouldn’t recommend applying some sort of coating over the existing ceramic tile that would make the installation waterproof and slip resistant. Nothing is “anti-slip.”

      If the tile installation is stable, you scarify the tile and apply a polymer modified mortar patch over it . Then apply a liquid applied waterproof membrane over the mortar. Then adhere a slip resistant tile over the membrane.

  16. Michelle says:

    A couple years ago I had a bathroom retiled correctly and beautifully with a membrane, etc. However several of the the joints are cracking now from settlement. As an experiment I tried sanded caulk around the sink, but it cracked. I wanted to use a polyurethane caulk in there because it holds to tile and stretches very well and I want to do the best job possible. Or, should I just use silicone?

    • Donato Pompo says:

      The transition joint between the sink and tile is a joint that will move so you need to use an elastomeric sealant to caulk the joint. The best long lasting sealant is an ASTM C920 sealant that can be either a polyurethane or a silicone sealant.

      They are much more difficult to work with than an acrylic caulk, but the performance is much better and last much longer. Make sure you use painter’s tape along both sides of the joint tightly against their surfaces. After applying the sealant, tool it immediately to the shape you want which is normally with a slight concave surface, and immediately pull the tape off at an angle away from the joint. Otherwise it can get messy if you wait too long.

Leave a Reply

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