Will the Wet Spots on my recent Marble Slab Installation go Away?


My contractor has installed moon face marble slabs approx 6 ft wide and 10 ft tall in my office entry. There are numerous unsightly wet spots still visible since installation 4 days ago. I’m told it’s the plaster leaching thru and will dry. Is this reasonable?


ANSWER - If you have a number of wet/dark spots on your newly installed marble slabs, it may be that the marble slab was installed in a spot bonded method.

If the marble slab was installed on a wall, it is possible that they mechanically attached the marble to the wall with wire.  When they do that they might use a plaster or a thin-set mortar to around the mechanical attachment to stabilize it and to allow then to adjust the levelness/plumbness of the slab on the wall.

If the marble slab was installed on the floor, then the spot bonded method is not considered an appropriate installation method.  This is because it leaves a lot of voids under the marble that would have a propensity to be damaged if those unsupported spots where subjected to some impact or heavy load.

A wet a thin-set mortar or plaster could transfer moisture to a stone and cause it to turn dark at the point of contact, but you would expect it to dry and go away.  Normally moisture will not migrate through a true marble because it has such a low absorption of 0.2% or less.

I have seen some cases where a certain type of mortar/plaster or mastic was used for the spots and it actually stained the stone where it was not removable.  This is a case of incompatibility between the adhesive and stone.  The installer or specifier of the adhesive should have verified the compatibility of the products.

The only think you can do now is wait and see if the spots diminish or go away.  I would put a measuring device next to them and take photos with dates on them, so you can compare them in a few weeks to see if they have changed.


14 thoughts on “Will the Wet Spots on my recent Marble Slab Installation go Away?

  1. Angelo says:

    Thinking of using marble tile for walls in a shower stall. It’s a cottage in Muskoka Ontario. Plan would be to seal the marble. My question is, what is the effect of natural lake water on marble vs treated city water.

  2. Donato Pompo says:

    Natural Lake Water chemistry will vary from lake to lake depending on its water source and what it has been subjected to. Same with treated city water as we know from recent situations where city water can be contaminated due to various situations.

    Some water will have higher concentrations of minerals and is considered hard water that has higher concentrations of calcium and magnesium. Hard water can cause buildup in pipes and can leave residuals on surfaces that need to be cleaned more frequently.

    Best thing to do is have the water tested. You can also experiment with it on samples of the material you want to use.

  3. Margaret Dowd says:

    We recently had our shower redone with white and gray Bianca marble tile. The tile is now turning a darker gray near the drain area and also two brown spots have appeared. The contractor says we must have used some cleaner but all I used was a wet rag. He says he has never seen this happen. Help

    • Donato Pompo says:

      Chances are that the darker gray area near the drain is due to the installer either did not protect the drain weep holes to allow water under the stone to evacuate into the drain or he used a drain that did not have weep holes. Only way to fix it is remove the stones around the drain and unplug the weep holes.

      The brown spots are likely rust spots. White marbles typically contain iron sulfide minerals that can cause rusting if they are exposed to excessive moisture. Or there could be a metal attachment beneath the stone that can cause rusting. If it is surface staining it can normally be cleaned with a poultice. Best to have a professional stone restoration person do the cleaning.

      • Jim says:

        Hi, we had Carrara Marble installed throughout our bathroom remodel. Unfortunately before completion, one of the new shower diverter valves leaked behind the shower wall when the temporary cap failed (probably for more than 24 hours). Now we have darker floor tile (Carrara Marble) all around the exterior of the shower curbing. I’m guessing water got below the shower pan, above whatever cement they applied, and above the subfloor. We have the ceiling ripped out below the shower and have been drying for a couple of weeks with fans and dehumidifiers (for the first week). I think the subfloor wood is dry. We also have fans blowing on the top of the floor tile, along with turning on the new in-floor heating (electrical). So far the darker tile has only expanded its reach, though perhaps it has stopped advancing in the last few day. The flooring contractor advises it may take a month, but it will dry out. The restoration company is mildly but not overly concerned about mold, because it is all new construction (we were down to the studs with the remodel) and it was clean water. Initially after the leak, we also had similar dark tile in the vertical wall of the shower underneath and adjacent to the leaking valve but that has virtually disappeared (most likely gravity and we have the back of that wall ripped out with fans drying it (it’s dry there now)). Thoughts? Suggestions? — Many thanks for any advice or insights.

        • Donato Pompo says:

          Carrara Marble should be very dense and absorb more than 0.2% per ASTM C503 standard. But it can absorb a little bit.

          Normally if marble gets damp it can darken a little, but it should dry out. Unless it absorbs a stain from something. If the water comes into contact with one of the other underlying materials it could possible pick up a stain that can be absorbed into the marble. When the marble dries the moisture evaporates and the stain could stay within the marble.

          This isn’t typical. Marble is used all the time inside showers and it doesn’t cause staining. Although sometimes marble can show some yellowish stain from the natural occurring iron sulfide (pyrite) that marbles often contain. This stain normally occurs if the marble is subjected to excessive moisture for a prolong time; if the marble has a higher concentration of the pyrite.

          I would not expect it to take weeks for a damp marble to dry out. It depends on temperature, humidity, wind and etc. to determine how long it takes to dry. If it is a stain you can try using a poultice to draw out the stain. There is no set method for removing a stain. It requires and trial and error approach.

          • Jim says:

            Many thanks. I think we will wait and see what happens before removing the flooring. Do you think removing some of the grout might help drying, perhaps especially along the joint between the shower curbing and the floor?

          • Donato Pompo says:

            The joint between the shower curb and floor should be filled with an ASTM C920 sealant (caulking) as that is a transition joint.

            If the marble is installed over shower floor and it is remaining dark, it might be due to the weep holes at the waterproof membrane are plugged. They are suppose to be protected so excess water can drain away. The membrane is suppose to have a slope to drain of 1/4″ per foot. If those conditions are missing then the mortar bed might be retaining excessive moisture and isn’t able to full dry.

            Only way to determine that is to remove some stones and inspect those underlying conditions.

  4. Cilla Zosa says:

    Hi! I have a Carrera marble bathroom countertop. I now know it’s not sealed. A dark grey ring formed I believe from water sitting on top under a soap holder. How long does it tak to dry out, and is there something I can do?

    • Donato Pompo says:

      People tend to refer to marble as a soft stone, but it is far from it. Per the ASTM C503 standard the minimum density should be 162 lb/ft3 and the minimum absorption is 0.2% which is below the minimum absorption for being impervious.

      Although marble being a a calcium carbonate material is susceptible to scratching and sensitive to acid that can etch its surface. Chances are the dark grey ring isn’t a damp spot because the marble is impervious, but it could be an etched condition. Etched meaning it chemically altered the material. So it should feel like it has some texture when you rub your fingers across the spot.

      The good news is natural stone typically can be restored to being like new. A professional stone restoration company can deep clean, grind and re-polish the marble. Sealers can help, but they won’t prevent etching or staining. What is important is that you wipe up any spills right away. The soap holder should be set on top of a cloth to protect the marble.

    • Donato Pompo says:

      Do not use bleach for cleaning the marble. It can bleach and etch the the marble surface that would require repolishing the marble to repair it.

      Use a neutral detergent in water to clean the marble floor. Use a soft bristle brush on an extended handle to scrub it clean. Use a wet vacuum to pick up the dirty water. Then rinse the the floor with clean water and pick it up with a wet vacuum. Then buff dry the tile with a lint free cloth.

      After the marble tile is clean and dry apply a penetrating sealer. Be sure to apply the sealer with a lint free cloth and immediately after applying the sealer, buff it dry with a lint free cloth not leaving any damp sealer on the surface of the tile.

  5. SARFRAZ says:


    • Donato Pompo says:

      To determine why some of the granite tiles turned dark would require performing a forensic intrusive inspection to evaluate the underlying conditions.

      Granite is normally impervious and do not readily absorb moisture, but here are some commercial granites that are not true granites that are more porous. Depending on whether the stone was installed and the application could suggest the stone is being subjected to excessive moisture depending the underlying conditions. Whether it is installed in an exterior application or an interior application in a wet environment or depending whether there is a vapor barrier under the house foundation can be factors. You can’t determine how to fix it until you have determined what caused.

Leave a Reply

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