Why able I getting Hairline Cracks in my Carrara Marble Shower?


I have a marble shower in which the wall marble tile is 12"x24" x3/8" cararra marble.

We are getting many "smaller than" hairline cracks in the marble in a half a dozen places. You can barely even feel them.
It's a large shower. The walls are 6' wide and 9' tall.

Any thoughts?


ANSWER - Generally speaking cracks are either reflective cracking where the underlying substrate has a crack or separation and it telegraphs through the marble or there is structural cracking where the structure has excessive deflection or other structural issues.  Or it is a compressive crack where the stone is not fully supported with the thin-set adhesive that leaves voids and there is a load applied to the stone causing the crack where it isn't supported.

Since you said it is smaller than a hairline crack and you can barely feel it, it leads me to think that you might have what we call indent fractures.  Typically this is caused by excessive shrinkage in the adhesive, if it is applied too thick, or within the substrate if it develops shrinkage cracks for one reason or another.

Initially indent fractures don't actual crack the surface. It is a very slight depression.  If you put a straight edge over it and shine a flashlight behind the straight edge you will see a small gap at the indent.  Typically you can only see these indent fractures from an angle when light shines on it, otherwise it isn't notice able.

Indent fractures can develop into actual cracks if it is subjected to enough stress since it is a weak point, but often they don't crack.  If it is an actual crack then you have to remove the tile and look for the evidence to explain what caused the crack, and the determine how best to remediate the problem.

7 thoughts on “Why able I getting Hairline Cracks in my Carrara Marble Shower?

  1. Lauren says:

    We just finished a long reno of a second floor bathroom (it was gutted). The floor tile is called a zebra marble (it has vey straight gray and white stripes running through it) and looks beautiful. However, during the course of the reno we started to notice that people who were working in the bathroom were leaving unsightly chips in the tile, which was alarming. That was the least of or problems, apparently. About a week ago I noticed what looked like a crack running from the exterior wall through the tile (the crack is underneath or inside the tile as there is nothing on the surface). Over the course of a few days (and to my horror) the crack extended through five more tiles running almost the entire length of the floor. It’s crazy—the crack runs a path and crosses grout lines, but no grout is cracked (at least on the surface). Another crack has also extended outward perpendicularly to the first, starting at the vanity. The contractor of course said he has never seen this happen in all his years in the business, and is coming to look at it with the tile setter in a couple of days. I am in shock as we just finished the project and the thought of everything being ripped out makes me want to cry. There is an electric radiant heat pad installed underneath—not sure if that has anything to do with it but it will obviously be destroyed when the tile gets taken back up. I have been reading some of your posts on indent fractures but although you can’t feel these on the surface you can easily see the big long crack as soon as you walk in, so nor sure it’s that. Project was started in June and recently completed (it took forever!) and I also wonder if the change of outdoor temp from hot to cold comes into play, although this has never happened before. We have a radiant floor system in a bathroom in the basement (porcelain tile) and the temps fluctuate much more down there, and we do not have a problem with it. I’d love your thoughts!! Thank you!

    • Donato Pompo says:

      If you see what appears to be a crack in your stone floor, but when you touch it, you can’t feel it, then it is an indent fracture. Normally the crack in the floor is only noticeable under certain lighting conditions. If you put a straight edge over the crack and put a flashing light behind it, then there should be a slight gap further indicating it is an indent fracture.

      Normally indent fractures occur from excessive shrinkage in the thin-set mortar adhesive or the mortar bed below it. Often the excessive shrinkage is due to too thick of thinset or mortar. Once the cracking occurs in the underlying materials then the crack will telegraph up partially into the tile which creates the indent condition.

      Eventually if the tile is subjected to a certain degree of stress, the intent fracture might turn into an actual crack.

      The only way to know for sure what is causing the condition and to know how to remediate it, is to have a tile expert carefully remove the affected tiles to look for the evidence.

      Normally stone will only chip if something heavy and sharp is dropped on it. So that isn’t normal.

  2. richard robinson says:

    We have laid a 8×20 dolomite marble in a bath and closet. Almost immediately we started seeing straight white lines that eventually turned into fine cracks. It so happens that there is a 3×5 grid pattern of cracks which, you can guess, is the location of the hardibacker seams. After reading your comments above I think it is clear that we have reflective cracking. Do you think this is indicative of lack of tape at the seams, lack of gapping between hardibacker sheets(do they really need them?), or could it also be structural deflection(it is over crawlspace). The tile sub is alleging that he did everything right, but we have yet to conduct a floor inspection. We were originally hoping that the tile vendor would reimburse us for the material, but they are the ones asking for the tile investigation report. I would love to know your thoughts.


    • Donato Pompo says:

      Obviously the marble wasn’t installed correctly or it would not crack.

      The fact that the cracks are every 3×5 feet the same dimension of the backer board it is likely they will line up with the backer board joints.

      The cracked marble isn’t the problem, it is the symptom of the problem. The marble doesn’t crack unless it is subjected to some sort of stress. Chances are they didn’t fasten the backer board down per the manufacturer’s directions. It needs to be screwed down at least every 8 inches and not closer than 3/4″ to the edge of the backer board. The board should have thinset mortar applied to the back of the board before it is fastened down with screws. The boards should have been stagger to the adjacent boards if there are adjacent boards. There should be a 1/8″ gap between the boards filled with the mortar and taped with 2″ wide alkali-resistant glass fiber tape and filled with mortar.

      The subfloor can’t have more deflection than L/720.

      Chances are they did more than one thing wrong that has led to the cracked marble condition.

  3. CC says:

    New construction (less than 1 year) white herringbone pattern, marble floor. It started with a very fine diagonal crack shortly after we moved in across a few of the 3×8 marble tiles, and now spans the entire bathroom. Builder wouldn’t respond. Engineers who designed the foundation did a structural check and said foundation looked great and the crack was due to deflection and no underlayment on the concrete slab and put that in writing in his report. Are we safe to replace those tiles? Should we be worried? Makes me sick thinking about this. Thank you for any thoughts.

    • Donato Pompo says:

      If this is an installation over a concrete slab substrate and the crack is in fact a reflective crack from a crack in the concrete slab that spans the entire bathroom floor, the question is why is happening now?

      Perhaps the marble installers bridge over a pre-existing crack? If the crack is straight, then maybe the installers installed over a control joint in the concrete slab?

      Normally cracking in a concrete slab occurs relatively soon after it is pored, unless there was some sort of seismic movement.

      If this is an on-grade concrete slab there wouldn’t be deflection. If it is a suspended concrete substrate then it might not have been properly designed or constructed to limit the deflection to L/720 as required by the standards.

      In theory, you can remove the cracked tiles and install a crack isolation membrane to isolate the cracking and bond the new tiles to it. There should be adequate movement joints in the tile perimeters and every 20 to 25 feet in each direction.

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