Slate Tiles Sound Hollow and Grout is Cracked


Slate Tiles Sound Hollow and Crack is Cracked - We purchased a house in Florida and renovated it. We have numerous places on our newly laid honed slate floor that have a hollow sound under parts of the tile (16"x24"). Usually at the corner. The tile is laid on a concrete floor that previously had ceramic tile on it. We have experience with slate in other homes and have never experienced this. We were told by the tiler and contractor that this is normal for slate but I am finding this hard to swallow. The grout is now coming out in the corners and areas where the tile sounds hollow. I noticed this when I was doing a through clean of the floor after moving in. As I wiped over the tile I could hear how it sounded different. I taped on it and it sounded hollow. As I cleaned I noticed that this occurred all over our 1000 sq. ft floor. I am concerned even though my contractor says it is normal. Should I be worried and if so what should I do about it? Hope you can enlighten me and give m e any advice on how I should proceed with this issue. I love my floor and would like to see it last. We have not made the final payment on our renovation due to a few outstanding issues but this issue scares me the most. Thank you for your time.

Response to First Answer below:

The tile installer came and looked at the issue yesterday. My contractor said he will come and look at it tomorrow. The contractor said they are planning to inject an epoxy under the tiles (over 60 of the tiles seem to have hollow sounds and about 30 of those have the grout starting to come out) to fill the hollows on Monday. My contractor said this is the solution to this problem. Here is hoping he is right.

Response to Second Answer below:

I know you haven't seen the job but we have gone through all the tiles and have found over 60 tiles with issues. About half of those have issues with the grout (mostly in the high traffic areas). If the epoxy is a cosmetic fix then what do you do to fix it properly. Do we have them lift the affected tiles and re mortar underneath? I do want this fixed properly. I just want to know what I should be asking for from my contractor.


First ANSWER - Just because a slate tile sounds hollow doesn't necessarily mean there is a problem.  It might be a reflection of the configuration of the tile assembly or substrate beneath it.  Although on the other hand it could be a symptom of a problem.

The slate tile might have voids under the tile that are causing the corners of the tile to sound hollow.  Slate is a cleaved layered material, so sometimes the slate can naturally be cleaved within its body that could give a hollow sound.  The hollow sound could be an indication that the tile is not bonded at the spot or area.  If could be adhesively debonded between the tile and substrate, or between the topping material and substrate, or cohesively within any of those materials.

Per your description you said the grout is cracked at some of these hollowing sounding tiles.  Looking at the photo you sent it appears that the tile may be loose and moving to some degree.  The cracked grout appears to be resultant damages from whatever is causing the hollow sound in the stone.

Grout should not crack.  The movement in the stone could be an indication that the tile has debonded at those hollow locations.  It might be an indication that the substrate beneath the stone is not properly constructed.  There may be too much deflection in the substrate.  If the substrate is a backer board then the backer board may not have been adhered and screed down properly, and/or the sheets of backer board was not staggered as they should have been, and/or the joints between the backer boards were not properly gaped, filled and taped.

The hollow sounds are only a possible indication that there are problems underneath the stone.  The cracked grout suggests that there are underlying problems.  The only way to determine what is the problem, the extent of the problem, and how to remediate the problem is to perform an intrusive inspection by removing various tiles under various conditions.  If it is practical to do so, you might consider retaining a forensic investigator like CTaSC to perform an inspection.  Click Forensic Investigations for more information.

Second ANSWER to second question above - Trying to fix the problem by injecting epoxy to fill voids in hollow sounding areas of the tile and to stabilize loose tiles is treating the symptom rather than fixing the problems.  I have seen this attempted many times, and it never fills all of the voids and never fixes the problem.  It normally buys them more time hoping the problem goes away.

Third ANSWER to third question above - there is no way to know for sure the extent of the damage with an intrusive inspection, but if you have cracked grout that likely means that tiles are loose.  It may or may not have anything to do with the tile bond to the substrate beneath it.  It might be because the substrate under the tile is defective in some way.  So injecting epoxy wouldn't help in that case.

There is a good chance that the tiles that are not hollow or loose now might become hollow and loose in the future because they might have the same underlying problem as the loose tiles.

The only way to know is to perform a forensic inspection by removing various tile under various conditions to see what the cause of the problem is, so the problem can be remediated rather than trying to remediate the symptom of the problem.

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