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06-21-2019, 10:17 AM
Tile installer trapped between rock and hard place. HELP!!!!
Customer selected Calacatta Gold marble micro-mosaic 3/8 inch tile for shower floor, packed so closely there is not even 1/16 inch grout line. I at first tried to refuse the installation, but tile store insisted "we sell this for bathrooms all the time and have not had any problems." So, I installed it over Schluter system with unmodified DitraSet and grouted with unsanded grout as recommended by tile store. So guess what? Tile now is remaining wet and grey in between showers (virtually daily, with husband, wife and daughter) and customer is unhappy because it is not dry, pristine white like the tile on the floor in the rest of the room.
I have consulted with our local Schluter rep, a CTI, who has advised tiling over existing marble as solution. Customer is consulting with different CTI who says that tiling over tile in a wet area is taboo.
Tile over tile is the ONLY cost effective solution to customer's problem. Tearing out tile over Kerdi sheet is nearly 100% disaster potential.
Tile store and mfr have been contacted and insist that the product is not the problem, that it is an installer problem. I have been searching and have failed to find documentation for either
a) testing that shows that tight-set marble tile is not OK for shower floors
b) testing that shows that tile-over-tile is not OK for wet areas.
Because the existing tile is marble and can bond well to mortar, my plan would be to grind the surface of the marble both to remove the light coat of Aquamix Sealers Gold that I applied prior to grouting to prevent staining from the light grey grout, and to roughen the surface to promote bonding. I would tile directly to the marble as there is already waterproofing under it. In the research studies I've found, absorption percentage for marble seems to be 0.2 to 0.8 %; air cured concrete (e.g. shower mud floor) is 7 to 10%. I have concluded that there should not be a moisture problem tiling over the existing marble even with the tiny, probably only partially-packed grout lines, since the marble is so much less absorptive than the usual mud-filled copper pan installation.
I would be particularly interested in a response from John Koessler, since he discussed this exact shower floor on Dr Fred's tile show under the moniker of 'soggy tile.' In it, Mr Koessler stated that the micromosaic marble "should not be used in wet areas - PERIOD." If this is written 'in stone' somewhere I would love to see it to refute the tile store's assertion that this material is routinely installed in shower floors with no problems noted.
Photo attached of tile installed next to ruler for reference.

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06-21-2019, 03:57 PM
RE: Suitability of marble micromosaic for shower floor

Since my post earlier today one of the CTIs has pointed me to the NSI installation handbook, Ch 13 on wet areas. This document has much of the information I need to go back to the manufacturer and tile store with concrete evidence that the standards-setting organizations (combination of NSI and ASTM) do not consider the tightly-packed micromosaic marble tile installed as suitable for a shower floor.

I also found, for anyone's reference needs, an extremely detailed list of marble hardness/porosity study that was done and re-issued by the Marble Institute of America (now the Natural Stone Institute) in January 2005 as their NSI Bulletin Volume II, Issue 1. 15 pages. Available as Adobe download.

Tool Guy - Kg
06-21-2019, 04:00 PM
3/8” mosaics with many that look like they are touching each other?

Oh, boy. I can't believe the store gave such advice. They should know better than that.


06-22-2019, 08:50 AM
I am not sure how all the recommendations makes you feel ...... But if marble in a shower is in question , i would ask --
Is your waterproofing System compatible with it , or careful evaluation of materials to be used is a must ?!

Dave Gobis
06-22-2019, 10:03 AM
NTCA has a white paper on this subject. Has been a problem for years.

Karls tile Inc
06-26-2019, 04:52 AM
They always blame the installer even if you did everything exactly to spec. I try to talk people out of natural stone for shower floors especially white and gray stones like carrera and calcutta gold. Good luck I have been in your position before.