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Stone tiles in showers – precautions

Question

We have done a high-end condo project with roll-in showers. The showers, pans and bath floors are dry-pack mud jobs. We then used two coats of roll-on waterproofing and crack-isolation product 6″ up walls. Flooring and shower pan material is Asian Carrara marble mosaics. Tiles are sealed with a penetrating sealer. After six months of use I am getting some complaints of darkening tiles. The moisture is wicking its way up the pan and onto the main floor. I assume this is capillary action. Any thoughts? The 1″x1″ mosaic has epoxy grout. See attached. 

Answer

Thank you for contacting us. I have received many emails and pictures similar to yours over the years. This has become a real issue and has prompted the members of the Technical Committee to include a precautionary letter in the NTCA Reference Manual, Chapter 11 – Precautions. 

What I can tell you is that at this time the cause, prevention or correction of this type of discoloration is unknown. We know discoloration similar to this can happen when the installer uses industry-recognized standards, methods, and best practices. There are many different opinions on why this could happen. We are encouraging our members to send the precautionary letter to potential customers who are considering natural stone in showers. 

If a customer is determined to use natural stone, six-sided sealing of that stone can reduce the chances of this type of discoloration. Chapter 8 of the NTCA Reference Manual discusses six-sided sealing and its benefits in water management for natural stone installations. 

I know members of our Technical Committee have discussed these issues with some of the leading setting material manufacturers, who agreed it was a problem and offered to provide funding to research the issue. However, I’m not sure if any research has been done and what results they may have. I have reached out to Pavlov Starykov. He is a tile contractor who has done some experiments on these topics and I believe was working with some setting material manufacturers on this issue. 

Pavlov is a Certified Tile Installer has advanced certifications with the Ceramic Tile Education Foundation (CTEF). He is a member of the NTCA and a Certified Tile and Stone Inspector with Stone Forensics. He is one of the most researched and educated people on the topic and he is included in this response.

Attached is a link to an article he wrote on the subject in TileLetter Magazine, July 2020, entitled: “Marble moisture discoloration: don’t blame the stone!” https://www.tileletter.com/marble-moisture-discoloration/. I hope this information helps.

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