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 and 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.

Answer Thanks for reaching out to the NTCA Technical Team with a great question.

As you know, TR420 is a TCNA method designed for repairing a shower pan. I have confirmed there are no changes to this method in the 2024 TCNA Handbook. A committee is reviewing this method.

That being said, this can be a very difficult method to achieve for a few reasons. One is that renovations in general are more difficult due to the fact that we don’t know what we’re going to find when we open the walls and remove the shower pan. Another reason would be because we don’t know the quality of work that was done and we are now being asked to fix.

Method TR420 is designed to help give a path on how to repair a shower that utilizes a new shower pan as well as a new bonded waterproof membrane (sheet membrane) on top of the mud bed while using a three-piece clamping drain.

There is currently no method showing how to repair an existing tiled shower using a bonding flange drain. What you are describing is a mix/hybrid of two different methods. (as an example – B415 walls + B422 shower pan).

I completely understand your reasoning for using a bonding flange drain, but you would be outside of a TCNA method, and would need to rely on written manufacturer instructions for how to properly achieve this repair.

One thing I think many contractors and installers may agree on is that when a shower is leaking, it is a best practice to do a full tear-out to guarantee and warranty your new work. Any time we are doing repair work as tile contractors we need to be very cautious about taking ownership of someone else’s work or offering a warranty on work that someone else performed years ago (i.e. in the case of you repairing the pan but the walls are original). Taking on this type of repair work would likely come down to a business decision. How much risk/liability is your business willing to take on? The answer to that question will vary from business owner to business owner.

I hope this helps add clarity to this method. If we can help in any other way, please reach back out.