Here is a picture of the Redgard.

Here are a few things you can reference:

1) The PDF for RedGard.

Reference 3), Suitable Substrates. There is no mention of any type of gypsum-based wall board being suitable. For clarification, it mentions "gypsum based cement topping" but that is gypcrete, a lightweight concrete-type mix. It is in no way related to gypsum wallboard.

2) I'm no expert on USG gypsum panels. This will probably come across as being a bit negative, but the reason is that they continually change the makeup and name of the different boards, pass them off as being suitable for certain applications, then when they fail 10 years, 25 years, or 20 years down the road, they reformulate the product, rename the panel, and start the game all over again.

I recommend you call USG and ask. I'm fairly certain that the MOLD TOUGH series is considered by the manufacturer as NOT SUITABLE for tub and shower surrounds.

If USG doesn't say that they are not suitable, but defers to TCNA standards, then they are not suitable.

In my world? It's not suitable. I'm a basics kind of guy. Cement board. Wonderboard or Durock in wet areas.

Good luck with your project.

USG contact info: for technical assistance please contact 1-800-USG4YOU (874-4968)

No, it's not an appropriate substrate. Just because he's been doing it that way for 15 years doesn't mean that he's been doing it right. It should be ripped out, and replaced with a cement board substrate.

If it meets ANSI 118.10 standards for a "Surface Applied Membrane," you should be good to go.(and I believe it does) Three applications ought to be adequate.

Thank you very much for your input.

If I ask him to replace the drywall with Durock, can he do so without removing the new shower pan? The pan was done properly and floor tile has already been installed over the pan.

Another option I thought of is to request another coat of Redgard or for Kerdi to be applied over the Redgard. I'm not sure how effective this will be where the walls meet the floor, which is where I assume most problems occur.

Out of curiosity....Did he replaced Greenboard with Durock? I'm asking because if anyone would read specs of Greenboard ... it says clearly "This panel also qualifies as a

water-resistant backing board for ceramic tile in residential tub and shower enclosures as permitted in the National Building Code of Canada 9.29.10.4". All you need to do is use waterproof membrane like Kerdi or RedGard. Even if they fail in 20 years tiles will look ugly anyway.

sure but as you quoted:

"Not suitable for use as a substrate for tile in wet areas such as tubs and showers, gang showers and other areas subject to direct water exposure"

There is no direct water exposure when board is properly sealed. RedGard waterproofing is 0.6 MPa so properly applied coats shouldn't leak any water.

You could easily multi-coat as this guy did:

It doesn't matter after you apply the coats of regard what substrate your using. Today's gen is quick to throw the tcna handbook at you cuz they don't understand or see the real working picture.
Regards when applied correctly is a waterproof liquid membrane no air no water will pass so that being said definitely no thinset will pass either.
The only judgement here could possibly be the greenboard moving with the Weight of the tiles on it. But that’s why you have to apply even more redgard to the seams.

"It doesn't matter after you apply the coats of regard what substrate your using."

In correct, it does matter according to building codes. Within the wet zone these types of products are not permitted to be used as a substrate to support the application of tile. Regardless of the applied waterproofing membrane the supporting backerboard must meet specific performance requirements of which a minimal level of water resistance is required amongst other requirements.

Well maybe I should become a kitchen designer this way I’ll automatically know all there is to know about waterproof substrates tile and how bathrooms are made.

Maybe you should take more care in your advice and how you word things. It is inappropriate to make statements that are fundamentally incorrect and by making such it is evident that your understanding of building codes, materials and their application is severely limited.

The comment you made concerning the fact that it does not matter what type of material is utilized as a tile backer board in wet zones because the applied waterproofing prevents the tile backer board from water exposure is clear evidence that you do not understand all of the related issues and the reasoning behind the building code requirements.

This does not mean you can't express your opinion, it's just that when doing so you should be clear that it is just your opinion versus implying it as factual.

Your from Canada I’m from New York you can do the math. There maybe building codes for your area but in the state there next to none for redoing finishing walls with tile.

Not sure what math I need to do.

You may not be looking at the appropriate code(s). If you are in the city of New York you will most likely need to refer to 3 codes in order to fully understand all requirements. Suggest starting with the IBC as this has been accepted by many regions as the fundamental building code for which other state and city codes are based upon.

" It doesn't matter after you apply the coats of regard what substrate your using "

100% innacurate , not true.

I'll bet that someone doesn't even know what a mil gauge is, much less own one.