Hybrid shower? Kerdi membrane on CBU with traditional pan on floor

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chrissyp

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Contractor wants to do CBU with Kerdi membrane on walls, but a traditional mortar shower floor (liner sandwiched between two layers of mud). Any thoughts on tying these two systems together? Any concerns or things to look out for? Any advice is much appreciated.
 

Jadnashua

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Not a great idea. Keep in mind that while cbu is not affected by being wet, it DOES get wet, and some of the can wick moisture up nearly a foot from your damp pan. Your mudbed above the liner will be damp...that's natural. Industry standards direct when using a surface applied waterproofing, it be a system, not a hybrid, and that the waterproofing extend up at least the height of the showerhead so that no water can get behind it and get trapped.

FWIW, Kerdi works just fine when applied over a single preslope layer of mud, but you need to use their drain. When you do that, the entire surface the tile is applied to is waterproof. The pan sees the most water...why wouldn't you want it waterproof? Otherwise, you'll have in excess of an inch of damp mud beneath the tile. Yes, when properly installed over a liner, it will weep any moisture to the drain, but with a bonded membrane drain, you just have thinset, and no porous deckmud to absorb moisture. Moisture doesn't penetrate all that far, and generally, evaporates in between uses. ANy small amount of moisture that does get beneath the tile on a traditional pan, gets drawn into the porous deckmud by capillary action and gravity.

Ask him to show you a TCNA procedure that says to do it that way...that's the industry bible if you want a tiled shower to work right.
 

rubadubdub

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I think the question I was about to ask is similar, so I'll ask it here instead if its ok. I have a traditional walk in shower built up to the final deck mud so far. Once I fix any inconsistencies in terms of dips/humps on the floor, I was planning on applying regard on the walls and floor. I already applied regard to the bottom part of the CBU that got buried by the mud. Is this similar to the hybrid system described above? If not, is what I planned on doing fine?
 

Jadnashua

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If you go to the www.custombuildingproducts.com website, you can download their installation guide on how to waterproof a shower using their products. Trying to reinvent the wheel is dangerous, since you won't have the experience and testing that the manufacturer has spent literally millions of dollars on over the years. This is a good start, but there are more documents and their Tech SUpport is good, too...
http://www.custombuildingproducts.com/media/59556507/sis_brochure.pdf

Any time that you mix different methods, you are essentially moving into uncharted waters. Things don't always play together well, often with unpredictable results. It's best to follow one method for the materials selected, and follow through exactly. It's not necessarily hard, but it is exacting. Most people don't want to read the novel, and want the short overview...that's dangerous when you're spending lots of time and money, and want it to last a long time without problems!
 

Atomic1

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Uncharted terriotory. Could probably work with proper detailing. I have seen this setup work with densshield instead on the walls with a proper gap between the wall and base.
 
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