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I have to redo my tub surround due to moisture leaks and failing grout.

One of the company says they are going to apply kerdi membrane on the drywall, then tiles onto the kerdi membrane, and as such as cement backboard would not be required.

Just wondering how effective is the tile on kerdi membrane vs the traditional cement backboard where tiles are glued directly to the drywall?

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    Cementboard sufficient to a tub surround is dirt cheap .vs. the cost of the tile job. But that doesn't address your question, so it's just a comment, not an answer as such. In my opinion based on my experience I will not apply tiles without backerboard. I have no experience with "kerdi membrane" but I have met enough failed tile over drywall to never want to go there when $40-50 worth of backerboard will give me a substrate I can depend on. – Ecnerwal Feb 13 '15 at 1:14
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    The Kerdi is essentially replacing the backerboard. Kerdi's systems are fairly reputable. – DA01 Feb 13 '15 at 19:59
  • There is a product called Kerdiboard you might want to look into. It's a lightweight polystyrene board faced with Kerdi that comes in various thicknesses that you can use in place of drywall or cement board. It installs with screws, then you tape the joints & screw holes and install your tile. It costs more but it;s worth it for the ease of install IMO. You probably have to go to a big box store's special order desk or find a specialty tile supplier to find it. – Sean Jan 5 '17 at 20:09
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First Kerdi can be installed on drywall or cement board. However if installed on drywall it is only protecting one side. The back side of the drywall fails and it is just as bad or worse than if it had an issue on the tile side.

All shower/tub areas should have concrete board (I prefer hardiboard). Whether or not you need a Kerdi membrane is a factor of use. In normal residential use I wouldn't worry about it. Backerboard plus a waterproofing membrane like Redgard is all that is needed for tub surrounds with thinset then tile. Kerdi really offers very little advantage over hardiboard plus membrane.

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    You need to have some sort of waterproof membrane, tile grout and backerboard are porous. @DMoore commemnt: "backerboard is all that is needed for tub surrounds with thinset then tile" would be correct if he had included a layer of waterproof craft paper (hung horizontally and overlapping from bottom up) behind the backerboard. – Jimmy Fix-it Feb 13 '15 at 17:38
  • @JimmyFix-it - hardiboard is not really porous unless it is completely saturated. We waterproof the outside the hardiboard in 99% of the install with something like redgard. craft paper or plastic is great if the inspector wants it but it is (should be) doing nothing for a tub surround that has HB/redgard/thinset/tile. – DMoore Feb 13 '15 at 17:43
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    @DMoore- OK, I agree that with redguard it would be redundant. However, the answer you provided above suggests that the OP could omit a waterproof membrane, that I do disagree with. – Jimmy Fix-it Feb 13 '15 at 18:06
  • @JimmyFix-it - You are right. Will add it in. I think it is engrained in me to do that so I forget to mention it. – DMoore Feb 13 '15 at 18:09
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As a contractor, I have been using Kerdi since it came out. I always install it over cement board or backerboard, never over drywall, even blueboard or the mold resistant greenboard. Redgard is a similar product. If done according to manufacturers instructions, it will seal nicely. Important to overlap seams in a shingle method. I've never had a failure. Even the tiling subs have moved away from mortar walls because of the labor cost and lack of craftsmen in the industry.

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There's a really great manual (with diagrams) that makes this all easier to understand. You absolutely need to include some sort of substrate (cement board, green board, regular drywall) with the Schluter-Kerdi waterproofing kit. The waterproof membrane isn't a substitute for the substrate. It is a topical waterproofing layer that is put over the substrate to keep water from getting to the wood frame and nails. My understanding is that Schluter-Kerdi, being a sheet topical membrane, recommends just going with regular drywall instead of cement board but you can certainly use cement board as the substrate. It doesn't matter but you absolutely need to use both a substrate and a waterproofing membrane. The site is called floorelf.com. The manual can be found here: http://floorelf.com/waterproof

I would say unless you are experienced in working with Schluter-Kerdi waterproofing membrane kit, then I would say work with an easier waterproofing approach, either liquid waterproofing or a combination of liquid and traditional waterproofing (plastic covering the wood frame). Here is a video off the Schluter-Kerdi website that describes how to apply the waterproof membrane on top of the substrate (gypsum board in this case): http://www.schluter.com/schluter-us/en_US/Shower-System/KERDI-SHOWER-KIT/Schluter%C2%AE-KERDI-SHOWER-KIT/p/KERDI_SHOWER-KIT

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Out of curiosity, why are any of you suggesting backerboard at all? that's home depot homeowner installation method. Any competent tile contractor would float the wall in mortar giving you a stronger, level, perfectly flat, and square surface to properly install tile. If your going to do the job do it right, and give professional advice pls. The Kerdi/Daltile TS underlayments do have some valid applications such as in steam shower installations where a bullet proof vapor barrier is required.

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    Hi David. Welcome to the site. #1 - Backer board has nothing to do with HD or any other big box. Mike Holmes uses backer on his show. Actually if you watch any pros work (not on crappy shows) on TV you will see them use backer board 99% of the time. #2 - Your suggestion has plenty of merit though. A mortar wall is better and can be customized easier. #3 This is a DIY site. We don't expect people to master this and in fact I know maybe two guys in my area (live in big city) that could do this. #4 You could be adding a good 1000 pounds to your install too. – DMoore Jun 30 '16 at 3:25
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    But I can't give you a +1 because telling someone to mortar all the walls in a large shower is like telling them to make an ice sculpture. You are telling someone "to have" an artistry that not only takes art skills, knowing how to mix mortar for different situations, and a good amount of practice. I have watched it done 10-15 times, have good skills, and if I tried to make large mortar walls myself it might like a little kid threw up concrete... – DMoore Jun 30 '16 at 3:31
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Not everyone knows how to do mud and lathe it's so old school plus it wears you out, ive been a setter for 27 yrs and I have yet to go back to one of my showers I installed using perma base or durock, with a liner, i have started using waterproofing such as redguard or aqua-defense

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