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Unread 07-20-2018, 10:18 AM   #1
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New kerdi shower 3ft x 3ft shower install, drywall or cement board?

Well after so much good advice and design ideas from everyone, I'm finally tackling my first shower. I'm actually doing the work for my brother in law. It's a small 5x7 bathroom with a small 3x3 standup shower stall. We have it ripped to the studs right now and the plumbing is getting done today. We are going with the full 38x38 kerdi shower kit. I'm getting conflicting advice on whether or not to use drywall or cement board. Some say just use drywall as its the kerdi is a complete water proof system and duroc would just be wasting money. How about that yellow drywall that is super water resistant on both the walls and ceiling? He doesnt want the ceiling tiled. We actually submerged this stuff in water to see how it performs and was pretty amazing. I just don't remember what it costs.

Ok that said, the floor we are going to do ditra as it's a second floor bathroom on a double layer of plywood with existing tile on it now. What thickness should the plywood be under ditra? The existing tile from a 1986 home has no cracks.

Last edited by little; 07-20-2018 at 12:52 PM.
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Unread 07-20-2018, 11:04 AM   #2
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Behind the Kerdi regular drywall seems the best, I'm thinking concrete board would have a tendency to suck the water out of the thinset?
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Unread 07-20-2018, 11:12 AM   #3
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Regular white drywall is acceptable by Schluter, so I’d go with that
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Unread 07-20-2018, 12:18 PM   #4
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I agree. Drywall is FAR easier to Kerdi and not as "thirsty" as CBU.

If you think hiring a professional is expensive, wait until you see the cost of hiring a amateur.
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Unread 07-20-2018, 02:49 PM   #5
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I'm going to recommend he use the purple drywall board if anything as he will be just painting the ceiling. It shouldn't really be necessary but if it gives him an extra piece of mind it then I'd go for it since not much more than standard drywall.
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Unread 07-20-2018, 03:55 PM   #6
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Purple sheetrock would be fine for the ceiling, but a moisture resistant drywall won't take Kerdi as well due to the coating on it. It wouldn't provide any benefit if the Kerdi was installed improperly, so there's really no upside to using it.

Here's the only downside to using sheetrock: if you ever have a plumbing leak inside one of those walls, the sheetrock would possibly be damaged and not repairable, so the shower would be toast.

Has it happened before? Probably so. Not to me or any of the other installers on here, that I know of. It's such a small risk that to me, it's not worth the expense and hassle to use cement board.

So that's your choice to make. I've used plain white sheetrock on hundreds of showers with success, and that's what I would recommend.

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Unread 07-20-2018, 06:12 PM   #7
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Jim, you wanna check the framing requirements for use of that purple gypsum board on your ceiling. Frequently the requirement is for closer joists when using those MR boards. You'd likely be better off using plain white gypsum board in your application.

My opinion; worth price charged.

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Unread 07-20-2018, 07:55 PM   #8
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The first Kerdi shower I built was back in 2004. I used regular drywall and did it all myself, plumbing too. I'm not much of a plumber but never had a leak with this. I like a fully tiled shower, ceiling included, so I had no exposed drywall in the shower to worry about.

We had a pretty big niche too and it would get flooded with water when cleaning it up. The back wall of tiles in the niche was installed to the back side of the drywall in the bedroom on the other side. Never a leak there either. This was back when a Masterblend thinset was recommended, later to be determined to be not a great choice. Didn't matter, still there fourteen years later. Installed by a DIYer.

Not only my first shower build but also the first time I ever tiled anything. Link to the project in my signature.
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Unread 07-20-2018, 11:15 PM   #9
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Watch some videos on the Schluter site. Make sure to wipe the wall down well with a wet sponge just prior to spreading the thinset...it makes a significant difference in the ease of installing the Kerdi. Make sure you don't try to use huge sheets as a first timer. Do embed things as well as you can, then, pull a corner back off to check...you should have complete coverage on both the membrane and the backer, whatever it is. If it was well done, it'll go back up in moments without trouble.

It really does make a difference when mixing thinset if you actually mix it the time specified in the instructions...a kitchen timer helps. You'd be surprised at the differences.
Jim DeBruycker
Not a pro, multiple Schluter Workshops (Schluterville and 2013 and 2014 at Schluter Headquarters), Mapei Training 2014, Laticrete Workshop 2014, Custom Building Products Workshop 2015, and Longtime Forum Participant.
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