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Unread 01-09-2021, 08:48 AM   #1
bcs001
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Preferred Kerdi Shower Wall Material

Hello again. I am starting another walk-in shower project and decided to use the Kerdi shower system. I’ll buy the kit that has the floor pan, curb, drain, rolls of membrane, corners and seals. Also planning to try the prefab rectangular bench.

Right now I am demo’ed down to the bare wall studs and 1x6 diagonal plank sub floor. Will put a layer of 3/4” plywood on the floor but looking for advice on the preferred wall material to apply the Kerdi membrane onto. I’ve always gone the traditional 1/2” cement board route with redgard but Kerdi appears to allow other substrates.

Is there one that is recommended or is it just personal choice based on cost and ease of installation? I don’t mind the added cost and effort of using cement board or plywood vs. drywall if there is some advantage. Two of the three wall are exterior if that makes any difference.

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Unread 01-09-2021, 09:22 AM   #2
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Plywood is not compatible with kerdi.
Kerdi can go on plane jane drywall and that's what is mostly used. Just take time to get walls plumb and flat and blocking in all the corners. If your shower size and drain location fit a prefab pan that may be good. But just so you know making a dry mudpan is really easy to do. Yes it involves some bags of cement and sand but if your tiling your going to be touching those heavy beasts anyway.
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Unread 01-09-2021, 09:47 AM   #3
bcs001
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I have done a mud pan and built-up curb before but from a lot of what I see the pros discussing here, the kits look easier to use. My pan area is 47x60 so the 48x72 kit can be cut down to fit and get the drain exactly where I need it.
If I use sheetrock, do I just finish seams the same way as cement board using unmodified thinset and mesh tape?
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Unread 01-09-2021, 10:12 AM   #4
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Welcome back, Bruce.

I would very strongly recommend you not finish the drywall joints anywhere you plan to install Kerdi membrane. The membrane, installed with thinset mortar, makes far stronger joints than you'll otherwise get and it's much easier to install the Kerdi over the clean, open joints, especially the corner joints.

My opinion; worth price charged.
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Unread 01-09-2021, 11:16 AM   #5
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I use plain white sheetrock. Two widths of Kerdi (overlapped 2" without Kerdi band) goes up about 78", usually to the height of the shower head.

At the corners above 78", I cut a 4" wide strip of Kerdi to cover the corners, 2" on each side. It's not for waterproofing, but just to stabilize the corner.

This assumes that tile will go to the ceiling and cover all the Kerdi. If you're stopping the tile short of the ceiling, you'll want to use drywall tape and mud so you can paint over it.
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Unread 01-09-2021, 06:09 PM   #6
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You want the walls flat, so the tapered long edge on the drywall can allow you to get an indentation unless you're careful, and what you use to embed the Kerdi across that joint is not long enough, or you misjudge and get an edge in the depression. FWIW, the paper on drywall is directional, and it really is designed to be installed horizontally, not vertically, so it attains its maximum strength. Make sure to use the specified screws at the specified spacing.
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Unread 01-11-2021, 08:30 AM   #7
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I appreciate the great advice as always. I'll plan to go with drywall as recommended and take some extra time to shim the studs as need to get good and flat. It sounds like I can just fill the long tapered drywall edge with thinset and use a wide knife to set the Kerdi flat across the seam.

Kman, Do you run the Kerdi horizontally to just save on how much material you use or is there some other good reason for it. The Schluter videos show it being installed vertically.

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Unread 01-11-2021, 02:27 PM   #8
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Quote:
Originally Posted by Bruce
Kman, Do you run the Kerdi horizontally to just save on how much material you use or is there some other good reason for it.
It's just whichever way fits the best with the dimensions of the shower vs. the width of the roll. I often run vertical, but if it causes me to finish with a lot of leftover material, I'll run horizontal.
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Unread 01-11-2021, 04:57 PM   #9
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Drywall is definitely stronger along the long side, as is plywood. Take a sheet of paper or newsprint and try to tear it. Then, try turning 90-degrees and tear it again...they align the fibers along the long length. Now, how much different, I've not measured it. This article says it is 3x stronger when run horizontally across the studs versus vertically. https://www.thisisdrywall.com/?p=448 Sounds like a viable reason to do it that way to me!
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Unread 01-11-2021, 07:02 PM   #10
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Can't argue with that, Jim. But we were discussing running Kerdi horizontally or vertically.
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Unread 01-11-2021, 09:38 PM   #11
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That's what happens when you skim messages...

Yeah, Doesn't matter which way Kerdi is run, and you don't need to do it via shingling, but it doesn't hurt...the seam will be waterproof if it's done right, regardless. Same with putting up banding, you can band a corner then put the sheets up, or just overlap the Kerdi if you're doing a larger tile where the slight difference in thickness might make a difference...you're only talking a few mils, so it really doesn't matter unless you've got a spot that has more than two layers.

What you don't want to do with Kedi unless you've got a lot of experience, is try to put up very large sheets to avoid seams. It's hard to do that while keeping the thinset from skinning over.
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Unread 01-11-2021, 10:20 PM   #12
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Curious why there's no recommendation for kerdi board? Is it the price?
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Unread 01-11-2021, 11:30 PM   #13
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When the OP mentioned he was buying the shower kit, recommending Kerdiboard didn't enter my mind. Nothing wrong with the product, but you are correct that the price can scare some people away.
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Unread 01-12-2021, 06:40 AM   #14
bcs001
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Its a great learning experience to see the back and forth between experienced members here. Yes, I did see the option of using Kerdiboard vs drywall but there is a very big price difference and I have done enough trowel work to be comfortable putting the membrane material up on the walls. If there are other advantages to the kerdiboard vs drywall with Kerdi, I'd be open to considering it.

I've also been reading past threads about the thinset to use for the Kerdi membrane and there seems to be quite a lot of differing opinions and also the effectiveness of the bond of the membrane and substrate. Since tile is then "hanging" off of the surface of the Kerdi, this substrate bond is critical.

I would also be interested to hear opinions about buying the Schluter Shower kit vs individual parts. It looks like a cost effective solution along with the pre-fabed 38" bench but maybe the issues I've read here with an adequate curb and bench slope on the Schluter product is a reason to consider something else?

I have already built one full shower in the traditional 2 layer mudbed, polyliner, cementboard, 2x4 curb & redgard waterproofing but was thinking the Kerdi kit would be less labor intensive.

Bruce.
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Unread 01-12-2021, 06:57 AM   #15
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Certainly nothing wrong with using Kerdiboard. It'll just cost you more, but will save you time. You won't have to worry about covering the walls, although you'll still need to cover the seams, penetrations, and floor.

You might also check the availability and price of the board in your area. I haven't priced it in a long time, but the difference was significant enough that I continued to use sheetrock. However, I was never working against the clock, so saving a half day was never a big concern for me.
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