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Unread 09-13-2021, 07:08 AM   #1
md9918
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Kerdi Basement Shower - What substrate to use on cinderblock wall?

Hey all-- I bought a Kerdi shower kit for my basement shower-- one wall is exterior cinderblock, which is 70/30 underground/above ground. For the cinderblock wall, I'm thinking of using furring strips and green board, then Kerdi over the green board. No moisture barrier, no foam insulation. Does this make sense? Or should spring for Kerdi Board? For what it's worth, the original 1958 mud/wire shower used furring strips on the block and it didn't seem at all to have been affected by moisture.
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Unread 09-13-2021, 08:41 AM   #2
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Welcome back, Mike.

A geographic location in your User Profile helps with questions such as this.

How was the outside of your wall treated against moisture intrusion?

You can fur out the CMU wall without a moisture barrier or insulation if you want, but I'd likely recommend the use of a CBU as your shower wall instead of the drywall in your application. If you elect to use drywall, I would recommend plain white gypsum board rather than the MR Board, which really provides no benefit at all and some of them don't play as nicely with thinset mortar.

Probably a worthwhile extra expense in your application would be the use of one of the foam boards as your tile backer. KerdiBoard or one of the other available brands. No danger from moisture on either side of those and you get at least a little thermal insulation.

My opinion; worth price charged.
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Unread 09-13-2021, 10:36 AM   #3
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Thanks, CX. Y'all so dang friendly around here.
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Unread 09-13-2021, 03:00 PM   #4
jadnashua
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You could put the Kerdi directly on the block wall if it's clean! Or, if it's not clean or straight, you might consider making that wall out of KerdiBoard, which would give you a thermal break and a bit of insulation...it's available at up to 2" thick and as thin as 3/16". If whatever's on the wall now will accept thinset, you'd have either choice, but if you used KerdiBoard, it would allow you to flatten and plumb the wall at the same time.

Furring strips and cbu would work, too, but not give you much of any insulation and only a little thermal break.
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Unread 10-07-2021, 07:40 PM   #5
md9918
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How to use an inside pipe cutter to cut pipe to length for Kerdi Drain

Hey all-- I made a math error when installing my new under-slab shower drain. I had terminated it in a coupling which was about 3/4" too high. I removed the coupling with a heat gun to see how much of the pipe would go into the drain hub-- only about 1/8". So, unless you all know a trick for lengthening a pipe by only 5/8", I think I need to cut the pipe down an inch or so and install a new coupling.

I've seen in the Liberry the suggestion to use an inside pipe cutter, but how do you deal with the backfill you'll need to remove (in my case about an inch worth)? Fortunately I used sand as backfill, so I'm thinking I'll try to shopvac it out. Any other suggestions?
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Unread 10-07-2021, 07:47 PM   #6
jadnashua
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A shop vac can probably do it, maybe a spoon.

Make sure to measure as keep in mind that the socket of the drain may not fully seat onto the pipe until you add the primer and cement (which melts the plastic) so you need to measure carefully rather than try to dry fit.
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Unread 10-07-2021, 08:18 PM   #7
cx
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Mike, it'll help if you'll keep all your project questions on one thread so folks can see what you're working on and what's been previously asked and answered. A moderator can give it a more generic title any time you'd like to suggest one.
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Unread 10-07-2021, 09:44 PM   #8
Shady at Best
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I think that's the perfect scenario for one of these.
https://www.pvcfittingsonline.com/2-...-s0301-20.html

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Unread 10-08-2021, 02:47 PM   #9
md9918
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Well the shopvac with crevice tool worked great as an excavation tool. Also, I highly recommend sand as a backfill material. I bought an inside pipe cutter, installed a new coupling, installed the Kerdi drain. Although the pipe extender would have been perfect-- it was just going to take a few days to get here and I didn't want to wait.

Seating the Kerdi drain was fine, although a word of warning-- the hub is especially deep on it (relative to typical fittings) and holds a lot of cement, which will push the drain out/off center as it cures. I ended up shooting two stainless screws in the holes on the collar to correct a slight slope that developed when I wasn't paying attention.

Incidentally, I ended up going with Durock CBU for the cinder block wall. It's obviously the most cost-effective route, and normally I spring for Kerdi Board or something similar, but I didn't have to make too many cuts and was able to do everything I needed with a circular saw with a masonry blade. Other than the potent smell, no regrets.

Thanks everyone for the input.
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Unread 10-08-2021, 05:30 PM   #10
jadnashua
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A box of tile on the drain while the cement cures will help prevent it from creeping up. The hub is slightly tapered, so it acts like a wedge with the cement acting like a lubricant, trying to push the two apart until the solvent evaporates and it gets solid...that can take a bit longer than you might think.
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Unread 11-01-2021, 07:19 AM   #11
md9918
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What's going on here?

The cement is cured, and I'm not ripping it out just to fix this, but for next time, I'm wondering what I did wrong to create the brick-like appearance of the shower wall tiles. I am conscious of lippage when setting tiles and if I can see or feel a definite difference in pitch of adjoining tiles, I'll adjust until everything sits flush, but apparently, my eyeballs/finger aren't cutting it. It doesn't help that it really doesn't show up until the grout is in.

I've done two other bathrooms in subway tile and did not get this appearance, but in those instances, I used grey grout and standard 6x3 tiles instead of the white ground and 12x3 tiles I used here. The recessed light directly above also isn't helping things here. But still, I'd like to improve my technique-- is there a trick to keeping tiles flat and all on the same plane?
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