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Unread 06-13-2020, 12:15 AM   #1
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Shower waterproofing

Hi All,

First off -- this is my first post as a new member, I’ve been lurking for several years and have benefitted very much from your collective expertise. Thank you all!

I’m working on my second complete bathroom remodel, and have a couple of questions about waterproofing the shower. I’m doing the work myself and feel generally comfortable doing so, but I’d welcome the opinion of those with more experience. Apologies in advance if my questions have been answered elsewhere, and if this is the case, please feel free to direct me there.

I’m planning on doing a mud-bed, tile floor shower, built over my home’s existing plywood subfloor. I plan to waterproof the floor with a PVC liner above a mud pre-slope, and I’ve notched out my wall studs to accommodate the thickness of the liner where it extends up the walls. I notched the studs up to 12” above the subfloor, which I expect will be roughly 8” above the finished tile floor. I removed about 1/8” of thickness from each stud (1/4” in the corners), though my chisel work on the gnarly-grained old 2x4’s ain't pretty at all. I’m concerned that the bottom of my shower walls – the CBU, thinset, and tile – may be not be adequately secured to the studs in these notched sections. Does anyone have any bright ideas of how to address this?

I know that one method is to install the wall CBU prior to building the top layer of the floor mud bed. In other words, install the wall CBU over the vertical portion of the pan liner, then pour the top layer of the shower pan, such that the bottom edge of the CBU is secured by the top layer of the pan. It seems to me that this design will invite water, over time, to ride down the CBU and saturate the top layer of the shower pan (as well as the wall CBU itself via wicking).

Assuming that my pre-slope and liner work as intended, is saturation of the top mud layer of my pan a worthwhile trade-off for securing the CBU to the studs?

Thanks, and thanks again for the time and mental weightlifting you all do on this site, it’s a tremendous resource. All advice appreciated.
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Unread 06-13-2020, 12:30 AM   #2
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Welcome to the forum, Quinn. Bet you never thought you'd get an answer to this question at 1:30AM, did you?

Fact is, your mud bed will be saturated no matter what you do. It's designed to be. The reason for using deck mud is that it's very porous and allows water to flow through it, over time, by gravity.

If your liner is sloped like it's supposed to be, then when water wicks through the mud and hits the liner, it'll then make its way to the drain.

Just be sure when setting the CBU on the walls that you hold it up off the liner about a 1/2" so that the bottom edge won't hold any water at the perimeter.

Put the bottom row of nails/screws at least 3" above the top of the curb. The mud will easily hold the bottom edge in place.
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Unread 06-13-2020, 08:39 AM   #3
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I might add that it's important to use true deck mud (dry pack). Sand mix mixed dry is not dry pack, it's too rich. In the "liberry", it shows how much sand you need to mix to sand mix to get deck mud. Or, you can use 5 parts of all purpose sand (HD or Lowes) and 1 part Portland cement to mix your own deck mud. This weak mix makes it porous so water will easily run thru it and find the weep holes.

Also, it's also important to protect the weep holes from being clogged with deck mud. Pea gravel piled up on the weep holes works well for this.

You'll find the "liberry" in the dark blue bar above. Find the 'shower construction info" thread.
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Unread 06-13-2020, 03:23 PM   #4
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You also cannot use a fiber-cement board on your walls when you embed it in the mud bed to anchor the bottom...at least not with HardieBacker, they list that fiber as cellulose (wood pulp).

My preference is to make the entire shower waterPROOF rather than just water resistant (except for the pan, but the part above it will be damp). That generally means something like a sheet membrane like Kerdi. You could also use something like KerdiBoard on your walls, which is waterproof and won't absorb moisture instead of CBU. Much lighter, easier to carry, and cut than CBU. There are several other foam panels out there that should work well, too, as well as sheet membranes.
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Unread 07-05-2020, 10:41 PM   #5
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Leak testing with mud curb in place

Thanks to all for the (very) prompt and helpful information on my last question. You guys are a terrific resource!

I’m continuing the same bathroom remodel, and have another shower question for you fine folks. I’ve installed my pre-slope, liner, drain, and wall CBU with poly sheeting behind. Then, for some dumb reason, I muddled my shower curb before I leak-tested the liner. I’m now concerned that I won’t be able to get accurate results from the leak test due to the shower curb mortar sucking up the test water. I used pre-packaged Quikcrete mortar mix (no. 1102) for the curb. Anyone come across this situation or have suggestions, other than order a new brain?

Thanks again!
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Unread 07-06-2020, 01:17 PM   #6
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Your curb will absorb some moisture. What you might have to do is fill the pan, let it sit for a couple of hours, then mark where the level is then, and wait.
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Unread 07-07-2020, 09:13 AM   #7
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Thanks Jim, that’s a clever idea. Will give it a try.
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Unread 09-14-2020, 10:01 AM   #8
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Mosaic and field tile thickness mismatch

Hi all,

I am planning to install a mosaic strip within the field tile of a shower I’m building. The mosaic tiles are 1/8” thick, while the field tile is roughly 1/2” after installation (including thinset). My question, which I’m sure is common, involves how to install the mosaic strip so it’s flush with the field tile.

The approach I’m considering is to use fat mud, set in a bed of thinset, under the mosaic tile strip. I would build the fat mud to the proper thickness, using a strip of wood milled to the thickness I need as a temporary screeding reference. Once the fat mud is dry, I’d install the mosaic tile on top, set in thinset as per usual.

My wall underlayment is Durock.

I’m hoping you fine folks might chime in if there are flaws in this approach, or an easier way of getting to the end result. I’ve heard of using Kerdi mat or similar products to flush mosaics, but I don’t have access to any of that good stuff. This seems like a quick, inexpensive way to perfectly dial in the thickness I want. Thoughts?

Thanks,

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Unread 09-14-2020, 10:22 AM   #9
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Welcome, Quinn.

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.

That's likely to be a bit thin for fat mud, but perhaps you can make that work. There are a large number of cementitious patching materials available from tile installation product manufacturers that might work better in that application, but that's up to you.

You're planning a screed stick similar to this?

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My opinion; worth price charged.
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Unread 09-14-2020, 10:38 AM   #10
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That's likely to be a bit thin for fat mud, but perhaps you can make that work. There are a large number of cementitious patching materials available from tile installation product manufacturers that might work better in that application, but that's up to you.
This may be a dumb question but could you use a layer of thinset, screed, then let it dry before applying another layer to bond the thin band of tile like this? Maybe embed mesh tape to add support similar to filling a cement board seam which has the same small amount of buildup?
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Unread 09-14-2020, 01:57 PM   #11
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Thanks for the replies.

CX, if there is a superior product to fat mud for this application, or something specific that you’d recommend, I would love to know. And yes, that screed stick method is essentially what I had in mind.

I’m hesitating to use thinset exclusively because I’ll be applying about 1/4” of it as the mosaic base, and then an additional layer to attach the mosaic tile. I suspect an application that thick may shrink, crack, etc., though I don’t know the degree to which building up multiple skim coats would solve this problem.
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Unread 09-14-2020, 06:02 PM   #12
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Fat mud works best in my opinion but you will need it to be about 1/4 inch thick or so. After it sets, you can add a thin layer of thinset to fine tune it if needed. For a mosaic strip like that, I like leaving about 1/16 space for the thinset that will bond the tiles.
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Unread 09-15-2020, 06:17 AM   #13
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Thinset mortar for that 1/4" "lift" will absolutely shrink.

It will also absolutely work. If I already had the thinset mortar on hand that's what I'd use. And did. Apply it, screed it, let it cure for a few days, then dress/fine tune it with a skim coat and let that cure for a day.
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Unread 09-16-2020, 11:16 AM   #14
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Alright, it seems gradually building layers of thinset will produce the result I’m after, although it also appears there are several products that would work for this. I’m planning to use a large-format tile thinset to perform the “lift”.

Thanks again everyone.
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Unread 09-16-2020, 11:19 AM   #15
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Do you have access to ardex products? X5 or even better X77
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