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Unread 10-09-2020, 06:45 PM   #1
pitts
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New bathroom, question with CBU

Hello all, first post, let me thank you for the support you bring to DIYers like me.

I just gutted my bathroom and was trying to get all the details in place before actually rebuilding everything.
It was 1/2" green drywall everywhere with a one piece acrylic shower and a corner bath. I'm going to use an acrylic shower base with tiled walls all around the shower and bathtub.

First, for my walls, I plan on using 1/2 durock with mapei aquadefense. I live in a condo and one of my shower wall is shared. They built the wall with cement blocks, vapor barrier and then built a 2x4 wood frame over it. Then there is another vapor barrier and horizontal 1x4 furring strips to which my drywall was attached.

1) Should I remove the vapor barrier behind the furring strips ? Doesn't this create something like a 3 layered moisture club-sandwich ?
2) Is is ok to install my CBU over furring strips across the studs (horizontally) ? Should I remove them and screw right into the studs ?


Then, my floor. My subfloor is made of a 5/8 plywood over the joist and something that seems like fibre cement board glued on top (no visible screws). My tile underlayment is 2x 1/2 plywood with the tile directly bonded to the plywood
My understanding is that plywood is a bad substrate for tile, but gives the floor it's structure (as opposed to CBU). So :

A) Would that be a better idea to remove the top plywood, install a 1/4 plywood and 1/4 CBU ? Or just clean the plywood and reinstall the tile right over it ?


Thanks for your time and opinion on this !
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Unread 10-09-2020, 08:33 PM   #2
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Welcome, CP.

My first advice would be to determine whether your plan meets with the requirements of your condo association. They can be pretty.....interesting, at times.

I'm afraid I don't fully understand the vapor barrier material over the CMU interior wall. But with it, I understand even less the second vapor barrier material on the face of the stud wall inside the interior CMU wall. I'm sure someone had a plan, but I got no eye-dee what it might be.

From a practical standpoint, I would eliminate at least the membrane on your side of the stud wall, but see again my first sentence.

I also don't understand the purpose of the horizontal furring strips. There may be one, but the only person I know on site is the guy asking us about the furring strips. I suggest you seek to determine just why those walls are constructed as they are, probably starting with your condo association.

I'd want to install my CBU directly to the face of the stud wall.

I think I don't understand your subflooring. You have a first layer of nominal 5/8ths" presumably T&G plywood, then a layer of unknown fiber/cement board, then a layer of nominal 1/2" plywood, fastened in an unknown manner, then another layer of nominal 1/2" plywood, also fastened in an unknown manner, which was used as your tile underlayment? I got that right?

While we (TYW) do not customarily recommend bonding tile directly to plywood, There is an accepted ceramic tile industry method for doing so and it's perfectly sound if done absolutely correctly. First consideration would be the composition of the subflooring under that layer of plywood and while yours might be adequate, it does not meet the criteria for the only industry published method.

If you take up the top layer of plywood, which will be required if you're concerned about final floor height, you would then need to determine just how that next layer of plywood was fastened through your Fiber/Cement(?) layer and just how stable what you have seems to be. May be OK, may not.

You would not install any 1/4" plywood, but you may be able to use a 1/4" CBU or perhaps even a membrane of some sort, but what you have for a subfloor would not, at least technically, meet the requirements of any manufacturer of such products.

You really need to know more about that subfloor, including just what joist structure is under it.

My opinon; worth price charged.
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Unread 10-09-2020, 10:17 PM   #3
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Without seeing them, those furring strips may be there for noise isolation and may not be simple furring strips. My guess is that it may decouple the shower wall a bit more than just attaching the wall to the studs. Have to see it. Is there any adhesive attaching the furring strips (it could be a noise isolation rubber like substance).

The CBU strength isn't directional, so doesn't really matter which way it is oriented.
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Unread 10-09-2020, 11:14 PM   #4
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Are those 2x4's attached to the concrete wall by the 3 1/2" side, or the 1 1/2" side. If it's the former, that may explain the furring strips.
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Unread 10-10-2020, 09:30 AM   #5
pitts
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Ok well, the association don't really care as long as I'm responsible and don't modify shared space without consent. They sure won't help or guide me…

For the membrane, it seems like all exterior/shared walls and ceiling have it behind their drywall, not only in the bathroom. Not sure if that would be a code requirement or something

For the furring strips, they are only nailed to the studs, no glue … Now I think I will add furring strips vertically on the studs between each horizontal one. That way I could screw the durock with 1" 5/8 screws through both the strips and studs. that way I would keep my wall to drain distance of 12" and eliminate the need to move my drain 3/4 ! Seems reasonable ?

The 2x4 studs doesn't seem fastened to the concrete, there's an air gap of maybe 1/4 … but I'd have to remove the insulation up to the ceiling to check for fasteners.

And for my floor, you got it right CX, Joist/Ply/fiber cement/ply/ply/tile. I'd have to remove my tile to get more info. Just curious, why do you say "If you take up the top layer of plywood, which will be required if you're concerned about final floor height" ?
My joist are engineered I-joist … Don't have much info with them. For what it's worth (probably nothing), the tile was installed since construction in 2004 and none were cracked, including grout.
For the unknown board, I've attached pictures of a scrap piece that was in the bath cavity. Maybe you'll recognize it
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Unread 10-10-2020, 10:21 AM   #6
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Adding a geographic location to your User Profile might help a bit, CP. And I'm afraid I don't quite understand how you can have an "exterior" wall between apartment units.
Quote:
Originally Posted by CP
why do you say "If you take up the top layer of plywood, which will be required if you're concerned about final floor height" ?
Because if you add a suitable tile substrate on top of what you've got you'll raise the finished floor above what it was previously. If you don't care, I don't care.

I'm guessing that unknown material between your subfloor layers is for sound/fire control and can't be legally (homeowner's association) removed. Looks like some good ol' asbestos board, which would be good for the application, but that's not likely. Makes the whole subfloor package something other than any industry accepted setup, but the industry hasn't seen all the potential combinations and, as you point out, it worked at least for a while.

My opinion; worth price charged.
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Unread 10-10-2020, 04:20 PM   #7
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How soft is that cement like layer and is it in discernable sheets, or does it look monolithic? Could it be fiber reinforced gypsum? That stuff can help to flatten the floor and would help in fireproofing and sound between units. In a thinner layer, it could also have been self-leveling cement. I've not seen that use fiber reinforcement fibers (it looks like there may be fibers there, but not positive).

Some condo associations have some very strict rules about noise abatement and if you tear out that layer and the people beneath now can hear noises from you they couldn't earlier, you might have to tear the whole floor out and start over...could get messy.

Laticrete does make a neat tile mortar that has shredded rubber particles in it that can perform like a sound barrier in a tiled floor.

It may not be necessary, but if noise on your walls is a concern, they do make some clips for the furring strips that can help isolate the wall from the studs. But, if the studs are not in contact with the block wall, that's not worth adding as that already is a noise abatement technique so noise isn't transferred between units.
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Unread 10-12-2020, 06:13 AM   #8
pitts
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Quote:
I'm afraid I don't quite understand how you can have an "exterior" wall between apartment units.
By exterior, I meant shared… All shared walls, even in the bedrooms, have this drywall over furring strips and double vapor barrier.

The "cement" layer could look like a gypsum reinforced board, because it has white compressed powder, but really denser and harder than say, X rated drywall. It's definitely in sheets, there is a flat paper on one side and waffle textured paper on the other. And yes there seems to be fiber in it, but more like some kind of paper mulch or pulp .. if that makes sense.

Anyway, since it's hard to identify my subfloor and joist structure exactly, I will stick to the method they used and install my tiles one the plywood. At least I won't be blamed for changing the noise isolation value between floors and won't change my floor height.
I'm going to follow a post on this forum (that I can't link to) and use Kerabond/Keralastic (Thread #125910 Post #9)

What thinset would you use on the walls for a mix of large and small tiles ? The keralastic is expensive as hell (90$ for 2GAL) so I would only use it where necessary.
Would Flexbond LFT be good a choice for both small and large tiles or should I use LFT for large tiles and "regular" for small tile ?
Or should I stay with a Mapei product to use over aquadefense ?


Thanks again! Can't wait to be over with my damn quarantine to go buy all my materials !
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Unread 10-12-2020, 09:29 AM   #9
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Define "large tiles" for us, CP.

The ceramic tile industry defines large format as a tile with any side longer than 15 inches and at that point they recommend a LHT thinset mortar. That's primarily to prevent the presumably heavier tiles from sinking into the mortar on floors after they've been set and to compensate for cupping and warping of the larger tiles. If your tiles are flat, and your substrate is flat, you don't need any such help, especially on wall installations. But that's entirely up to you. The Keralastic certainly costs more than mixing with water, but about the same price as you'd pay for a good quality exterior house paint, to put it in perspective.

And while the Kera/Kera is certainly a good mortar for tiling directly to plywood for a floor installation, you're correct that it is essentially overkill for your walls. I expect MAPEI requires the use of a modified thinset mortar (ANSI A118.4) for bonding over their waterproofing membrane but any brand bearing that identification will work. If you're concerned with warranties, which you're not, it's always best to stay with products from the same brand. Otherwise, choosing good quality products from the brand most easily available to you is what I think you'll find most of us do.

My opinion; worth price charged.
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Unread 10-22-2020, 02:53 PM   #10
pitts
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I removed a piece of my subfloor and it was really a type FRX gypsum board pretty much like guessed by Jim.

My tiles are 4x16 and 6x36, so I'll definitely use LHT for both.

My walls have sistered studs up to 1"1/2 off the ceiling. When reading PermaBase+ instructions, they say to screw between 3/8" and 5/8" at the edges... I'm thinking to shim the gap between the studs and the ceiling but wonder if it really is necessary. (see picture)
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