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Old 06-12-2012, 09:17 PM   #1
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Cement board install questions


I am currently in the process of remodeling my bathroom and I have some issues I need help with. After removing the tile from the tub/shower I found lath and plaster walls beneath what looked like regular sheetrock, probably not a good thing. My first question is: Should I remove the lath to start fresh from the studs, and if so, what material can I add to make up that 3/8" - 1/2" dimension I'll lose before I install cement board? Any info would be greatly appreciated, thanks!

Can I install new 1/2" plywood to take the place (dimension) of lath & plaster? I need the tile to hang over the edge/lip of the tub/shower, which will be 7/8"-1" away from studs. If I remove the old lath and install Durock, membrane, then tile, I think it might fall 1/4" short in one spot, so I want to make sure the wall, when finished hangs over the back edge, if that makes sense. This is my first time trying this, so I'm hesitant to remove something that should stay. Any help would be appreciated! Thanks


Last edited by poplar12; 06-12-2012 at 11:45 PM. Reason: More info
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Old 06-20-2012, 02:15 PM   #2
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Originally Posted by poplar12 View Post
I am currently in the process of remodeling my bathroom and I have some issues I need help with. After removing the tile from the tub/shower I found lath and plaster walls beneath what looked like regular sheetrock, probably not a good thing. My first question is: Should I remove the lath to start fresh from the studs, and if so, what material can I add to make up that 3/8" - 1/2" dimension I'll lose before I install cement board? Any info would be greatly appreciated, thanks!

Can I install new 1/2" plywood to take the place (dimension) of lath & plaster? I need the tile to hang over the edge/lip of the tub/shower, which will be 7/8"-1" away from studs. If I remove the old lath and install Durock, membrane, then tile, I think it might fall 1/4" short in one spot, so I want to make sure the wall, when finished hangs over the back edge, if that makes sense. This is my first time trying this, so I'm hesitant to remove something that should stay. Any help would be appreciated! Thanks
My somewhat educated guess:
Are you NOT replacing/moving the existing tub/shower? If that's the case, sounds like you will need to fir out the wall on the back edge so your durock and (eventually) tile will sit above/in front of the flange.
1/2" ply should work, strips of 1/2" ply on each stud would work too. Replace 1/2" ply with whatever amount that the wall will need to come out so that your durock will sit above/in front of the flange, but not on the tub deck. Good luck

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Old 06-20-2012, 03:15 PM   #3
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Just use furring strips--do not cover the entire wall with plywood--cement board and tile are not waterproof--some moisture will get through to the ply and cause trouble.

It is a very good practice to waterproof the surface of the rock with a paint on waterproofing like Red Guard or Hydroban--Then you are truly safe--Mike---
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Old 06-21-2012, 12:43 AM   #4
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I appreciate you guys responding!

jimbriam - Thanks for responding. I'm not replacing the tub, so I'll use strips to bring it out, thanks again! Is it alright if the furring strips touch the top of the flange? I should allow a 1/4" space between the cement board/tile and tub deck, correct?

oh'mike - Thanks for responding. That's what I'll do, use strips to bring it out. I was planning on using Durock's tile membrane kit, but the products you suggested cost a lot less, so I'll look into it. Any preference between the two? How many coats? Also do you suggest a particular mortar for the joints, mesh, and tile... considering some probably don't work well with Red Guard or Hydroban? I should allow a 1/4" space between the cement board/tile and tub deck, correct? Thanks again.
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Old 06-21-2012, 02:55 AM   #5
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Assuming I am imagining the situation correctly, the ply strips should extend behind the tub flange so that it has framing to secure directly to ... My preference is for the tile backer board to come down to rest on the top of the flange and then the tile itself comes all the way own over the flange to the rim of the tub. This way if the silicon around the tile/tub join ever fails the backerboard is not sitting in a puddle, so to speak. The reason I do not bring the backerboard down over the flange to only 1/4" off the rim is because in doing so the bottom of the backerboard is being forced out slightly (by the thickness of the flange) giving you a bit of a slope which can then become a pain ... Especially if installing larger format tiles which we often do. When we install the tile we then back butter enough thinset at the bottom row of tile to fill the small gap between the backerboard and the tub rim so that there is no hollow spot there. This technique also means that if the silicon fails there is a barrier of thinset as opposed to an air gap so is an extra line of defense against leaks. I hope you can visualize what I am describing ;-)

We used to use Red Guard a lot but switched to the Mapei equivalent after finding better coverage results. It's very similar but bright blue ... Goes on with a 4" nap roller. We use the mildew resistant mesh tape and a decent polymer modified thinset to seal all joins and corners. Once dry (next day) we sand it smooth and go over with the roll on membrane. Usually 2 or maybe 3 coats depending on the type of backerboard and how porous. I am using Denshield a lot these days so actually only the joins/corners need waterproofing but I prefer to do the whole thing with 2 thin coats as blends the joins/corners in better and makes for a slightly easier tile install as everything is flush. Overkill ... A bit. Watertight ... You bet
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Old 06-23-2012, 09:31 PM   #6
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Thanks for responding Bonzai, I appreciate it!

I have a new question if someone can help... Is it alright to take the CBU(I'm using Permabase) up the 3 tub surround walls 60" and finish the remaining 25" or so with Gold Bond XP Gypsum Board? I don't plan on tiling all the way to the ceiling, so I was wondering if this is acceptable. I wasn't sure if you can paint Permabase well, this is my reasoning for the Gold Bond XP Gypsum Board. Any info would be appreciated! I'm still wondering what tile membrane product to use as well...Redguard, Hydroban, Kerdi, etc... I guess it depends on the answer to my first question. Thanks in advance!
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Old 06-23-2012, 09:55 PM   #7
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The reason I do not bring the backerboard down over the flange to only 1/4" off the rim is because in doing so the bottom of the backerboard is being forced out slightly (by the thickness of the flange) giving you a bit of a slope which can then become a pain ...
Thanks for your post. In context to the above, would it be advantageous to fur the studs out a bit further so that the CB would drop down past the flange?
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Old 06-23-2012, 11:22 PM   #8
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I've seen that done too and actually done it myself when using small mosaic tile everywhere as need the more solid flush backing all the way to the tub flange. I've even wrapped Kerdi band around the bottom of the cement board for added protection.
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Old 06-24-2012, 07:25 PM   #9
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I've seen that done too and actually done it myself when using small mosaic tile everywhere as need the more solid flush backing all the way to the tub flange. I've even wrapped Kerdi band around the bottom of the cement board for added protection.

So is extending the furring so that the CB actually goes past and a bit below the flange the way to go (if the oppotunity is available)?

Also, in sealing the bottom of the CB, would one seal (silicone) just the back part of the CB to the receptor, as below (of course, in this image, the CB meets the top of the flange)? My thinking is that if the entirety of the bottom of the CU is sealed, the water has nowhere to go.

Thanks for your patience with my novice questions...my quest to do the best job possible can manifest itself in that way.

Cement board install questions-durock_kohler2.jpg
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Old 06-24-2012, 07:48 PM   #10
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The problem with that diagram is that the tub is not tight to the studs or anything else for that matter as there is an air gap ... you need to screw through the tub flange in to something solid ie. the stud of the surrounding wall or shims if as in your case there is a gap between tub & studs.
The extra silicon caulking behind the tile at the base of the cement board is not required but it can't hurt. When I do my own places I similarly go a bit over board ... in one I did all the walls of the entire bathroom as well as the ceiling in cement board. Skimmed over the non-shower walls and ceiling. That puppy will never rot ;-) Feel a bit sorry for the new owners if they ever want to remodel, but it will not be 'cos it fell apart.

The cement board should come down to the tub in this scenario (over the flange) but not actually touch the tub ... stop about 1/8"-1/4" shy of the tub. This gap will get a colour matched silicon caulking. If that ever fails then your cement board is not directly sat on a wet ledge and you have a vertical flange as well as additional silicon behind the bottom of the concrete board.

Tip: after doing the silicon around the base of the tile/tub let it cure & then fill up the tub to make sure the extra weight does not cause the tub to sink such that the caulking splits. I've seen cases where apparently the tub installer ran out of screws and a tub was held in place by 3 drywall screws (which are also the wrong type of screw to use ... we use decking screws as they are waterproof). It's the little things like this that I find can make the difference between a job which will last a while and one which will outlast the owner.
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Old 06-25-2012, 10:44 AM   #11
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Thanks again for your comments.

Quote:
Originally Posted by Bonzai View Post
The problem with that diagram is that the tub is not tight to the studs or anything else for that matter as there is an air gap ... you need to screw through the tub flange in to something solid ie. the stud of the surrounding wall or shims if as in your case there is a gap between tub & studs.
This actually is not a tub, but rather a Kohler cast iron 9026 Kathryn shower receptor (4'x3'). The shape of the receptor will not allow for the flange to sit directly against the studs, and in fact, the illustration is from Kohler's pdf manual (minus the blue markings I placed), thus the shims are required to extend the CB out to an acceptable position. In fact, I have already placed shims (1/4" give or take as required) to the position shown in the diagram. Replacing them with thicker shims is not an issue if it means a better install/result. Is it worth the effort?

Another pertininet fact I failed to mention is that the floor the receptor sits on is reinforced concrete, so no danger of sinkage there.

The air gap has concerned me for other reasons though, in that I have air sealed the space and all stud cavities, as I live in the deep south. The gaps, of course, provide a passage for air to flow from cavity to cavity, which is something I wish to avoid. I suppose I'll wind up bringing the shims down to just above the flange and place a bead of silicone between flange and shim.

Again, thanks for the benefit of your (and others here) experience. And since I wasn't the OP of this thread, I hope that others will benefit as well.
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Old 06-26-2012, 10:45 AM   #12
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Quote:
Originally Posted by rightit
Thanks again for your comments.

This actually is not a tub, but rather a Kohler cast iron 9026 Kathryn shower receptor (4'x3'). The shape of the receptor will not allow for the flange to sit directly against the studs, and in fact, the illustration is from Kohler's pdf manual (minus the blue markings I placed), thus the shims are required to extend the CB out to an acceptable position. In fact, I have already placed shims (1/4" give or take as required) to the position shown in the diagram. Replacing them with thicker shims is not an issue if it means a better install/result. Is it worth the effort?

Another pertininet fact I failed to mention is that the floor the receptor sits on is reinforced concrete, so no danger of sinkage there.

The air gap has concerned me for other reasons though, in that I have air sealed the space and all stud cavities, as I live in the deep south. The gaps, of course, provide a passage for air to flow from cavity to cavity, which is something I wish to avoid. I suppose I'll wind up bringing the shims down to just above the flange and place a bead of silicone between flange and shim.

Again, thanks for the benefit of your (and others here) experience. And since I wasn't the OP of this thread, I hope that others will benefit as well.
Perhaps you should have created your own thread as this certainly was confusing ... I thought I was still responding to the OP.

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