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Old 09-22-2010, 11:27 PM   #1
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Replacing shower stall plaster


Remodeling the bathroom. Most of the walls are drywall covered with a thick layer of plaster - about 1" total. The shower/tub surround is old cement board covered with an extra 3/4" to 1" plaster and two layers of tile on top. For some reason the plaster gets thicker as it comes down to the level of the tub rim. It was not really noticeable until I started to demo.

I have removed the tile and broken away the plaster around the tub rim and it looks like the nails and chicken wire in the plaster are very rusted and need to be torn out. Just a foot or so above this area the plaster, nails and wire look fine.

Rather than demo out the entire tub surround, I was thinking of just making a horizontal cut and removing the two feet of bad stuff just above the tub rim and then shimming/layering some cement board to match the thickness of the remaining wall. It will all be covered in tile again so I'm not worried about having a perfectly flat wall.

Does this sound reasonable?

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Old 09-23-2010, 09:06 PM   #2
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Replacing shower stall plaster


You would be better off demoing out the entire tub surround. You probably will not be happy with your tile job if you do it the way you are talking about. If the walls are not flat, the tiles will not lay flat.

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Old 09-23-2010, 09:54 PM   #3
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Replacing shower stall plaster


I agree, its just that the upper part of the wall is so solid. Demoing that wall would take longer since there is another bathroom tile shower on the other side which uses the same wall studs. I think I would have to cut the plaster into smaller pieces and carefully pry out the wallboard nails.

I didn't want to mess with the ceiling either which extends over the whole room. I suppose I could cut the wall higher up above the level of the tile or maybe close to the ceiling. Course if I demo the ceiling I won't have to strip off the paint. Always a trade-off.
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Old 09-24-2010, 04:00 PM   #4
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Replacing shower stall plaster


I guess the important thing is to make sure there is no chance for water intrusion between the old wall and the new cement board. That lower part where it meets the tub is going to get the most water anyway. I'm just finishing a tile tub surround and its a lot of work to go through if you are going to end up unhappy with the results.
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Old 09-24-2010, 07:32 PM   #5
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Old 09-25-2010, 12:04 AM   #6
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Replacing shower stall plaster


Well I removed a section of the old surround extending 5ft above the tub rim. Demoing is hard work. I used an angle grinder with a diamond wheel but it couldn't cut deep enough and left 1/2" of cement board and chicken wire. It took a couple hours with a pry bar and wire cutters. That stuff was so solid I think I should have stuck to my original plan.

At least it should all be very flat now. If I shim the backer board correctly, it will sit proud of the old plaster at the top. That top row of tile will overlap the top seam. The old wall varies between 3/4" and 1" in thickness, so the grout seam is going to vary too. I think you would need to be 7ft tall to see it though.
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Old 09-26-2010, 10:23 PM   #7
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Replacing shower stall plaster


so you are supposed to use cement board in bathroom to block moisture
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Old 09-29-2010, 08:22 PM   #8
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Replacing shower stall plaster


To match my thick plaster walls, I plan to use 2 layers of 1/2" hardi backer. I did get the longer screws for that. I also have an aluminum box shelf insert that goes between the studs.

I'll be using mesh tape and thinset for the backer board seams. If I understand right I should use silicone caulk for the corners. Do I need any thinset on the face of the 1st board layer to stick the 2nd layer on or are the long screws enough?

Would it be better for the box insert to mount after the first layer (box flange sandwiched between layers), or after the second board layer?
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Old 10-03-2010, 03:20 PM   #9
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Quote:
federerso: "you are supposed to use cement board in bathroom to block moisture"
Cement board DOES NOT "block moisture". Cement board is not damaged by moisture but it DOES NOT block moisture.

Quote:
To match my thick plaster walls, I plan to use 2 layers of 1/2" hardi backer.
Good Luck with that!

Quote:
I'll be using mesh tape and thinset for the backer board seams.
Good idea!

Quote:
If I understand right I should use silicone caulk for the corners.
I wouldn't be using 100% silicone anywhere in that assembly. Silicone sticks to everything however nothing sticks to silicone. You don't want any silicone anywhere in that project.

Quote:
Do I need any thinset on the face of the 1st board
Nope!

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Would it be better for the box insert to mount after the first layer
Nope!

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or after the second board
Yup!
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Old 10-03-2010, 10:22 PM   #10
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Replacing shower stall plaster


To match my thick plaster walls, I plan to use 2 layers of 1/2" hardi backer.

Quote:
Good Luck with that!
Do you see a specific issue, or are you just commenting on the extra work required? An extra layer of CB is easier for me than floating plaster.

If I understand right I should use silicone caulk for the corners.

Quote:
I wouldn't be using 100% silicone anywhere in that assembly. Silicone sticks to everything however nothing sticks to silicone. You don't want any silicone anywhere in that project.
I have the first layer of CB in place. I have only used silicone at the tub flange and to cover the edge of the plastic barrier. The instructions I was following suggest caulking the corners and tub rim, but they probably mean after the tile is in place. I assume caulk=silicone. Are you suggesting thinset for everything?

Last edited by tns1; 10-03-2010 at 11:42 PM.
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Old 10-04-2010, 04:06 PM   #11
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Quote:
Do you see a specific issue, or are you just commenting on the extra work required? An extra layer of CB is easier for me than floating plaster.
You have few choices. Matching two layers of cement board to an existing thickness of plaster isn't always do-able or the way to go. Sometimes it works, sometimes it won't work.

Quote:
Do you see a specific issue, or are you just commenting on the extra work required? An extra layer of CB is easier for me than floating plaster.
I understand where you are coming from but in my experience I have always found it easier to remove all of the plaster of an entire wall, then there is no matching to be done.

A second choice would be to float the wall with cement and that really isn't too difficult. I know it sounds impossible but it is fairly easy to do.

Quote:
I have the first layer of CB in place. I have only used silicone at the tub flange and to cover the edge of the plastic barrier.
Silicone in the interior is no big deal and a good idea in fact. But, when it comes to expecting thinset to stick to the silicone - it won't. On the surface panels before the tile you should tape and thinset those seams. The addition of waterproofing would be good also, but that is where I would not apply any silicone. Siliconized caulk maybe but not 100% silicone.

Quote:
The instructions I was following suggest caulking the corners and tub rim, but they probably mean after the tile is in place.
Correct. I still wouldn't use silicone there either. Caulk works just as well and is easier to tool and finish and you can get caulk in colors to match your grout.
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Old 10-04-2010, 11:15 PM   #12
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Replacing shower stall plaster


After the 1st layer of CB was up, I took an angle grinder and created a step where it meets the old plaster. This way I was able to tape and thinset the two wall sections together. The seam is not perfectly flat, but I have cut my my 2nd layer CB panels so they lap over the 1st layer seams by at least an inch or so including this top joint. I'll use chair rail tile pieces or some other trim to hide the mismatch.

The Hardibacker screws do not screw down flush with the surface. They protrude about the same as a roofing nail head, maybe a tad more. Is this OK? You would have to counter sink each hole to get them flush.
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Old 10-04-2010, 11:34 PM   #13
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Originally Posted by Bud Cline View Post
Cement board DOES NOT "block moisture". Cement board is not damaged by moisture but it DOES NOT block moisture.




Yup!
so what are you supposed to use instead in the bathroom behind showers and sinks?
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Old 10-05-2010, 12:23 AM   #14
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Replacing shower stall plaster


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Originally Posted by federer View Post
so what are you supposed to use instead in the bathroom behind showers and sinks?
This came up in another thread I recently posted, and I had the same thought you did - that cement board was water proof. It is to the extent that water won't damage it, but the water will still slowly soak through the cement board and reach the studs.

Apparently you should place some type of water (vapor) barrier between the cement boards and the studs they're mounted to, or you can skip that and just coat the cement board with Redguard after it's been installed into place.
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Old 10-05-2010, 09:50 PM   #15
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Replacing shower stall plaster


The screws I am using are Rock-On 2.25" (phillips head). These were the only ones long enough to hold on two layers of CB. Getting the screws flush to the surface is a common problem for CB in general. I pre-drilled the CB and used a dab of caulk on the screw. This was still not enough so I countersunk each hole slightly with a carbide cement drill. All the screws are now flush.

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