Cement Board Installation

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Old 10-18-10, 11:00 AM
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Cement Board Installation

When installing cement board on shower walls, is it ok to butt the cement board against the top edge of the nailing flange, or should the board overhang the flange? I'm concerned that if I don't overhang the board, the bottom course of tile won't have anything to grab, but I'm also concerned that if I overhang, the cement board will be bow outward at the bottom and not line up with the adjacent drywall.
 
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Old 10-18-10, 11:12 AM
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It should overhang about 1/2"-3/8" from the tub surface I believe...then the tile is placed about 1/8" from the surface...so the unsupported tile is only about 1/4" or so. You could adjust that slightly depending on actual tile size. The gap from tile to tub is caulked.

And yes..you will need to use thin shims attached to the studs to bring it out to avoid the bowing.

These things I know from reading many, many posts here..not from experience. The measurements I quoted are approximate...one of the Pros will be more exact.

Oh...I believe you run your moisture barrier over the tub flange as well, then trim it with a razor after the board is up? That way any moisture that penetrates is redirected back into the tub.
 
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Old 10-18-10, 03:33 PM
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Hi guys,

It can be done either way. However, I've found that in order to over hang the flange you often need to shim it 1/2" or even more. It's no problem shimming the back wall 1/4", but the side walls often need more. Also, even 1/4" of extra grout necessary can look tacky to many people.

Alternatively you can shim the walls out with the same 1/2" backer board, either cut into strips or solid. This method closely mimics the edging appearance of an old fashioned mud job. Of course you can only do this if "mud caps" are available to match your tiles which in many cases non are to be found.

So, it's usually best to stop the concrete backer at the flange, caulk that area well and let the tile come down to within 1/8" of the tub supported by a little extra thin set mortar to fill the void. Obviously this doesn't work so good with small tiles.

As to the moisture membrane, I'd rather skip the paper or plastic on the studs and apply a surface membrane on the concrete backer then thin set the tiles. This makes a much better job waterproofing the surface so the substrate has no chance to get wet.

Jaz
 
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Old 10-18-10, 05:10 PM
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Ok, so I trimmed a 1/2" off the bottom of the cement board to so it no longer bows outward and now rests against the top edge of the lip/flange and is flush with the drywall. So now I need to caulk the bottom edge, but won't this cause moisture to build up at this point rather than drain back into the pan, or am I misunderstanding this whole moisture barrier thing? Seems to me that if moisture/water is collecting on the plastic vapor barrier so much so that it is dripping back into the pan, the caulk is going to stop it, thereby defeating the purpose. And is the cement board really going to get so saturated that water will start dripping down the plastic? Seems unlikely.
 
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Old 10-18-10, 06:08 PM
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The concrete board is thicker than the tub flange, so any moisture will drip down or simple wick into the board. You're only caulking the bottom of the board. BUT, I suggested applying a surface membrane so none of the substrate would get wet.

I like surface membranes (Kerdi is my favorite especially for showers), because unlike with other types of moisture control methods, the wall board is protected, while with paper or plastic on the studs the substrate can get wet. The latter method is to protect the cavity of the wall and adjoining walls. Surface membranes also allow the walls to dry faster so they're not likely to mold, mildew or cause the lumber to swell causing cracks or worse.

While I like Kerdi, you can also choose a liquid membrane, it's applied like paint. Just be sure to follow directions for mil thickness or it may not be as affective as it should be.

Jaz
 
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Old 10-18-10, 09:11 PM
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I already installed a 6 mil vapor barrier and installed the cement board over top. I'm simply looking for an explanation as to why I need to caulk the bottom when I have a vapor barrier behind. Caulk is to keep moisture out, but that doesn't matter in my case because moisture can't get past the barrier anyhow. It seems to me that caulking the bottom when using a vapor barrier would actually trap moisture behind the board (between the vapor barrier and cement board) near the bottom so maybe I should just tile it and grout it that way everything can dry out without being impeded by the caulk/silicone. Now caulking around the perimeter of the tub makes sense to me because there is a small gap and no vapor barrier behind.
 
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Old 10-18-10, 09:26 PM
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You're right, don't caulk yours since you already have the plastic on the studs.

For others reading this, do it the other way if you're using a surface membrane.

You're also correct that you will have no actually water running back into the tub. If you did, you'd have big problems. It's more of a condensation/vapor protection. Be sure to leave a little of the 6 mil dangling over the flange. If you had any moisture it would be wicked by the board since that gap will be caulked after the tiles are installed, but just in case......

Jaz
 
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Old 10-18-10, 10:07 PM
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Ok. Thanks. Sorry, but again, silicone on the outside of the tile vs the inside is going to do the same thing isn't it--prevent moisture from escaping on the bottom. After all, the silicone is to prevent moisture from getting behind the tile, but since I have a vapor barrier, this isn't necessary and is actually not a good thing. Grout along the bottom would allow moisture to escape (right???). So as of now, I'm not going to use caulk/silicone at all for the shower, only the tub (which has no vapor barrier).
 
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Old 10-19-10, 04:00 PM
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I wouldn't recommend silicone, use acrylic or siliconized acrylic caulk. The people that make your grout also make caulk to match in color and texture.

You need the caulk at the tub/wall, inside corners and where dissimilar objects meet. If you use grout at the tub/wall joint, it'll crack guaranteed.

Shower and tub? Did I miss something? I thought this was a tub surround, so you're also doing a separate shower stall?

Jaz
 
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Old 10-19-10, 05:21 PM
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OK, as I was posting my last answer, I realized that what we've been talking about is not a tub enclosure but rather a shower with a plastic base. I assumed it was a tub cuz Gunguy said "tub" a couple of times and you didn't clarify.

It doesn't change that much except you should have instructions with that plastic base which may tell you how to handle the bottom. Some want you to caulk it all except for a small areas near the front on both sides. Right where a groove is, (yours may not have it.) Follow what they tell you to do.

In any case the barrier on the studs is to protect the wall cavity, there won't be any actually water there. High humidity yes, but no water. The concrete board, being against the barrier, will wick any moisture and it will evaporate out the grout lines.

Jaz
 
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