cement board in shower, corners crumbled

Frizzle71July 25, 2013


I am getting my bathroom remodeled, and the contractor put up the cement board around the tub and the sheetrock outside the tub area today. Tomorrow he is coming back to do spackling.

I noticed that a couple of the corners of the cement board look broken or crumbled in spots. Is this something to be concerned about?

Also, he left a small space by the side of the tub where there is no cement board. Please see the photos.


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photo 2

    Bookmark   July 25, 2013 at 4:09PM
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photo 3

    Bookmark   July 25, 2013 at 4:10PM
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photo 4

    Bookmark   July 25, 2013 at 4:11PM
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photo 5

    Bookmark   July 25, 2013 at 4:12PM
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There are many posters who know more then me about these things. I hope they will post tonight for you.

Pic 1&5: Not sure if the board is coming down over the tile flange but I understand that is what should happen. Look at the instructions you have for the tub.

Pic 5: I don't like the wood where it is, but I am not an experienced remodeler. I would have cut the cement board around that area.

Pic 3: I think that hole can be filled in with thinset and fiberglass tape (not spackle). Otherwise the boards aren't supposed to be too tight together. All seams are to be fitted with fiberglass tape, and thinset applied.

What kind of vapor barrier are you using? If there is poly behind the board there shouldn't be Redgard or Hydroban on top of the board. If no poly behind the boards than make sure that everything is sealed with the paint on vapor barrier. I don't think the paint on barriers will stick to silicone, so don't silicone caulk those openings at the tub.

EDITED to state that these comments may be inaccurate, and I will be happily corrected by those with more experience and knowledge.

This post was edited by enduring on Thu, Jul 25, 13 at 20:42

    Bookmark   July 25, 2013 at 8:38PM
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Hi enduring, thanks for your input.

I don't think the board is coming over top of the flange, but if it did, wouldn't the board be kind of angled in at the bottom?

There is nothing behind the board, so I will make sure that something is done on top of it.

Wish I knew more about all this stuff :)

    Bookmark   July 26, 2013 at 1:15AM
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I think the board is in poor shape and I would not have used it in my DIY job.

The tub, do you know what kind it is? You can find installation instructions for tubs on the internet. If it is a Kohler you can go to any site that sells them and they also link instructions, as does the Kohler website. Is this a tub with or without a shower?

The wall either can be shimmed out with furring strips, or the 2x4s notched in so the tub tile flange can be recessed a bit. But I don't know if it is needed with just a tub installation, as the walls won't be wet like in a shower.

As I look on the Kohler web site they set the board down on top of the flange like in your picture, siliconed the tiny gap than tiled over the board with the tile providing the overlap. Then the gap where the tile meets the tub gets caulked with silicone. They don't mention any vapor barrier like Redgard, hydroban, or poly. The installation is probably where there is no shower involved. Again I am not an experienced remodeler, just a DIY on a few things. What you don't want is water to be creeping back behind the wall board to get into your wall/stud space.

Now the cast iron shower pan that Kohler has, is installed with furring strips on the studs that rest down onto the the top of the tile flange. Then, the cement board is dropped down over the flange and down onto the deck. The furring strips that are shown in these instructions look to be several feet long. I don't know how this is done in real life. I would have thought that the strips would need to be the entire wall length.

EDITED to post link that I had saved. This link has a picture that Mongoct posted, its the 5th post I believe. It is an excellent diagram of how to treat the wall, tub, and tile without using firring strips. This is how I did it on my non-showered tub.

Here is a link that might be useful: Is Kerdi-Board Overkill for Tub Surround

This post was edited by enduring on Fri, Jul 26, 13 at 7:55

    Bookmark   July 26, 2013 at 7:44AM
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Paint-on water/vapor barriers like Redgard, Hydroban, and Aquamix will "fill in" small cracks and corners. You just glob 'em on pretty thick and that does the trick.

On the little crumbly places, as long as they're little and the cement board isn't compromised, like with a long crack where the board bent or something, thinset will fill it in and hold the tile well.

I'm in the process of tiling a tub surround and set the cement board on top of the flange (to go in front of it you'd need to fur out the studs so it didn't bend in at the bottom) and then painted Aquamix over the joint. On one side of the tub the face of the cement board was almost even with the flange and on the other side it hung over because we didn't set the tub exactly centered between the 2 walls.

I would have your GC add a small strip of cement board over top of that wood in the last photo. I'm assuming that's wood that sits 1/2" behind the drywall and cement board, no? It needs to be covered with something just to bring it out flush with the adjoining walls.

Just my amateur DIY two cents to add to enduring's. :-)

    Bookmark   July 26, 2013 at 9:07AM
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I believe you generally tape joints in cement board and skim some thinset over. It helps to fill in crumbly spots. I know I tried really hard to keep my cement board in good shape when installing it, but sometimes the edges just crumble like that.

    Bookmark   July 26, 2013 at 2:07PM
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Thanks for the responses everyone!

The tub is a Kohler Villager cast iron, and I think you are supposed to have the board right on top of the flange like they did it.

I'll make sure they fill in all the crumbly spots properly, and also make sure they put a little piece of cement board in that space there.

    Bookmark   July 26, 2013 at 6:44PM
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You can have the board on top of the flange, as long as it is still proud of it. The tile will hang below the bottom of the board, 1/8" above the tub deck. Your waterproofing should extend at least 3" beyond the face of the tub. If you're using a liquid membrane (as opposed to Kerdi), that means you need to extend the cement board, not use drywall. Where the wood is showing is very prone to failure in any tub surround, so you want to make sure it is waterproofed very well. I would have them re-do the end wall to have the cement board extend the proper distance beyond the tub and down the front edge of it (the dog-leg). One piece there is better than cobbling it together by adding on more small pieces. Make sure the mesh tape is alkali-resistant.

    Bookmark   July 27, 2013 at 7:22AM
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