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Thread: tiling gap between backerboard and tub

  1. #1
    DIY Junior Member suellen's Avatar
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    Default tiling gap between backerboard and tub

    We had a new acrylic tub installed which has a flange that attaches to the studs. The tub installation instructions said to install the cement backerboard with a 1/2-inch gap between the tub flange lip to prevent wicking, with the tile then installed extending down over the flange. I've just finished installing the backerboard. However now that it is time to tile, this seems to create difficuties. I have a 1 & 3/4 inch gap from where the backerboard ends to the top of the tub lip (one half inch gap open as per directions plus height of flange). I can see how the tile will adhere to the backerboard, and I can caulk a 1/4 inch section were the tile meets the tub. However, how do I grout the vertical seams between the tiles? It is cavernous behind the bottom row of tiles....Also, this seems to potentially expose my studs to more moisture than if I had brought the backerboard down to or over the flange...

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    The recommended procedure is to install a vapor barrier behind the cbu and lap it over the tile flange. You can adhere the tile with thinset IF at least half of the tile is on the cbu (i.e., if the tile is over 2x1-3/4" in your case). If the tile is too small, then you'll have problems. While a little messy, you should be able to grout that area.

    Another possibility (too late without a lot of work) is to notch the studs so the flange is flush with the stud face, then run the cbu down to about 1/4" from the top of the tub (over the flange). Or, you can shim the studs then install the cbu over the flange.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    DIY Junior Member suellen's Avatar
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    Thanks - we did put a vapor barrier behind the cement backerboard and the tiles are 12" long so that part should be okay...I'll have to think about if we can see a way to get the board down further without having to do a total removal of the majority of the backerboard. If it was just a matter of unscrewing and recutting a few pieces it wouldn't be so bad but we already taped and mortared the seams, etc.

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