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Thread: Cement board over drywall

  1. #1

    Default Cement board over drywall

    Is it acceptable to screw 1/4" cement board over greenboard as a base for ceramic tile in a tub/shower surround? If I do, is it necessary to put a vapor barrier between the cement board and the greenboard?
    I thought I saw that this was ok in a posting somewhere but I can't find it now.

    Thanks.

  2. #2
    DIY scratch-pad engineer leejosepho's Avatar
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    I would be more inclined to remove the drywall and install 1/2" concrete board, and I believe your tile job should make a great moisture barrier!

  3. #3
    Plumber jimbo's Avatar
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    Cement board should have a moisture barrier under it.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Contrary to common perceptions, tile and grout are not to be considered waterproof, and you should always put a vapor barrier behind them, and if you are using cbu, then it should go on the walls first. 1/2" on the walls, use the specified screws or hot dipped galvanized roofing nails (not electroplated), and use the special alkali resistant tape on the seams to tie the sheets together. You can install that tape while tiling (this makes it easier to keep it from getting a 'speed bump' from buildup).

    Another common misconception is that greenboard is much of any good as a waterproofer. While some local codes still call for it, the latest national ones do not. Wet, use cbu, dry use normal dryall - there's no spec for greenboard in there at all anymore.

    The best way to do this is to use a vapor barrier, bring it down over the tiling flange of the tub. For the cbu, depending on the size of the tile you are going to use, if they are big enough, you can stop the cbu just above the flange. If the tile are small, though, you want th ecbu to come to within about 1/4" of the top of the tub. To do that without it flaring out and making it hard to make the tile look good, you need to either notch the studs so the flange is flush with the outter side so the cbu will go over it without flaring out, or use shims on the walls to allow it to fit over the flange. Normally, it is cleaner to have it notch the studs, then matching thicknesses of the tiled areas and the drywall outside of the tub is easier.

    Check out www.johnbridge.com for tiling help.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  5. #5
    Homeowner geniescience's Avatar
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    Quote Originally Posted by Radagast
    Is it acceptable to screw 1/4" cement board over greenboard ...
    Yes, 1/4" is good for this. You can leave the drywall (greenboard in this case). If you don't want to remove it.

    By the way, just to let you know, 1/4" CBU used to be the only thickness sold by the James Hardie company, and they sold it for walls. Nobody has ever said that it wasn't good enough, or wasn't thick enough. I repeat that 1/4" Hardibacker can be used for walls without anything else underneath it.

    All cases: you need a vapor barrier. Yes, you can perforate it with screws. To be good and thorough about the job you are doing, you could add a gooey product into the hole, by unscrewing each screw a little and adding a dab of goo, to seal the hole, before re-screwing. I have done that.

    David

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    Jack of all trades frenchie's Avatar
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    There's also vapor barriers that go over the cbu (or even directly onto the drywall, no CBU required), so there are no screw penetrations.

    ...just hit John Bridge's.
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  7. #7

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    Thanks for the replies. I will probably put it over the greenboard, but I do have one more question about this. I have removed a portion of the drywall in one bathroom and can see that there is a 1/4" gap between the bathtub flange and the stud. A small wood spacer was placed in there to brace the tub. The bottom of the drywall was just notched out in the back to accommodate this 1/4" gap. If I were to use 1/2" cbu direct on studs how would I accommodate this gap? I assumed I could just notch out a small section of the cbu at the bottom to make it slightly overlap the flange. Is this reasonable (and necessary)?

  8. #8
    Retired Defense Industry Engineer jadnashua's Avatar
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    As I mentioned, it depends on the size tile. If more than 1/2 will be supported on the cbu, then stop the cbu at the tiling flange, but make sure the vapor barrier laps over the flange.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    Homeowner geniescience's Avatar
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    Don't try to fill that space. Leave the gap, and leave the CBU hanging (screwed firmly to the studs of course) in mid air above the tub's tiling flange. Leave the CBU edge far enough away from the tub's tiling flange that no water will ever touch it or get close enough to be sucked into the CBU. Your plastic sheeting behind the CBU is the thing to hang down low, past the edge of the CBU, so that it can be placed inside the tub's tiling flange, like a curtain that "puddles" as they say in designer-speak.

    Hope this is clear.

    Then, when you tile, the lowest tiles (i.e. the bottom course or bottom row), are what you make sure to align with the tub surface, and nobody ever sees the gap of space behind the tile where there is no CBU. You silicone the small small gap you left between the tub and the tiles.

    Hope this is clear.

    David

    p.s. if you use a more expensive kind of waterproofing like an orange sheet membrane or a bucket of paintable gunk, you do the same thing with the CBU, leaving an air gap, and on top of the CBU you paint or trowel on that liquid-applied membrane or you thinset on that orange sheet material.
    -dr

    p.p.s. The moral of the story is that you do not let any CBU or any drywall touch the tub or come close enough to it that liquid h2o that might puddle up behind the tile would reach it and get sucked into the structure of the board.
    Last edited by geniescience; 08-29-2007 at 08:37 AM.

  10. #10

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    Yes I understand. I'll just let the tiles cover the flange. That will make it easy.

    One of the bathrooms has a window in the tub surround. Anything special I need to do around this or just the same process of moisture barrier and cbu? Should the window sill slope slightly down into the tub?

    Thanks again.

  11. #11
    Homeowner geniescience's Avatar
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    Quote Originally Posted by Radagast
    ..One of the bathrooms has a window in the tub surround...
    slope sill, yes. What to cover it with could be a long discussion. You can tile it or paint it a lot with boat paint. You can do a lot of other things to it.

    Two bathrooms? one is a tub-shower, and the other isn't?

    David

  12. #12

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    I'll be doing a total of 6 bathrooms in two houses. Four are bathtub/showers, two are showers only. The one I am starting with is a bathtub/shower with a window in the 5ft wall. The next one doesn't have a window in the surround. I was assuming tile should go in the window area, but I'm new to this so if there is a better way I'm open to it.

    Thanks.

  13. #13
    DIY Member Sincraft's Avatar
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    Someone told me once that the drywall and the cement board on top of each other cause each other to expand and contract in 'shifts' causing cracks in grout. IS this true? I'd like to know because this may come up for me soon here. Although might be easier to just remove the drywall and have the peace of mind but would like to skip that if possible.

  14. #14
    Retired Defense Industry Engineer jadnashua's Avatar
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    Tile and the cbu expand and contract at the same rates. The screws that hold the cbu to the wall or floor will actually cause it to be crushed a little, enlarging the hole around the screw over time, but the tile and cbu will float together. Do not glue the cbu in place, and make sure to use the special alkalai resistant mesh tape on the seams to tie adjacent sheets together. You can do this during tiling. No need to taper the joint like on drywall since looks aren't important, the reinforcement is.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  15. #15
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    Default No grout cracks?

    I too had someone recently tell me that drywall and the cement board on top of each other cause each other to expand and contract in 'shifts' causing cracks in grout. Jim answered about cbu and tile expand and contract at the same rate, but the drywall is different (might be why lot's of people say take the drywall out completely.) But I have the exact situation noted here, 1/4" durarock on top of greenboard, vapor barrier over the greenboard, cbu properly screwed in and taped. but I wondered like the last question, will the grout crack prematurely if all else is done right?

    This "pro" further told me the cbu needs to breath and the vapor barrier can stop that hence cause the cracking, and that's where I devalued his opinion. But I don't want the grout to crack! ...Studs, Greenboard, vapor barrier, 1/4" cbu, good thinset, grout, sealer...any concerns about cracking? Guess I should add I'm in a basement with pretty constant tempature and humidity.

    Thanks!
    Roger

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