Backer Board only or on top of green board?

Discussion in 'Shower & Bathtub Forum & Blog' started by tnflyboy, Jan 15, 2007.

  1. tnflyboy

    tnflyboy New Member

    Jan 15, 2007
    I am finishing my basement. I have installed a bath tub with a shower. What I need to find out is the proper way to install tile around the tub. I was told to use cement backer board and place it inside the lip of the tub. Then at the top, butt dry wall against it. Then when the tile was laid, the tile would over lap the drywall.(I'm using 1/2" drywall and 1/4" cement board)
    I started trying to figure out how to hang the cement board. But it looks like if it is hung on the studs and goes inside the lip of the tub, it will have a pretty significant curve to it. And then in the corners there would be a decent size gap between the cement boards.
    I have heard about putting the cement boards ontop of drywall, which looks like it would help with keeping the cement boards straight.
    A third option I have heard is to butt the cement board to the top of the lip of the tub and allow the tile to hang over the lip.
    Can someone tell me the best way to do this????
  2. jimbo

    jimbo Plumber

    Aug 31, 2004
    San Diego, CA
    All of your suggestions are wrong!

    First, do not put backer over greenboard. There needs to be a vapor barrier behind the backer, heavy mil plastic or 15 lb felt. 1/2" cement board is the better material. But in any event, the backer does need to overlap the tub flange. You need to fur out the studs with lath spacers so the the backer board sheets hang plumb. All seams and gaps in the backer installation are taped with fibrglass tape and waterproof joint compound, or thinset.

    For excellent further advice on your project, check out this tile forum:
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  4. jlohrenz

    jlohrenz New Member

    Jul 19, 2006
    If the lip of your tub can't be notched into your studs so it is even with them then this is what I'd suggest you do:

    Get either roofing paper (tar paper) or 6-8mil plastic and cover all of the studs around the tub area with this. it should come down over the lip of the tub. This is the vapor barrier, and if any moisture penetrates your tile, it will run down this and to the tub surface.

    Secondly put the cement board on the walls and this can come to rest right above the lip of the tub if you cannot get the surfaces flush. If you can get the surfaces flush, stop the cement board a 1/4 - 1/2 above tub surface.

    Done properly there will be no 'bulges' near the tub lip. Your bottom row of tile will cover the gap at the bottom.

    I'd also check out as well as there are tons of tips there.

    Good Luck!
  5. geniescience

    geniescience Homeowner

    Nov 27, 2005
    humid summers hot, humid winters cold
    use the CBU as its own furring strips

    you can fur out the 1/4" CBU by cutting furring strips of the 1/4" CBU itself. This is what the James Hardi website has said, and one long-term tiler signed in at JBF has mentioned it as the only way to fur out Hardiboard back when 1/4" was the only one available. BTW, this is better than buying Hardi-500 which is not a true 1/2" in thickness; it's more like one centimeter.

    the way you described it initially was mostly correct, imho, if i read you right. The greenboard was to go above the height of the shower head, i presume. And then the tiles were to go up to and just beyond (higher than) the junction of the CBU and the drywall. You were missing the vapor barrier. It must go inside the tiling flange so that even invisible water (moisture) can creep down and get exhausted (evaporated) without getting sucked into the wood of your studs. Grout is porous and moisture will get behind the tiles. How much is anybody's guess, but moisture there will be.

    Did you know that greenboard is paper-faced and it going to let mold eat it from the inside? Even tho it has a mold retarding agent in the paper. So it is best to use CBU all the way up to the top, imho.

    What jimbo and hlohrenz said is 100% right too. Nothing i said here is contradiction. Hope this helps you "find out the proper way to install tile".

  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    New England
    Forget the greenboard...just use the cbu and if you want, regular drywall. If you are not going to tile to the ceiling, the drywall is fine above the showerhead with a good paint job.
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