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Thread: 1/8" backerboard gap? Really?

  1. #1

    Default 1/8" backerboard gap? Really?

    I installed two 3x5 sheets of backerboard in my tub alcove today. As I read from this forum I should leave a 1/8" gap between the two sheets and fill with thinset and mesh tape.

    Now that I have installed this, I keep looking at that gap and fearing that I am inviting problems for a leak. Wouldn't it be better to leave NO gap as that would protect better against water leak?

    I've attached a picture. Can someone reassure my fears?

    Snooze (Seattle)
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  2. #2
    Plumber jimbo's Avatar
    Join Date
    Aug 2004
    San Diego


    I don't know if 1/8" is necessary. Check on www.johnbridge.com to see what they recommend. But ANY gap would leak if not prepared properly. This is where the mesh tape and waterproof joint compound come into play.

  3. #3
    DIY Member GregO's Avatar
    Join Date
    Oct 2004



    An 1/8" gap is perfectly normal to allow for any movement without migrating through to the tiles. Thinsets are supposedly much more flexible to compensate for such movement. Therefore, you should tape and thinset every gap. I don't see any tar paper or plastic moisture barrier behind your cbu, do I??? CBU is not waterproof, but is water and rot resistant and doesn't degrade like gypsum drywall.



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