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Thread: Maximum "void" under cement under layment

  1. #1

    Default Maximum "void" under cement under layment

    Hello all. I'm deep in my bathroom remodel and am wondering how big a hole in the subfloor would be acceptable without compromising the tile floor. Specifically, I removed a 2 inch PVC drain and am left with a 2.5 inch hole in the 3/4 inch plywood subfloor. I plan on using 1 ft square tiles. Would this hole, under my cement underlayment, be a problem? I can't patch it from below, so I'd have to cut out a large area, add bracing, and replace with new plywood. My joists are 2x12, 16 inch OC. Thanks.
    Last edited by tomcat22; 06-04-2008 at 07:59 AM.

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
    Join Date
    Sep 2004
    New England


    I'd be somewhat worried. CBU isn't structural. You might want to ask this question over at www.johnbridge.com. As a general rule, on a floor tile, you want a minimum of 80% thinset coverage (ideally, 100% support) which assumes a 100% subfloor. The biggest worry would be if someone in say high heels stood just in the right position...but see what the tile experts have to say on the referenced site.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    Moderator & Master Plumber hj's Avatar
    Join Date
    Aug 2004
    Cave Creek, Arizona

    Default tile

    Place a piece of sheet metal over the hole and then tile it.

  4. #4
    In the Trades AZ Contractor's Avatar
    Join Date
    Apr 2007
    Phoenix, AZ


    Quote Originally Posted by hj View Post
    Place a piece of sheet metal over the hole and then tile it.
    16 guage or heavier.

  5. #5


    You did not state where the hole is located. If it is under a sink, tub, or vanity where no weight will be placed on it, the CBU should be fine. Anywhere else, the sheet metal will work.

  6. #6


    Thanks for the responses. The hole is in a traffic area. I hadn't thought about sheet metal. Great idea.


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