Maximum "void" under cement under layment

Discussion in 'Remodel Forum & Blog' started by tomcat22, Jun 4, 2008.

  1. tomcat22

    tomcat22 New Member

    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: Jun 4, 2008
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    New England
    I'd be somewhat worried. CBU isn't structural. You might want to ask this question over at 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.
  3. hj

    hj Moderator & Master Plumber Staff Member

    Cave Creek, Arizona

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

    AZ Contractor In the Trades

    Phoenix, AZ
    16 guage or heavier.
  5. PEW

    PEW DIY Senior Member

    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. tomcat22

    tomcat22 New Member

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