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Thread: Bathroom subfloor damage

  1. #1
    DIY Junior Member nickyg's Avatar
    Join Date
    Feb 2006

    Default Bathroom subfloor damage

    We are remodeling a small bathroom. I pulled all of the old flooring up and finally reached the subfloor. Around the toilet was some water damage and the plywood was slightly delaminating. The top of the subfloor was damaged and weak, but from the basement the wood looks great. I decided to put floor leveler all over the floor to help add some strength instead of replacing it. I came back after the leveler had dried and there were some cracks near the toilet in the weak floor areas. I was just planing on putting backer board and ceramic tile over the leveler, but now I am worried.... Is the floor sturdy enough??? Will the tile crack just like the leveler did??? What can I do to make sure I get a solid surface to finish the floor??


  2. #2
    Aspiring Old Fart, EE, computer & networking geek Mikey's Avatar
    Join Date
    Nov 2005
    Central Florida

    Default Replace the subfloor

    Not sure what "floor leveler" you used, but typically that stuff offers no strength other than in compression. I would cut out a piece of the old subfloor big enough to span 3 joist bays, then sister the outside joists and screw in a new piece of plywood. That might require replacing the toilet flange, which should be mounted on top of whatever the finish floor is. SInce you have access from below, that shouldn't be as bad a job as it sounds.

  3. #3


    It would have been smarter to just replace the bad subflooring in the beginning. But if the subfloor is strong, and that's a big "if", I wouldn't worry about the crack in the floor leveler. I've seen litecrete floors that had so many cracks it would make your head spin, and the floors were strong enough. But the key once again is how strong is the sub floor. If it's weak, the tiles will probably crack eventually.

  4. #4
    Retired Defense Industry Engineer jadnashua's Avatar
    Join Date
    Sep 2004
    New England


    Before going to all of the trouble, you need to determine if the floor itself is strong enough for tile. At www.johnbridge.com you can access a deflection calculator to assess your floor joists. They will also help guide you through a tile install. THen, you need to make sure your subfloor is strong enough (i.e., the plywood). Depending on the spacing of the joists and the type of tile (ceramic or stone), will determine how thick the subflooring needs to be and how it is installed. Cement board (cbu) is not structural, so don't think using 1/2" is better on a floor, it isn't for any practical purposes. The cbu must be installed in a bed of mortar and screwed or nailed down properly before you tile.
    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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