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Thread: subfloor question

  1. #1
    DIY Senior Member coach606's Avatar
    Join Date
    Apr 2006

    Default subfloor question

    I had to cut a few access holes in the new subfloor in my new attic bath to make a few changes. I will be using 12 x 12" porcelain tiles on the floor. My contractor suggests that I need to add a second layer of 1/2" plywood screwed in over the first, then a layer of cement board or other moisture barrier.

    He says that because the tile is larger sized, it will tend to come loose unless I make the floor more rigid with a second layer of plywood.

    Anyone heard of this? It will be a real challenge for me to get it done for a variety of reasons. Also, I have a double sink and I stacked two san tees to make the arms. I calculated the distance from the floor carefully and it could change things.

    Does this really need to happen to keep my tiles from coming loose? I've never heard of it before.


  2. #2
    In the Trades Bob NH's Avatar
    Join Date
    Oct 2005
    New Hampshire

    Default Visit the Tile Experts

    Visit the tile experts at www.johnbridge.com

    I think you will find that the experts recommend two layers of plywood under 12x12 tiles.

    The problem could be cracked tiles, which is more serious than an occasional loose tile.

    Also, attic floors often have minimum size joists on long spans. That is another condition that is not good for porcelain tile. They will also advise you on spans and joists.

    You should keep a good stock of matching tiles to replace cracked ones.

  3. #3

    Default subfloor question

    We used chicken wire and a thin layer of thinset when we did a stone job in our breakfast room. Pretty stable. Try it!

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


    Penny, what you did is affectionately called a NJ mudjob. The vast majority of this type of installation fails. It may take anywhere from days to years, and usually does. Use it at your own risk, there are much better ways to do it. It is not an approved method of installation. The Tile Council of America sets the gold standard for tile installations in the USA, and it has been tested and found wanting...if you want something to last, don't do it. You might be lucky, but the labor is the biggest part, and there are much more reliable, approved methods.
    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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