tile underlayment over subfloor

Discussion in 'Remodel Forum & Blog' started by pipewoe, Sep 25, 2007.

  1. pipewoe

    pipewoe New Member

    Messages:
    22
    Hi,

    I've gutted down to the old subfloor in my bath remodel and want to reinforce it a bit w/plywood overlay prior to backerboard and tile. The room is 8x8 w/customary 5 foot tub, one closet, one w/d alcove. Is it acceptable to cut multiple rectangles for all the little spaces, or do I need to use 2 4x8 sheets and "cut around" all the little angles, etc. Is there a minimum thickness? I'm trying to stay fairly level w/the floor outside the door which is seems to be only 11/16 ths (hard to measure accurately.) Assuming 1/4 inch backer and 3/8 in tile plus whatever thinset is, can I use 1/2 inch plywood or should/could I go down to 3/8?

    Thanks
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,765
    Location:
    New England
    Need more information about what is there now. If the existing subfloor is planks, then the minimum thing you need to add is 1/2" C-faced or better plywood with exposure 1 or exterior rated plywood - no D face. If it is plywood, then the minimum thickness you should consider is 3/8". You probably can't find thinner stuff that meets the specs to install tile on anyway.

    But, all of this is mute if you don't have enough strength in your joist structure. To get this, you need to know the species of the wood, its condition, how thick they are, how tall they are, and their spacing. Then, the deflection can be determined. The size of the room is irrelevant, it is the structure under the floor that is.

    CBU works, but I really like Ditra from www.schluter.com. It is lighter, easier to install and cut, and provides more insurance for the floor. Especially on a small room, it is far easier, and the cost isn't a big delta.

    For the plywood to actually help, it needs to be in big pieces.
  3. pipewoe

    pipewoe New Member

    Messages:
    22
    Holy cats, that's a lot I need to find out. I guess I'll have to beg someone to go into the crawl space--I'm sure not going under there. From above, I can see that the subfloor is planks, 7" wide lain on the diagonal with a groove between each plank.

    I looked at the ditra but wasn't clear as to whether it's a plywood substitute or backer board material. Looks like HD has it--I guess I can go find out there.

    Thanks
  4. transition height is not a big deal. A body gets used to it easily.

    You will have to use the right materials, and they will take the space they take.

    david
    Last edited: Sep 26, 2007
  5. frenchie

    frenchie Jack of all trades

  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,765
    Location:
    New England
    I forgot the most important thing...how long the joists are between supports!
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