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Thread: Moisture protection beneath CBU floor?

  1. #1
    Geotechnical Engineer Fistor's Avatar
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    Mar 2008
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    Default Moisture protection beneath CBU floor?

    Hi -

    Just a quick question here... I plan to put down some cement board over the subfloor, prior to placing tile.

    My question: do I need to add some sort of moisture protection (e.g. poly sheet) beneath the cement board?

    I know this is advisable behind cement board wall for a tub surround, but then again, there is much greater chance for water infiltration. Still, spills, etc. can occur on a bathroom floor, so I thought I'd see what everyone thought...

    thx!

  2. #2
    DIY scratch-pad engineer leejosepho's Avatar
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    I did not put any under mine, and I do not recall ever hearing I should ... and personally, I believe I would rather have a spill pass on through into the sub-floor rather than pooling all around my heating wire.

  3. #3
    DIY Member B2CHR's Avatar
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    No vapor barrier. I allways set the backer board in thinset so you can't use a VB. If I have to tear up the floor first then I put 30pd felt paper between the two layers or plywood.

  4. #4
    Retired Defense Industry Engineer jadnashua's Avatar
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    Nothing on the floor! Embedding the cbu in thinset is mandatory - this fills in any minor imperfections and provides 100% support for the tile...voids and possible movement are death to tile. Also, use the special mess tape on the seams.

    I'd rather use something like Ditra, though...SO much easier to install, and you can carry all you need for a big room on your shoulder in one trip. www.schluter.com
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  5. #5
    DIY Member B2CHR's Avatar
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    Will have to check into this Ditra. Here alot about it here.

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