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I've read that OSB is not suitable as a flooring substrate for tiles. But if the substrate will be cement board, is it ok to use OSB beneath that for subfloor repairs?

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  • It should be, since the tiles are not on it. Main thing for tiles is to have a solid base that does not move under any normal weight. 1/2 or 3/4 inch sub might not be enough for tiles, by itself.
    – crip659
    May 12 at 17:56
  • How thick is the OSB?
    – FreeMan
    May 12 at 18:15
  • OSB is 19/32 nominal 0.578 actual. It will be shimmed up on joists to match existing floor that is 1-1/4" thick . Then the entire floor covered in 1/4" cement board.
    – jay613
    May 12 at 18:25
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    I have used backer on OSB many times, I like to use thin set to the floor for a rock solid base but with OSB I use mastic, it has to dry for 24-48 hours but adding an additional bonded level makes the floor tougher and less prone to damage.
    – Ed Beal
    May 12 at 19:43
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    OSB is approved under schluter ditra and other mats as well applied with thinset so its perfectly fine with backer board IMO
    – redlude97
    May 13 at 21:47

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I have used backer on OSB many times.

I like to use thin set to the floor for a rock solid base (OSB pulls the moisture out of the thinset two fast and if kept wet it swells please don’t ask how I know (oops)) with OSB I use mastic it has to dry for 24-48 hours (the brand I used)but adding an additional bonded level makes the floor tougher and less prone to damage.

I was taught to bond the backer to wood flooring and have seen the difference it makes in my own homes (I am a really big guy) I found the tiles if also back buttered much less susceptible to bullseyes when things are dropped on them.

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  • Good advice. I'm not optimistic I can get my tiler to do the mastic and then he and everyone else pause for 48 hours. On the other hand I'm using 24x24 floor tiles so your suggestion is worth a lot. Good food for thought.
    – jay613
    May 13 at 23:33

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