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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?

jay613
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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 '22 at 17:56
  • How thick is the OSB? – FreeMan May 12 '22 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 '22 at 18:25
  • By shimmed up, you do mean doubled up, not just some shims on the joists to raise it? – crip659 May 12 '22 at 18:44
  • I did not mean doubling the OSB, but now you mention it that is very close to the thickness of the existing floor so if that works (I don't have it yet) I'll do that for sure. But otherwise I did mean putting furring (not shims) along the tops of the joists. – jay613 May 12 '22 at 18:49
  • Would not just use furring on the joists. Tiles need thick solid floor base between joists. Tiles do not bend well. – crip659 May 12 '22 at 19:24
  • I had a look, 2 layers of this OSB should work well. So, two layers of 19/32 OSB with 1/4 inch cement board on top? I'm not doing this myself, my tiler has done hundreds of floors and I trust him. The *question* is whether it is ok to use OSB beneath cement board *at all*, versus plywood. I know not to use OSB directly under tile because it expands. – jay613 May 12 '22 at 19:38
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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 '22 at 19:43
  • @EdBeal good answer ? At least I think so. – jay613 May 12 '22 at 19:45
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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 '22 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.

Ed Beal
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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 '22 at 23:33