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Before we begin, English is not my first language so my terms could be a bit wrong.

Considering I'm planning to install 1/4" concrete backer-board over the sub-floor before tiling it again, is OSB okay as a sub-floor or does it have to be plywood?

I'm in the process of remodeling our upstairs bathroom in a condo unit. What is there now is a 3/4" OSB sub-floor (over which tile was directly installed). Before we bought the condo, the toilet leaked and got shaky (and unusable, as it wanted to go through the floor; fortunately, there was a joist just underneath to support it); it's dry now but the OSB sub-floor around where the toilet was is completely rotten and has to be replaced.

I believe the original contractor who built the house did a sloppy job in there, 1st by installing OSB in the bathroom instead of using plywood and 2nd not putting any backer-board before installing the tiles and 3rd by the way the spread the thin-set under the tiles (the "curvy/old" way).

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At minimum I would replace the OSB that got wet. It doesn't do well when it's wet. Theoretically that should never be an issue but life happens and sometimes OSB gets wet. Personally I would replace it with plywood.

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  • I thought about replacing the section around the toilet with plywood instead of OSB to make sure the same issue does not happen again. But then, I'd like to avoid having to replace the whole sub-floor if possible :/
    – user25447
    Commented Sep 23, 2014 at 15:40
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    OSB has a higher tendency to resist moisture, but this also means it holds onto more moisture leading to rot. Plywood underneath a neglected leaking toilet will rot as well. The issue is not OSB vs Plywood; the issue is improperly installing the toilet and not addressing the leak. Keep in mind also that thinset and grout are permeable which is why it's not a great idea to install tile on wood (and why it's a terrible idea for a shower or location that regularly gets very wet), and again, given time, OSB vs plywood is irrelevant.
    – gregmac
    Commented Sep 23, 2014 at 16:18
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OSB was an idea introduced in the late 1970s and caught fire in the 1980s. After a few decades of use, compared to plywood, OSB just does not hold up as well. You would have thought wood manufacturers would have learned from manufacturers of plastics, and moved toward fiber-mat combination for strength and durability... guess not.

I have observed plywood homes which have solid (but squeaky) sub floors which are several decades old and still in excellent shape. On the other hand I have observed sub floors which are OSB and just a few years old and crumbling into their original configuration (saw dust).

Environmentalist may very well like OSB but for wet applications, its not desirable. In wet applications mold, mildew and rot are the constant battle. Bathrooms in homes are just not configured to deal with these issues, Bathrooms in residential construction don't disperse water/humidity from the air, and usually have very little air movement, most being tied to only one register outlet of the central air system. It helps if one keeps their central air system fan in constant "on" mode as opposed to allowing it to cycle off.

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Can't add a comment. Just to add I read that OSB edges tend to curl (expand upward) if OSB gets wet which causes havoc with any flooring. If I remember correctly the same article said there is ether newer OSB on the market or there will be OSB products coming out to address this issue. I can't remember where I read this, maybe finehomebuilding.

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