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My home was build in 1906. I am installing tile and plan to use thinset and hardy board underneath. One room has a 1/2” particle board (not OSB) subfloor over 1”thick x 8” wide planks (probably fir) running in the opposite direction as the joists. Its pretty flat and sturdy but I am concerned about the thinset moisture and one edge of particleboard is 3/4” lower now. I considered just screwing hardiboard into joists. If I replace the particle board with a 1/2” plywood (same as adjoining kitchen) do I screw into the joists or planks? Previous owner attached to planks w nails, but I fixed those squeaks already. Does it matter which direction to lay them? I would prefer to avoid adhesive if possible. Or can I avoid the extra work and keep particleboard? (I think I know that answer unfortunately) Thank you!

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  • Adhesive is the #1 way to avoid future squeaks...
    – FreeMan
    Commented Jul 17, 2023 at 17:46

2 Answers 2

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The moisture from thinset will not be a problem. The real issue is if the old floor is perfectly flat. I've never met an old house with plank subfloor that is flat, but you can be the lucky one (but see below). You can remove the 1/2" and replace, or screw new one on top. If the planks are 1" thick and not moving, fasten cement board to planks.

Again, the real issue is that you need perfectly flat floors. I always say "flat and level", but always get comments from people saying "flat, not necessarily level." This is technically true, but in my decades as a carpenter, I have only once met a old house that had floors that were flat but not level, that is, the house sank evenly 6" on one side. Every other house has settled in a way that creates uneven floors in more than one direction, and I know of no easy way to make a floor flat but not level. There are better carpenters than me out there I'm sure.

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Hardie specs a 5/8" ply or 23/32" OSB subfloor. What's not clear is whether or not a 1x plank subfloor is equivalent to that. Personally, I'd replace the particle board—which IMO has no place in a house other than flat-pack furniture—with the ply/OSB.

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  • Particle boards were used for decades (still?) as a way to flatten floors. It's not about stiffening the floor, but going over cupped planks, etc., to then put finished floors on top. I almost always remove these layers when I find them, but only because I usually also remove the planks and redo subloors. But it's not that they have "no place" in a house. Sometimes people just want a new kitchen floor, not to gut the whole thing.
    – Cheery
    Commented Jul 17, 2023 at 18:54

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