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!
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.