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I'm wondering the best strategy to prepping this for tile. It's 2x9 (actual) joists with 1x8 subfloor and 1/2x2 flooring. The strip flooring is full of gashes and cracks and the whole subfloor has been removed in places for access, as shown.

I’m looking for a voice of experience on whether to remove one or both existing layers or, if I leave them, whether to patch. Every approach has its challenges. I think my preferred approach is:

  1. Remove the 1/2", leave the 1-by subfloor.
  2. Repair the opening parallel to the joists (over the 3" pipe) with shims and OSB to the level of the subfloor. Do not repair the openings across the joists, leave them.
  3. Cover the entire floor with 3/4 inch T&G plywood oriented across the subfloor, parallel to the joists, over the 10 inch openings.
  4. 1/4 inch Cement board on top of the plywood.

Alternatives I can think of:

  • I could NOT remove the 1/2 strips, I want that mainly so the floor doesn't get too high. I could remove EVERYTHING but that seems pointless and I'd have to build the level back up.
  • I could fill all the openings but the plywood is meant to sit on 16" OC joists so it can span 10" openings. I would lay it with the strength axis across these openings. If I fill them I'd have to make it perfectly level with the surrounding floor.

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2 Answers 2

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  1. Ensure that the subfloor/joists are 100% solid! Even the slightest bit of movement will crack your tiles and/or grout over time. If this means you need to pull up all of the existing floor, then do it.

  2. Remove any damaged or otherwise unsuitable planks and replace them. Your tile will be no better than what's underneath it. Best case would be to remove them all and replace with the tile manufacturer's recommended subfloor material.

  3. If you leave the existing floor, after fixing it, rent a floor sander and sand it smooth.

  4. Carefully clean up all the sanding residue so that it doesn't interfere with your tile job.

  5. You may still want to consider underlayment material. Again, use the tile manufacturer's recommendations here. They know best and not following their recommendations may invalidate your warranty.

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Having done this once, almost successfully, my approach next time would be…

Fill, level, sand & screw down the existing floor. Fill the missing sections with matching planks. If you want to lift & relay, screw & glue when replacing.

Then two layers of large area board, one 8' x 4' or 8' x 2' particle [chip] board or ply, glued & screwed through to the joists & also screwed spacing evenly between joists, just into the existing floorboards - measure & mark before laying & watch out for screw lengths where the pipes are.

A second layer of large board, mapped so the joins are not near the first. This time, definitely plywood. Again, screw & glue right through the lower layers into the joists plus intermediate layers.

That will give you a floor that can float as a single piece against any building movement.

I would also recommend membrane between each layer, lapped up [& under] the drywall at the edges - that's where I went wrong before; the entire floor moved as a single unit, but the edges started to leak slightly. This was on a wet-room where we expected a lot of water on the floor, but underestimated the importance of the edge sealing/tanking. It gradually damaged the large sheet underneath & now needs to be replaced.

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  • Good ideas for a wet room but in my case, only part of it is wet and that will be done with layers of mud and membrane. For the rest I'm not quite following the logic of your suggestions 1) leave all existing layers and add TWO more layers of subfloor ... this will result in a 1.5 inch step up from the next room! and 2) Why sand if I'm covering with two layers of new subfloor using glue and screws? None of these are terrible ideas but it seems like overkill.
    – jay613
    May 21, 2022 at 21:05
  • 1) You can lose the existing boards to help if you want. They're not going to be adding much to the overall strength. If you don't want any step, you're going to have to start by planing the planned floor depth out of the existing joists. 2) You sand it for better adhesion & to make sure it's absolutely flat. ...When you say you're going to use mud & membrane… over what? You can't do that without your two bonding layers first or it will all simply move & crack away.
    – Tetsujin
    May 22, 2022 at 6:39
  • Yes the shower floor is built up from the joists, reusing none of the old flooring layers.
    – jay613
    May 23, 2022 at 10:20
  • So you're going to end up with two segments, not connected?
    – Tetsujin
    May 23, 2022 at 10:31

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