How should I handle reflooring the bathroom of an old house?

We are updating the bathroom in our old house and after removing the old tile and concrete we found that it was sitting on 3/4" subfloor planks, and some of them are not in the best shape. We are planning to use Ditra or Ditra XL depending on the needed thickness for the underlayment. We have 1/4" porcelain tile (size 12"x23"). The gap between the old subfloor planks and the hardwood at the transition point is 1-1/2".

Questions:

1. The planks have quite large gaps between them in some spots, will this be a problem?
2. How thick should the total subfloor under Ditra and Ditra XL be? Would the old 3/4" planks with another 3/4" ply (at least) be enough?
3. What would be the best option to get to the same level as hardwood (1-1/2")?
• For example: add 3/4" ply + 1/4" ply + 1/8" Ditra + 1/4" tile
• Or: add 3/4" ply + 5/16" Ditra XL + 1/4" tile
Both options will be ~ 1/8" shy from making a perfect transition. Maybe there are better options?
1. Should we glue (carpenter's glue) and screw additional plywood or will screws (grid pattern 4"x4") be enough?

Photos of the floor:

• how far apart are your joists? Jan 23, 2021 at 0:00
• @norcaljohnny 16" between joists
– Tart
Jan 25, 2021 at 21:08
• see my posted answer. Jan 27, 2021 at 4:16
• Screws are enough, unless you don't want it to squeak in a couple of years, in which case glue is highly recommended (and simple, cheap, quick insurance). Jan 27, 2021 at 14:29
• add 1/4" ply + 1/2" CEMENT BOARD + 1/8" Ditra + 1/4" tile Jan 27, 2021 at 18:34

Two things here.

1. If you don't want squeaks... First you should remove plywood and add blocking to the joists. The swaying of the joists, even slight allows the screws to move and eventually squeaks. Second you need to glue the first layer of plywood down.

2. Your calculations are off - kind of by a lot - AND you are committing poor design thoughts at the beginning.

• you should have a threshold in the bathroom between the hardwood and the tile
• this threshold should sit higher than both - 1/4"
• the reasoning is a flood should stay in the bathroom
• yes you are building a mini pool!
• Big Issue - no calculation for the thinset. With bigger tiles this can get to be over 1/4". Rather significant. You will be troweling and back buttering.
• Another Big Issue - no calculation for the thinset for the Ditra install!!! This will be at least 1/8"
• So with 3/4" plywood, ditra, and tile you are at 1.5" conservatively... possibly over.
• so this threshold needs to be at least 1" so that it may hide the fact that your tile might be slightly higher than the wood.
• if back buttering the tile, you are probably going to need to put down less thinset with your trowel (under spec).
• thank you for the answer. Could you explain how to add blocking to the joists? What do you mean?
– Tart
Apr 5, 2021 at 14:53

If your joists are at 16" apart 3/4" sub-flooring will hold the weight and pose no problem. I suggest instead of adding odd sizes of ply down, make it up with the thin sit after laying down the ditra or ditra xl. At that point you dont even have to worry about the gaps between the sub flooring as it will not transfer up and affect the integrity of the ditra, thin set or tiles.