Take off all of the subfloor. Examine the joists. Calculate the d election. Reinforce the joists by sistering and cross bracing to eliminate deflection. One good 3/4” ply over the top of the joists, another 1/2” then Ditra and your tile.

Calculate your deflection. The smaller the tile, the less chance of cracking. https://www.johnbridge.com/vbulletin/deflecto.pl

Thank you, Sophie! I'll run this by them as well. It would be a bit more than the Wonderboard (3/4") versus your gameplan would be about 1.25". But still, less than 2" which I think is just nuts.

Im working with people fairly set in their ways, so may just need to find a pro more confident in these newer applications.

Your plan will fail. All decoupling membranes do is decouple. They do not add any additional stiffness. Winderboard is a decoupling membrane, and would need a layer of thinset underneath it to even perform that job. It does zilch for creating a stiff enough subfloor suitable for tile.

How to find someone who knows something. https://www.ceramictilefoundation.org/homeowners-guide-to-hiring-qualified-tile-installer

This is the industry bible that will tell you everything that you need to know. https://www.tcnatile.com/products-and-services/publications/218-english-publications/188-handbook/957-2018-tcna-handbook-for-ceramic-glass-and-stone-tile-installation.html

And for the record, Wonderboard, or ANY cementitious backer unit is NOT an uncoupling membrane. It's a bonding surface, ALSO with approved methods in the TCNA Handbook. Uncoupling membranes would include things like Ditra or Stratamat.

I’ll check. But 1925 House. Old joists too far apart. I’m betting all of that is going to need intensive care to make that happen. CB with the proper thinset underneath does have an uncoupling from the subfloor by bonding the tile to it, in a homogenous whole entity.

The joists are averaging 16" oc and are 2"x9.5" They run approx 9.5' long. I say averaging because there is one set that are 17" oc and all others are 15,75" oc.

One of the arguments against using ANYTHING but mud (per family head of project and contractor) is that the joists are "too far" apart. As far as I can gather, 16" oc is totally standard. Wonderboard recommends 16"oc joists, and 3/4" plywood then thin set, wonderboard, thin set, tile.

Our joists were sistered, shaved on top and the subfloor dropped so with a mud bed the tile floor was even with the adjacent floor. However it was built that way. It could be done on existing to reduce the height but it's a lot of work.

Those joists are MORE than fine. I was gonna say-- my house was built in 1908, and if anything, they OVERBUILT to today's standards.