I have a one hundred year old house, planning to put in a new 3/4 bath and laundry room on the first floor (no framing done yet) and finish off the entry way in the same now-open room. Issue has to do with readying the floor for tiling. The joists holding up the current floor are about 20" on center 2x6s. On top of that are, from the looks from the bottom, thin 14" planks of undetermined depth. On top of those, I can see from the top there are wood planks, and on top of that the former owner put half-inch plywood for rugs. We'll be putting down tiles, and the plywood already creaks when I walk over it. From what I've read, tiles do not like much flex in the floor and I worry that I will do the tiling and have them crack.
An additional wrinkle to this is that the floor is not level and I will likely be using self leveling compound to level it out.
Would people recommend that I strip the floors down to the now existent subfloor and build them back up myself so that they are a bit more stable? Are the 20" OC 2x6s going to be sufficiently supportive over time to, in particular, hold my washer/dryer stacked or should I sister or otherwise buttress the joists?
Any help people can give me would be much appreciated.