Long time lurker, first time poster.
I think I might have a solution to my problem but I'd like to get some peer review on it before I start work. I'm remodeling a master bath (50sf, 10x5) in a 1966 ranch and would like to add some nice tile. I pulled up the 5/8 particle board underlayment to inspect the subfloor and found it has too much deflection. The subfloor is in decent shape, however the 2x6x1 1/2 tongue and groove boards span a post and beam setup by 48". Everywhere I looked said this is a no-go for tile and I agree. Since the subfloor spans under walls and such, I don't want to rip it out and start from scratch so I want to add joisting from underneath. Here's my idea: 1. Span a double hung 2x6 between the beams on both sides of the project area, 48"ish span. 2. Hang 2x6 joists 16"oc between the double hung 2x6 cross beams from step 1, adding construction adhesive to the top of the 2x6's. Span is about 60", perpendicular to the subfloor planks. 3. Screw the subfloor planks into the new joists from above before the adhesive sets up.
From here, can I get away with 1/4" ply underlayment glued and screwed with 1/4" cement board mudded & screwed to it if the new joists stiffen things up well enough? A flat threshold is really on my want list. I'm open to a sloped transition, however.
Any other "gotcha's" that I might be overlooking? I thank you in advance for reviewing my question. Please let me know if I left out any details that leave ambiguities in my setup.