Need some guidance on preparing a floor for tile. I'm am currently remodeling a medium sized second story bathroom in a home built in the early 1900's. Here’s the Low down. I demo'd the room down to bare bones including pulling up the subfloor exposing the 2X6 Joyce’s on 16 inch centers. I installed 3/4" T&G plywood subfloor. The floor has a pretty noticeable sag to it (3/4" to 1") at its worst. Now for the questions at hand. Will self leveling compound be enough to fix this and be suitable for 12X12 tiles? Also I plan on using a concrete backer board, what thickness do you recommend for best results and would I use the leveling compound before or after I install the concrete backer?

have done tons of searches an now have found this wonderful forum where everyone curses Arcyl Pro and now I know why. At the recommendation of two Home Depots and Lowes I used Acryl Pro for tiling ceramic tile over a very old hard vinyl tile in my basement and guess what they are popping up. I would of removed the old tile but it was way to secure to remove..the mastic back in the old days holds well as we all know. Now some of the tile in this 400 Sq2 room has set very well, I learned this by trying to remove it and cracking some tiles. I am going back and fixing the bad ones. My question for the experts is now that I know I should use a thinset mortar. Is there any distinct difference between the following: Versabond FlexBond Speedset Which would be the best for bonding to existing tile in a somewhat cold setting?