I'm new to this site but hope you gents can offer some advice. Sorry about the lengthy post.
I just removed 540sq of cracked 12x12s on 1/2" CBU over 3/4" OSB and it stuck really well. I added a pound of deck screws to the OSB, an additional 1/2" layer of ply, glued and screwed every 6" (every 3" on perimeter avoiding the joists) and blocked the floor every 16" on seams from underneath. That's the good part.
When I pulled the old tile up I found a considerable slope(1") where SLC had been added under the CBU. A real PITA to get off even with the air chisel. It looks like the builder(and I use that term loosely ) miscaculated a load bearing wall and tried to sister som LVLs down the center of the kitchen. I since added 2 seperate 10" LVL beams on floor jacks. The longest span is now 13'7" with 2x12s.
My question finally: I am planning to use hardi to build up the low area and run lath with SLC to feather to finish height. I f this is O.K. to do, is unmodified Custom mortar O.K. or do I need a better modified? Since I'm pouring some SLC and will be using a latex primer over the ply does this affect anything? Can I tile on the ply since it's sealed or....should I be considering Ditra$$ or Noble$$$ as I am close to matching 3/4" hardwood nowand do not want unnecessary height. Oh, and I don't want to pull this up any time soon.....
I would recommend using the Ditra as that will help the tile from crackin up. Large tiles can crack much easier and thus extra protection will help you. Or you could go with a 18" Avaire tile and that would not need the protection. If the floor wasn't perfect then it would be ok with that product.