We have solid slats, then plywood subfloor on top of that. Plywood is in fairly good condition but will need to be replaced in one small section. My problem is in figuring out what to put on top of the plywood subfloor before the tile (6" porcelain). The floor has a bit of a bow in the middle of the room, I would say about 1/4" and is a little squeaky. So, what I will do is drive a bunch of screws into the current subfloor to tighten it up. Now...should we use 1/4" or 1/2" backer? Should we use hardi backer, durock or wonderboard? We have already purchased 1/4" hardi backer and we were told to affix it with liquid nails and lots of screws. However, now I am worried that 1/4" won't be sufficient and perhaps 1/2" would be better. That extra 1/4" may make it so that our new dutch door will not clear the counter tops when installed so if it can be avoided I would love to avoid it. I guess I just need some advice to set me straight! Going nuts second guessing just about everything with this kitchen project. Thanks for your help!