Here's my current tiling / backer board story: We have knocked a new door through from a 2nd floor room to the adjacent bathroom. The plan is to tile throughout. When we dismantled the cabinets in the bathroom, we found that the existing tile was sitting on 3/4 of cement like stuff. I removed a few existing tiles and the cement stayed behind in good condition. My error was in assuming that this would be the case in the whole room. To get the new tile level in both rooms, we clearly needed to put down a 3/4 base, which we did using 1/4 Hardibacker over 1/2 Hardibacker. This stuff is expensive and seriously hard work, even getting it in the truck at HD is a major ordeal. Anyway, having got the new area all ready, I started removing the rest of the existing tile and guess what, the cement fell apart. In fact, the only bit that stayed intact was the site of my initial investigation. Argh. So now I have to put down the same double layer of Hardibacker in this room also. If I'd known this up front, I would surely have only put in one layer, probably the 1/2, saving myself a lot of expense and effort. My only consolation is that with the double layer, and staggering the joints between sheets, we have a very good quality base for the tile. So I guess the lesson is look before you leap; and look really, really hard. Cheers, Norman Houston TX