Bottom Line Up Front (BLUF): How smooth does the subfloor need to be before you put down backer board? Long version: Renovating the bathroom. Ripped up the vinyl and found a layer of 12" linoleum tiles on top of a sheet of linoleum (except the 2'x4' area where the old vanity was). So I started chiseling it up. It was really slow going because the linoleum tiles were brittle and were flaking instead of coming up in big pieces. Then I noticed that the surface color actually appeared to be grey (it was hard to tell with the glue on them) and that, instead of being black like regular linoleum, the whole tiles were grey all the way through. Then I realized that they weren't, in fact, regular 12" squares. They were rectangles of varied dimensions. Finally it hit me : The "tile" was actually 1/4" backer board! So now I've got a bathroom where the middle 1/2 is stripped (more or less) down to the wood. And the end 1/4s are covered with a sheet of linoleum under 1/4" cement backer board. (You lay down cement backer board to give a smooth, strong, flat surface to put your tile on. At this point am I stuck pulling up the rest of the floor, or can I just put down a layer of thinset before I install my backer board to provide an even surface for tiling. How smooth does the floor under the backer board need to be anyway, because with the irregularities of the wood and the areas where the face of the linoleum came up, but the rest of it remained glued to the floor, it could be an involved project to get my floor perfectly smooth.