I understand that ceramic tiles can easily be laid directly on top of a concrete slab, assuming that there are no dips or humps, i.e. the floor is flat (but not necessarily level). However, the level of the bathroom floor needs to be raised about 1.25" to match the flooring in the adjoining rooms.
According to various sources, concrete backerboard is the best choice for a DIY'er in a bathroom setting. I have read how to lay concrete backerboard underlayment on a plywood subfloor, but I have some questions about my specific application.
First, exactly how is the backerboard affixed to the concrete? Is a thinset adhesive used, along with a series of nails/screws?
Second, the concrete slab is currently painted with, I believe, a waterproof paint. Do I need to strip the paint off before laying the backerboard?
Third, since the floor needs to be raised about 1.25", I assume 2 layers of backerboard needs to be laid. How is the second layer affixed to the first layer?
Finally, what size gap is required between the backerboard and a wall to accomodate expansion/contraction? 0.5"?
Thanks in advance.