We are putting tile in our bathroom. We just removed the OSB particle board from our bathroom floor. We have several questions. (1) We could not remove all of it, there is a 2 inch strip along two walls and the tub. It is one half inch thick. The thickest Hardiboard is about 3/8 inch thick. We are looking for ideas on how to increase the thickness so that the butt joint is smooth/flush. We were thinking of using 1/8th inch thick press board. We would tack the press board down with a few small nails. Then install the cement board with 1.5 inch stainless or galv. screws. Does this sound okay? (2) If the combo of the press board (or other) and hardiboard still dont exaclty match the existing OSB, is there some plaster type stuff that we can use to level it? What should we use to fill in cracks, waterproof, smooth out joints? (3) How far apart/ how many screws for installing cement board?