I have an old house and I am wondering when I replace my tub, do I put the tub in place and put the Cement Board (Durarock) over and outside the lip edge of the tub or how does it work if my house does not have that Cement Board (DuraRock) there now? I understand it is used behind tile and wet / dry locations. I need to replace my cast iron Tub and just trying to do it right. Not up on Bath Room Remodels but I need to get it started soon. Any info would be welcome. Is it like drywall just used behind the Tile areas of a shower / Bath Room. And you put the tile over that and match the tile up with the top edge of the bath tub? And groute it in and then chaulk the space between the tub and the tile for expansion and contraction?