I'm totally remodeling a bathroom ... I'm down to studs and bare floor. I'm putting in a new Kohler tub and 5 piece tub surround. question is this: after I put some plastic sheet on the wall, must I put backer board up before installing the tub surround, or can I attach the surround directly to the studs? I have some backer board because I was originally going to do ceramic tile, but it seems like a waste to use that behind a plastic surround.