I'm installing a Kohler acrylic tub in a three wall alcove, and have a few questions. My plumber told me to build a frame that is 16 5/8" high to support the tub (the actual height of the tub from the floor to the top of the tub). But in the instructions, it says to leave a gap of 1/16" between the tub lip and the frame surface, which would make it 16 9/16". Now is the idea that you want the tub to sit on the floor (set with sanded cement) and not have the tub lip touch the frame at all? What is the purpose of this? Is it ok if the tub rests lightly on the frame? Also the instructions say to lay a plastic sheet over the motar and the tub. Any advice would be appreciated.

I'm installing a bathroom in our basement and have a 48" shower unit that will be installing. While framing the walls for the shower, we set the shower base in the alcove to make sure everything would fit okay, and I could see that the shower unit doesn't sit level on the concrete. The concrete floor was sloped slightly when it was put in (40+ years ago) to direct water to a floor drain. The shower unit is ribbed underneath, so it's designed to be installed without pouring any support underneath it. I was kind of figuring I would run into this, and now that I have, I'd like to know the best way to level out the base. I've read that some people shim underneath it, and others put either grout, or quickcrete under it. Anybody have any tried and true methods for doing this?