Im putting a Swan brand fiberglass shower floor base in my basement bath. I intend on using concrete board and tiling the entire wall and ceiling within the shower. My question is what is normal installation procedure for installing shower base onto concrete basement slab?The plumber has hooked up the floor drain to the shower base but the base is not level on the floor? The base rocks a bit diagonally from corner to corner so I assume the slab is not level. Should he have shimmed it or used construction adhesive? Will the concrete backerboard hold the base in place once I put it over the lip of the base and tile?