I have a 30 x 15 feet concrete slab in the basement which dips considerably (4 inches) from the outerwalls towards the center drain. As I don't want to lose ceiling height, I'll be installing ceramic tiles directly on the slab. Is there anything I can do to level out the slab for such a large area and in such a thickness? In the end I just want to create a less significant dip towards the center drain (in case there should ever be a flooding). Any advise would be much appreciated!