I am looking to remove a fiberglass tub and shower and replace it with a tiled shower only. I found a system called quick pitch and pre-pitch that helps with getting the slope right when doing the base. My question is do I have to do a sloped pitch before I put in the liner and slope it to the drain? I am asking this because of the cement slab I will be putting it on. I know the slab is not sloped, but since I am going to be putting in a liner and sloping it then is the first slope necessary?