The old 1-1/4" tub drain had no slope, it runs level, cut into the top of the floor joists to the stack. I think shower drains are supposed to slope downward to the stack, like at least 1/8" per foot? I'm not putting in a tub, just a custom tile shower, so the new drain will be a couple of feet over. Thinking of running the new 2" drain down to the basement where the stack is PVC so I don't have to mess with the cast iron.