jarvis design· bathroom guru
While it takes a little more time, there is no reason why you can't rough in any tub or shower drain from above.
Huh??You should have a drop ceiling just above the existing tub (that conceals the p-trap for the tub upstairs) and a high hot. Remove the high hot and take a look with a flashlight. You should have 1-1/2" pipe going from tub to a 2" p-trap below the slab. Replace 1-1/2" pipe with 2" and make sure to install a fire collar to maintain slabs fire rating.