I have a bathtub that has horrible drainage and its because of an old galvanized horizontal pipe that connects to the stack.
I cannot replace this pipe with PVC with a better slope without compromising the floor joist structure in the ceiling. Home is from early 1900s.
However, I do have the ability to run a new drain all the way down to the basement and add a y connector that would keep the old galvanized pipe as mostly a vent.
I am a little concerned with the space I have in the ceiling to pull this off but I think I can do it. I am more concerned about the small 2 in drop where the pipe exits to the stack if I use it as a vent. (Please see images below)
Can someone give me an opinion of what I have proposed? Would the existing horizontal pipe work as a vent in this situation or would I be making any regressions here?