Dear Experts,
Hope you are all doing well. Speedball , you are a plumbing info God. Your sense of logic would make a Vulcan proud. You helped me a great deal with my first stage of my plumbing and I now need some help with a venting question. I have a basement bathroom that I'm putting in that needs to be vented. I ran the sink vent pipe, toilet vent pipe and shower vent pipe into 1 pipe going up to the ceiling. It's a 2" that will reduce to a 1 1/2" which will run 21 ft across ceiling then around my HVAC unit and up to the 1st flr half bath where it will come up alongside the 1 1/2" vent pipe, connect to it with an upside down T joint that curves up. This pipe goes straight through the 2nd flr(doesn't connect to anything there) and goes straight to the attic and out. I'm concerned that I will not have room to pitch any of these pipes. Would I have to?? Does rain water drain through that vent pipe?? It's a 5 year old townhouse. Live long and prosper... Spock

Ahhh! Mr. Spock! Welcome back and thanks for the nice words.
I'm concerned that I will not have room to pitch any of these pipes. Would I have to?? Does rain water drain through that vent pipe?? You are "bang on". You must pitch the vent line back to the source because rain water does, indeed, drain back down the vent. If you can't manage 1/4" to the foot then drop to 1/8th" but you must have slope. You are correct again that you will revent back in the attic to a existing roof vent using a inverted tee.
But before you begin this project we have woirk to do on the planet below. I'm taking a "away team" down myself. Sullu, Oh Herra, Spock! You're with me! ENERGIZE! Have a great Sunjday! Tom