If the horizontal is more than 1/3 the vertical, you go up one pipe size.
1 shower, 2 sinks, 1 whirlpool tub and 1 toilet. Can all these vents connect in the attic and return to the main 3" vent stack? Distance they must travel in attic is 25' approx.
If the horizontal run is 1/3 the vertical rise. Is that what you mean?
I guess I'm confused on what you mean by horizontal and vertical. Thx for taking this time to help.
Your total length of the horizontal run must be less than 1/3'rd the total length of the pipe's vertical run, or you need to bump the pipe size up one. The idea is that any horizontal pipe run still needs some slope to it, and you need to still have an air path for it to work as a vent in case for some reason it got some moisture in there (say rain, condensation, or whatever).
Important note - I'm not a pro
Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014
I think I'm picking up what your putting down. I understand the horizontal. But when you speak of vertical are you referring to the vertical rise of that horizontal pipe. This horizontal vent is say 30'. Is the vertical the distance this pipe goes from its beginning to where it ties into main stack? Sounds like just running 2"-3" up there is a safe call.