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Joining vents running downhill?

I am adding a bathroom to my upstairs, but it has never had plumbing on the second level. I don't have a "stack," just a vent pipe running up through the roof. I'll have to run at least a 3" drain down for the toilet (on the other side of the house), but I'd like to see if I can tie into my existing vent so I don't have to cut another whole in the roof. Unfortunately, my existing vent goes through the roof at a level below my kneel wall, so If I tied into it, my new vent would run up over the 2nd floor ceiling and back downhill for about 2 feet till in intersected the old vent. This doesn't seem up to code, but I thought maybe it would be okay because the downhill part would slope into the existing vent, so any condensation would still be able to drain out. If that doesn't work, I either have to have two vents through the roof, or I guess I could move the existing vent further up the roof (closer to the peak) and join both vents above the 2nd floor ceiling. Please advise, and thanks for the help.

One note - I just saw this in a book. There is a venting option called "loop venting." Essentially, that's all I'd be doing, it would just be a big loop. The original vent "stack" would allow any condensation to drain back down to the main drain in the basement.

Re: Joining vents running downhill?

Holy mackeral I got lost in the description.
Seems like your focusing to much on condensation draining in the vent instead of the vent function.
I can't say if your proposed method is allowable but one thing i learned is keep it simple. If that means putting in an additional vent up through the roof then at least it would be a straighter run.

Re: Joining vents running downhill?

waste vents cant run downhill hill period!

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