Jadnashua
Retired Defense Industry Engineer xxx
Short answer...if the pipe exits out the roof, it is likely a vent! Now, you don't need a bunch of vents out the roof, they can be combined, but the rules are you can't combine them just anywhere...it must be 42" above the highest drain or 6" above the flood plane of the sink, whichever is higher. Once a drain, always a drain...same thing on a vent. Now, sometimes it isn't that hard to add a vent so you can use an old one for a drain, but again, there are rules.