: We have a new house with a partially completed radon midigation system. In trying to figure how to hook it up, I followed it from the basement into the attic in which it vents through the roof. At the bottom of the vertical pipe which extends through the roof there's a drain pipe which I would expect is there to drain rain water. This is where the mystery pipe comes into play: there's also another pipe running up into the attic which is also hooked to this vent pipe which I have no idea what it's for(I know it's not a bathroom fan vent). Does plumming need to be vented? Could that be what it is? (We have city sewer and water if this helps solve the riddle)

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I am pretty sure it is not a sewer vent pipe, that would be totally against code.
My guess it is to prevent the radon gases from building up.
You can always run a snake down it and see where it ends up...