I have a leak on vent pipe where it offsets in the 1st floor ceiling from the 2nd floor bathroom wall. It doesn't leak when the shower is running or when the toilet is flushed. It only leaks when it rains! I checked the attic where the vent stack exits thru the roof & didn't see any external leak on the vent stack when it was raining heavily, but yet it continued to drip on the 1st floor. Can a leak develope on the vent stack if not cemented properly? Can I place a vent trap on the roof so that rain water cannot enter the vent stack?
Most likely culprit is a bad jack on the roof, allowing rain water to run down the outside of the pipe, then when it reaches the offset, gravity wins, and the water drips off the pipe. If you are certain this is not happening, I would suspect an unglued joint at the offset, thus supporting your idea that the rainwater is leaking from inside the pipe. Get a hooded roof jack, like for a heater vent, and temporarily slip it over the plumbing stack, stopping water from going down the inside of the stack, then see if this solves the problem.The correct fix would be to replace the bad joint, but if you cannot, I would externally coat the joints heavily with the proper glue, then in a few days seal them again with a quality marine sealant like 3M 5200.