I have what I feel is an odd problem, and I've asked a few people that are not sure the answer. About a year and a half ago, my bathroom ceiling had a huge water spot on it. Went into attack and the Vent Stack coming down from roof was wet. I had someone replace the boot on the stack on my roof and that cleared it up. About a month ago, I had a new Roof installed on my house. A couple days later, it rained quite a bit. About three days after that, my daughter let me know that the bathroom ceiling stain was back and the wall drywall was bubbled up. I assumed roofers didn't seal the boot so called them, and they came out to seal it better. The wall bubble had settled down a little before they came out to re seal. The next day, it rained quite a bit. Bubble back and i went up into attack and the Vent Stack from the roof was dry. HOWEVER, I have a big reducer to drop to a smaller PVC down to one bathroom. The other side of the reducer is a horizontal PVC to the smaller vent pipe down to the other bathroom. The leak were at the joints on both sides of the reducer to the smaller pipes (vertical down to one bathroom and at the horizontal pvc going towards the other bathroom vent). I dried all around it with a towel, but it was quickly wet and dripping again, at both joints. None of the Joints have glue, at least that I could see. I realize Rain Water can get into the Vent Pipe, but it was still dripping at these joints 4 days after the rain ??? A friend recommended some JB Weld, WaterWeld. I put that at both sides of the Reducer. I have not had any more drips since then and we've had a lot of rain. What would cause this to drip water for 4 days after a rain (prior to the JB Weld)? I'm now wondering if I have some sort of blockage at the reducer, like leaves, and water collected, and was slowly moving down and dripping out the joints that had no glue? Now I wish I hadn't put any JB Weld on it as I could have removed the horizontal pipe and put some fish tape into the reducer to see if it was blocked. Any ideas why I'd have drips at those joints when this is just a Vent Stack? It just seems too coincidental that it JUST started after the new roof was installed, but again, the vertical vent stack out of the roof is completely dry when it's raining.