the other day my wife found water droplets hanging from the ceiling in the bathroom, and i went into the attic and found that there was moisture in the area around the plumbing vent. i also found the plumbing vent was moist, so i suspect there is a leak in the roof around the plumbing vent. can someone give me an idea as to how to fix this? it is an asphaly shingle roof. thank you very much.