More than likely, you have a plumbing vent flashing boot that has failed. Our building codes require plumbing pipes that are designed to remove gases from our sewage systems. These pipes generally exit through the roof. They are kept watertight by installing a plumbing vent flashing boot at the point where this pipe exits the roof. UV rays will degrade the neoprene seal leading to a crack that will allow water to enter the house. Water will then run down the pipe and show itself on the bathroom ceiling.