Sun exposure and heat especially during the summer can cause the rubber boot to crack creating the space for water to leak in.
Quick patch roof leak.
Insert a nail in the hole where water is leaking through the roof.
One of you goes up on the roof and the other stays in the attic to see if they can spot any leakage.
Start low soaking the area just above where the leak appears in the house.
Go over the roof section by section.
Isolate areas when you run the hose.
Look above the ceiling leak point and scan the sheathing leading towards the peak of the roof.
With the help of a friend use a garden hose to try to discover the leak s source.
The person in the attic watches carefully while the one on the roof floods the roof with the hose starting at the bottom the eaves and slowly working uproof until water from the leak appears in the attic.
The nail will provide a path for the water.
Once the leak is found push a nail up through the hole to mark its location for rooftop repair.
Step 2 place a bucket underneath the nail to catch the water as it runs down the nail.
Unfortunately the rubber boot damage often becomes the main cause of roof vent leaking.
It may not be a direct line but it will always be above the ceiling leak location.
For example soak the downhill side of a chimney first then each side then the top on both sides.
Run water on the outside with a hose.