Once the leak is found push a nail up through the hole to mark its location for rooftop repair.
Quick patch roof leak.
It may not be a direct line but it will always be above the ceiling leak location.
Step 2 place a bucket underneath the nail to catch the water as it runs down the nail.
Sun exposure and heat especially during the summer can cause the rubber boot to crack creating the space for water to leak in.
Insert a nail in the hole where water is leaking through the roof.
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.
Have one person climb on the roof and spray water above the suspected leak location.
Isolate areas when you run the hose.
Go over the roof section by section.
The nail will provide a path for the water.
Run water on the outside with a hose.
If a leak is difficult to find enlist a helper and go up on the roof with a garden hose.
For example soak the downhill side of a chimney first then each side then the top on both sides.
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.
Unfortunately the rubber boot damage often becomes the main cause of roof vent leaking.