Roofing and Roofers Company #3) vents and flashings.

#1 Valleys

#2  Chimneys

#3  Vent and Plumbing Flashings

Generally Plumbing Pipe Flashings are made either of lead or incorporate a rubber seal around a metal base. The rubber flashings often deteriorate long before the roof needs to be replaced. These can be sealed temporarily but should be replaced shortly.

If you suspect a leak near a roof attic vent (those are often the square vent hoods close to the roof peak or ridge) check to see if they are plastic or vinyl, their life expectancy is usually less than the roofing itself. Check all vents for cracks or debris built-up behind the vent. Make sure the vent base flashing has a waterway cut into the shingles which will allow the  water to pass easily around the vent. Also check to make sure that bottom half of the flashing are exposed and actually cover the shingles.