A leaky roof is an inevitable nemesis of owning a home; it is one of those problems that hard to handle 
due to poor accessibility as well as sometimes not being able to find the leak. Most roofing leaks occur in the same places, such as at any protrusion like a vent pipe or skylight and vent pipes are frequently the culprits. Luckily, such leaks are fairly simple to repair and do not take long or cost much compared to most other roof repairs.
Leak Prevention
Any pipe that comes through a roof should have a way to prevent it from leaking into the building. Vent flashing comes in various styles and material to be used in different climates and roofs although the basic idea is the same. It is a collar that fits around the joint where the tube comes through the building top, to keep water from getting inside the opening. Even a small amount of water constantly dripping down the sides of a pipe is more than enough to eventually cause damage inside the building.
Flashing Types
The two types of flashing most often used are: galvanized steel which is outfit with a rubber collar that fits around the pipe; and lead which has a lead collar or tube that slides over the pipe. Depending on proper installation by a roofing contractor, sealing the flashing with rubberized roofing cement may or may not be necessary. Some collars can also be installed with caps, which is a small hood for the pipe itself to keep the end of the pipe and the flashing beneath protected from rain. Lead flashing is generally pressed from one piece of sheet lead, while galvanized comes as one pressed piece with a rubber boot that fits snugly around the pipe to act as a seal.
Replacement
Both of these methods are inexpensive and easy to install. Unfortunately, it is not a lifelong repair due to the rubber. Both the boot rubber and rubberized cement only last a few years when exposed to the elements. Rubber will eventually dry or crack, rendering it useless as a seal. Where cement has been used, a fix may be as simple as re-applying a thick coating of it around any exposed edges where the flashing meets roof or caulking exposed seams again. When flashing with a vent boot is installed, the entire rubber boot pops out and may require replacement.
The best time to do this type of repair is before it fails, preventing even the slightest bit of water damage from occurring. Re-sealing any flashing or replacing rubber vent boots should be included with regular roof maintenance and done every two to three years, depending on the local climate. By keeping up with roof maintenance and preventing rubber seals from drying out, it is much less likely that one will ever have to experience the inconvenience and potential damage caused by a vent pipe leak!