Short Answer
Common failure points include deteriorated boots, cracked flashing, improper installation, and sealant failure.
Detailed Explanation
Plumbing vent pipes penetrate the roof surface to allow air into the plumbing system. Each penetration is sealed with a pipe boot (also called a pipe flashing or vent flashing) — a rubber or neoprene boot that fits around the pipe and seals to the roof surface. Over time, the rubber boot deteriorates from UV exposure and temperature cycling, becoming cracked and brittle. Once the boot cracks, water runs down the pipe and through the crack into the attic. Other failure points include improper installation (the boot not properly sealed to the shingle surface), cracked metal flashing, and failed sealant at the pipe-to-boot joint. Pipe boot failure is one of the most common causes of roof leaks, and it's a relatively simple repair if caught early.
Why It Matters
Pipe boot failure is one of the most common and easily missed roof leak sources. Regular inspection of vent boots can prevent significant water damage.
When Repair Makes Sense
The pipe boot is cracked or deteriorated but the surrounding roofing is in good condition — the boot can be replaced.
When Replacement Makes Sense
The pipe boot is failing along with widespread roof deterioration — the boot will be replaced as part of the roof replacement.