Vents And Stacks
The rubber collar around a plumbing stack is the cheapest thing on a roof and one of the most common leaks, because it perishes long before the shingles do.
Short answer
A plumbing stack passes through the roof with a metal flashing and a rubber collar sealing against the pipe. The rubber cracks from ultraviolet and from freeze-thaw within fifteen years or so, well before the roof is finished. Replacement is quick and inexpensive, and a split boot is one of the easiest leaks to confirm.
Why the boot fails
Ultraviolet degradation of the rubber, which hardens and then cracks.
Freeze-thaw cycling, which works the crack open.
Movement of the stack with temperature, which stresses the seal.
Animals, particularly squirrels, which chew the collar.
Ice forming around the stack and lifting the collar.
And the age mismatch: a boot has roughly half the life of a modern shingle, so one roof needs two sets of boots.
Easy to confirm
From the attic, look at the underside of the sheathing where the stack passes through. Staining in a ring around the pipe is conclusive. From the roof, a cracked or split collar is visible from a few feet away. Of all roof leaks, this is the one most easily diagnosed and most cheaply fixed.
The other penetrations
Bathroom fan roof terminations, where the flashing and the damper housing both fail.
Kitchen range hood terminations.
Static roof vents, where the flange seal or the fastener seals fail.
Powered attic vents, which have a bigger opening and a motor housing that deteriorates.
Satellite dish and antenna mounts, which are fastened through the roof and are the classic retrofit leak.
And anything added after the original roof, which is where the owner should look first on a leak that started recently.
Doing it right
Replace rather than cement. Roofing cement around a split boot buys a season and makes the proper repair messier.
Lift the shingle courses to install the new flashing properly, rather than surface mounting it over the shingles.
Use a boot appropriate to the pipe size and material.
Consider a metal collar with a replaceable seal or a lead flashing on a roof intended to last, because it outlives rubber.
Replace all the boots at once on a roof of that age, because they are all the same age.
And do them during a reroof rather than afterwards, which is both cheaper and better detailed.
Common questions
How long does a stack boot last?
Roughly half the life of a modern shingle. One roof needs two sets.
Can it be sealed instead of replaced?
It buys a season and makes the proper repair messier. Replace it.
How do I confirm it?
Staining in a ring around the pipe on the underside of the sheathing.