The components involved
A proper chimney flashing system includes soakers, step flashing up the sides, a back gutter behind the stack, and an apron at the front — each doing a specific job.
Why the back gutter matters most
The back gutter, positioned above the chimney where water running down the roof would otherwise pool against the stack, is one of the most failure-prone parts of the whole system.
Installing or renewing the full system
When we replace chimney flashing, we typically renew the whole system together rather than one piece in isolation, since they're designed to work as an interlocking unit.