How step flashing works
Each piece overlaps the one below it, following the roof's slope up the side of a chimney or wall, so water is always directed down and over the next piece rather than finding a gap.
Common failure points
Step flashing can lift if fixings corrode or mortar joints it's tucked into fail, and even a small gap at one step can let water track down behind the flashing.
Repair and replacement
Because it's made of multiple pieces, step flashing can sometimes be repaired section by section, though widespread corrosion usually means the whole run is better replaced together.