The answer
Corrosion-resistant sheet metal (galvanized steel, aluminum, copper) installed at roof joints and penetrations. Failure causes:
- Thermal expansion/contraction loosening fasteners
- UV degradation of exposed caulking/sealant (fails in 5-10 yrs)
- Installation errors (missing counter-flashing, reverse laps)
- Galvanic corrosion from touching dissimilar metals.
Sources
This answer is drawn from the standards and guidance below. It is general information about how these systems work — not a diagnosis of your property.