Flashing
Thin pieces of metal or other waterproof material installed around roof penetrations, valleys, and edges to prevent water infiltration.
Where It's Used
Flashing is installed at every point where the roof plane is interrupted — valleys (where two roof slopes meet), around chimneys and skylights, at pipe penetrations, along roof-to-wall intersections, and at drip edges. Each location uses a different flashing style (Step Flashing, Chimney Flashing, valley metal, Pipe Boot) because each sheds water differently, but the underlying job is the same: direct water over the shingle surface instead of letting it find a gap.
The Most Common Failure Point
Shingles themselves rarely cause leaks — flashing does. Metal flashing can be undersized, improperly lapped, sealed with caulk instead of properly integrated into the shingle courses, or simply corrode and separate from age. Because flashing failures are often small and slow (a pinhole, a lifted edge), they can leak for months before a stain appears on a ceiling, by which point decking damage has already started.