Flashing is the thin metal that seals the joints and edges where a roof meets something else, such as a chimney, a wall, a skylight, or a valley. Its job is to direct water away from those seams so it cannot sneak underneath the shingles. Flashing is one of the most important leak-stopping parts of a roof, and most roof leaks start at failed or poorly installed flashing rather than in the open field of shingles.
Shingles do a great job shedding water across the open slopes of a roof, but they cannot seal the tricky spots where the roof changes direction or runs into a wall. That is where flashing comes in. Pieces of bent metal are fitted into those joints and tucked under the shingles so that any water running down is guided back out onto the roof surface instead of into the home.
Flashing shows up in every spot that breaks up a smooth roof plane. There is step flashing along walls and chimneys, valley flashing where two slopes meet, and flashing around skylights and vent pipes. Each of these is a hand-fitted detail, and each is a place water will find if the work is rushed. This is exactly why a complex roof with lots of chimneys, dormers, and valleys costs more to install well.
For a homeowner, the practical takeaway is that most leaks are flashing leaks. When a roof starts dripping after years of service, the shingles are often fine and the real culprit is rusted, cracked, or pulled-away flashing. A good repair finds and reseals the failed flashing rather than just patching the shingles around it.
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