Flashing
Thin metal or membrane material installed at roof penetrations and transitions to prevent water from entering the structure.
Flashing is a water-management material—usually galvanized steel, aluminum, copper, or rubberized asphalt membrane—installed wherever the roof plane meets a vertical surface (chimney, dormer, vent pipe, skylight, or wall) or changes direction (valleys, ridges, eaves). Its purpose is to direct water away from these vulnerable joints and onto the roof surface or into gutters.
Properly installed flashing is step-flashed at chimneys and dormers (individual L-shaped pieces woven into each shingle course), counter-flashed at masonry chimneys (embedded into the mortar joint), and kicked out at the bottom of a wall-to-roof intersection to direct water into the gutter rather than behind the siding.
Failed or missing flashing is one of the most common causes of roof leaks. Repairing or replacing flashing at a chimney costs $300–$1,500 depending on complexity; valley flashing replacement during a re-roof is usually included in the roofing bid.
Real-World Example
The leak at the living room ceiling traced back to corroded step flashing where the dormer wall met the main roof slope; the roofer replaced all the step flashing for $650.