Flashing Failure

Flashing failure is any loss of watertightness in the components that bridge the joint between a roof covering and something that interrupts it: a wall, chimney, skylight, vent pipe, valley or roof edge. Because these joints concentrate water and movement, flashings are among the first places a roof leaks, often long before the main covering is worn out.

Flashings may be sheet metal (galvanised or coated steel, aluminium, copper, lead, zinc), flexible membrane, or preformed rubber boots. Many details use two parts: a base or step flashing integrated with the roof covering, and a counterflashing (cap flashing) fixed to the wall or chimney that laps over it.

Common failure modes

• Sealant used instead of laps. Details that rely on caulk or roofing cement rather than shingle-fashion overlaps fail as the sealant ages and cracks.
• Missing or short step flashing along sidewalls, so water runs behind the covering.
• Counterflashing pulling out of mortar joints or surface-mounted counterflashing whose sealant has failed.
• Corrosion of thin or incompatible metals, or galvanic corrosion where dissimilar metals touch.
• Cracked rubber pipe boots, which degrade in sunlight.
• Missing kick-out (diverter) flashing where a roof edge meets a wall, sending water into the wall rather than the gutter.
• Mortar flashings (fillets) on tile and slate roofs that crack with movement.
• Physical damage from ice, foot traffic, or reroofing where old flashings were reused.

Signs

Inside, failures show as stains at ceilings next to chimneys, skylights and walls, or wet sheathing around a penetration seen from the attic. Outside, look for gaps, lifted edges, rust, cracked sealant, displaced counterflashing and staining on siding below roof-to-wall joints.

Repair approach

Lasting repairs restore proper overlaps so water is shed by gravity. Re-caulking may stop a leak temporarily but is generally treated as maintenance, not a permanent fix. On many reroofing jobs, flashings are renewed rather than reused, particularly when they are corroded or were bent during removal. On historic slate and tile roofs, traditional lead soakers and cover flashings are usually replaced like for like.