cap flashing

Cap flashing is a metal covering, typically fabricated from aluminum, copper, stainless steel, or galvanized steel, that sits atop and overlaps a base membrane flashing. Its primary job is to shield the top edge of the underlying flashing from weather and UV exposure, preventing water infiltration at the joint where the base flashing meets a vertical surface like a parapet wall, column, or roof edge.

The cap flashing overlaps the base flashing by a minimum of 4 inches on most installations, and the two are secured together mechanically or with sealant. The cap extends outward at an angle, typically 45 degrees or shallower, to direct water away from the critical seam. On parapets and walls, the cap flashing is often bent into an L-shape or hook profile so the upper leg can be embedded into mortar joints or behind cladding, anchoring it firmly while the lower leg overlaps the base.

Common variants and materials

Aluminum cap flashing is lightweight and economical but requires protective coating in corrosive environments. Copper develops a patina and resists corrosion naturally, though it costs significantly more and can stain adjacent materials. Stainless steel (typically 304 or 316 grade) offers superior durability in salt-spray or industrial atmospheres. Galvanized steel provides a middle ground in cost and performance but will eventually rust if the galvanized coating is breached. Sheet metal thickness typically ranges from 20 to 26 gauge for most applications.

Cap flashing failures occur when the overlap is insufficient, when fasteners corrode and loosen, when sealant dries and cracks, or when the cap is installed without proper slope. Water collecting behind a failed cap flashing can saturate insulation, corrode structural steel, and cause interior damage that remains hidden for months. In freeze-thaw climates, water trapped beneath cap flashing expands and can lift or buckle the metal, creating dams that accelerate deterioration.

The term reflects the flashing's position and function: it caps or covers the top of the base flashing assembly, much as a cap covers the top of a bottle. In roofing and wall trades, cap flashing is distinct from base flashing (which is adhered or mechanically attached to the membrane) and from counter flashing (which is sometimes used interchangeably but typically refers to flashing that fits into a building substrate rather than over it). Proper specification and installation of cap flashing often determines whether a membrane system lasts 15 years or 30 years.