Flat Roof Cap Flashing

What Is Flat Roof Cap Flashing

Flat roof cap flashing, also known as coping or coping cap, is the metal covering installed on top of parapet walls, roof curbs, and other low walls that extend above a flat or low-slope roof surface. Its purpose is to prevent water from entering the top of the wall assembly. Without cap flashing, rain falls directly onto the porous masonry, concrete, or wood framing at the top of the parapet and saturates the wall core. Cap flashing wraps over the top of the wall in an inverted U-shape, with flanges extending down both the interior (roof side) and exterior (building face side) of the wall. Drip edges at the bottom of each flange prevent water from wicking back up under the cap and down the wall face through capillary action.

Cap Flashing Materials and Profiles

Cap flashing is typically fabricated from sheet metal bent to fit the specific wall width. Common materials are galvanized steel, aluminum, copper, and prefinished (factory-painted) steel. The gauge is heavier than standard roof flashing because the cap must resist wind uplift at the building's highest point: 24-gauge to 22-gauge for residential, 22-gauge to 20-gauge for commercial buildings. The profile includes a flat top surface that may be slightly crowned or sloped to shed water, and two flanges that drop down the wall faces by at least 2 inches. A hemmed drip edge at the bottom of each flange forces water to drip free of the wall rather than running down the surface. Sections are typically 8 to 10 feet long and joined with overlapping or standing seam connections sealed with polyurethane.

Installation and Attachment

Cap flashing attachment must resist the wind uplift forces at the top of the parapet. Concealed clips are the standard attachment method. Metal clips are fastened to the top of the wall with screws or concrete anchors at 24 inches on center or less. The cap sections hook over the clips and lock in place. This concealed fastening prevents water from entering through exposed screw holes on the cap surface. At joints between cap sections, the upwind section overlaps the downwind section, and the joint is sealed with polyurethane sealant or connected with a standing seam fold. Corner pieces are installed first, with the straight cap sections lapping over them. On long parapet runs, expansion joints are included every 20 to 30 feet to accommodate thermal movement of the metal without buckling.

Insurance Implications

Cap flashing is one of the most wind-vulnerable components on a flat-roofed building. Displacement of cap sections is common after high-wind events and often results in significant water intrusion into the parapet wall before the damage is noticed. Adjusters should inspect parapet cap flashing on every flat roof claim, paying attention to displaced sections, open joints, and missing corner pieces. If cap flashing has been displaced, the interior parapet wall may have absorbed water, requiring inspection for sheathing damage, insulation saturation, and mold. Xactimate estimates should include the cap flashing material and linear footage, any damaged clips or fasteners, sealant, and corner pieces. If the parapet wall interior is damaged, sheathing replacement, insulation, and interior finish repairs should be scoped as additional line items.