flashing
Components used to weatherproof or seal roof system edges at perimeters, penetrations, walls, expansion joints, valleys, drains and other places where the roof covering is interrupted or terminated.
flashing: metal or membrane barriers that stop water intrusion
Flashing is a strip or sheet of material installed to redirect water away from joints, seams, and penetrations where the roof covering ends or is interrupted. It sits between the roof membrane and whatever interrupts it, creating a continuous weather barrier that forces water to run off rather than pool or seep inward. Without flashing, water finds its way into walls, attics, and structural cavities within days of a heavy rain.
The most common flashing materials are sheet metal (galvanized steel, aluminum, copper, or stainless steel) and synthetic membranes (EPDM rubber, PVC, bituthene). Metal flashing typically ranges from 0.019 to 0.032 inches thick and is formed into L-shapes, T-shapes, or custom profiles depending on the application. Membrane flashings are often bonded directly to the roof membrane and the vertical surface they protect. The choice depends on substrate compatibility, climate exposure, and cost. Copper develops a protective patina but costs significantly more; aluminum is lighter and cheaper but requires isolation from copper and dissimilar metals to prevent galvanic corrosion.
Flashing must cover roof-to-wall intersections, pipes and vents penetrating the roof, gutters and drains, skylights, chimneys, expansion joints, and roof valleys where water naturally concentrates. At each location, the flashing overlaps the roof membrane on one side by at least 8 inches and extends up the vertical surface on the other by at least 8 inches. The top edge is typically sealed or tucked under siding. Counterflashing (a separate piece) is often installed over the top edge of wall flashing to shed water back onto the roof rather than behind the wall.
Flashing fails when edges are left unsealed or unflashed, allowing water to work sideways under the membrane. Poorly sealed penetrations around fasteners create paths for capillary action. Flashing pulled away from the substrate by wind or thermal movement exposes the gap beneath. Corrosion from incompatible metals or from trapped moisture also compromises integrity. Improper overlap and slope allow water to be driven upslope rather than downslope during wind-driven rain.
The term flashing comes from the way the material flashes, or projects outward, from the roof plane. In roofing trades, flashing is considered a critical component separate from the membrane system itself, even though modern installations increasingly use integrated membrane flashings molded as one piece. Proper flashing installation often determines whether a roof system lasts 15 years or 30.