Metal flashing materials and metallurgy
Metal flashing performance starts with matching alloy, temper, thickness and coating to exposure, forming method and adjacent materials. The exam tests why a metal fails in service, not just whether you can name copper or aluminum.
Material selection
- Copper: durable, soldered or brazed, develops patina; runoff can stain and corrode zinc, aluminum and steel. Use compatible fasteners and isolation.
- Stainless: type 304 for general, 316 where chlorides or coastal salt prevail; harder to form, needs dedicated tools and welding procedure.
- Aluminum: light, corrosion-resistant in many atmospheres, but anodic to copper and vulnerable to alkaline runoff and copper staining. Factory coatings protect appearance, not cut edges.
- Zinc: long-lived, preweathered or mill finish; requires ventilation and compatible details; soft and dent-prone.
- Lead: highly conformable, heavy, low melting point; toxic, so avoid potable-water contact and control dust/fumes.
- Coated carbon steel: galvanized, Galvalume or factory-painted; cut edges and scratches need repair.
Worked example
A coastal job specifies mill-finish aluminum wall flashing within 18 in. of a copper roof and type 304 stainless fasteners. Is that acceptable? No. Wind-driven chloride makes 304 prone to pitting, and copper runoff can plate onto aluminum and initiate pitting. Use 316 stainless or copper for the flashing, isolate fasteners, and detail drainage so copper-bearing water does not run onto aluminum. If aluminum is unavoidable, factory-coat it and keep it out of copper runoff; coating does not make cut edges noble.
Traps
Treating all stainless as equal; ignoring copper runoff; using galvanized or copper fasteners on aluminum; assuming paint protects sheared edges; specifying lead near potable water.