Flashing is made of sheet metal or another impervious material. Its purpose is to prevent water from penetrating a building at a joint or angle, generally on the roof. The most common locations for roof flashing are at valleys (where two downward sloping roof planes meet), chimneys, eaves, rakes, ridges, roof-to-wall intersections, and at roof penetrations such as skylights, stovepipes and vents. Flashing must be durable and rust-resistant. Traditionally, materials such as copper, leaded-coated copper or lead are used for flashing.