Flashing is any piece of water-resistant material installed at transitions to keep rain and snow melt out of roofs and walls. There are as many different types of flashing materials as there are uses for flashing. Lead is a traditional flashing material that is still used today for flashing around chimneys. More modern flashing materials include the peel-and-stick tapes commonly used during window installations. You can buy boots to flash plumbing vents and gaskets to flash electrical penetrations. Commonly used metal flashing for horizontal building details, like the one shown here, are usually copper, galvanized steel, or galvalume, and are available in many colors.

This detail shows what is often referred to as a cap flashing or drip edge. Cap flashing is used over horizontal elements on the exterior wall of a building, such as the top pieces of window and door casings, above horizontal trim details like a skirt board or belly band, and over horizontal breaks in panel siding. The flashing prevents water from pooling on the horizontal surface and provides a slope to drain water toward the exterior.

Cap flashing has three main parts. The upturned leg attaches to the exterior wall. The base extends out and over what is being flashed. The drip edge turns down toward the ground.

The upturned leg is attached to the wall sheathing with corrosion-resistant fasteners, and then covered with house wrap that is taped in place. If the water-resistive barrier (WRB) is something other than house wrap, and can’t be lapped over the flashing, the upturned leg is taped to the WRB. Whenever possible, lap WRBs and flashing “shingle-style,” with the higher material over the material lower material on the wall. This measure protects against water getting into the assembly.

The base is sized to the horizontal detail it…