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Flashings are components of the exterior building envelope used to intercept and direct the flow of water to designed drainage paths. They are required to minimize the entry of water through joints and between materials. Flashings are usually installed wherever there is a discontinuity on exterior surfaces, such as:
at the top of exposed walls;
at roof-wall junctions;
at roof-chimney junctions;
around penetrations through roofing;
at valleys in roofs;
within walls above and below doors, windows and other wall penetrations; at foundation level to lead water out of the wall cavity; and
at locations where water might enter the building through a juncture between two materials.
Flashings are also used to direct water away from building elements susceptible to deterioration, staining, erosion, frost damage or corrosion when exposed to excessive wetting. As discussed in the previous chapter, flashings are part of the primary and secondary lines of defence against rain penetration.
DESIGNING FOR FACTORS THAT INFLUENCE WATER FLOW Flashings must deal with the forces that drive the flow of water, because water can move upward and sideways as well as downward. These forces include gravity, surface tension, capillary action, kinetic energy and differential pressure.