All of the elements of a roof move to some degree as weather conditions change. In cold climates, for example, materials shift significantly as sunlight heats the roof by day and later, when temperatures plummet at night. Also, temperatures on rooftops may vary considerably from those at other levels of a structure. The temperature and season when the flashing is installed are also significant. Tightly fastening metal when temperatures are warm means it won t have room to contract when temperatures drop.
Metal moves more than nonmetallic building elements, such as wood or concrete.
Metal flashings must be large enough to do the job but small enough to prevent unnecessary movement. Dimensional changes are accommodated by specifying materials that move at similar rates or by otherwise building some flexibility into the design. If the latter approach is used, the sections of metal must be adequately lapped at the joints and they must be short enough to limit movement. Cover and backer plates, though they require more installation time, shield lap joints where water leaks are likely to occur. High-performance, elastomeric caulk also helps keep water out.