The Probert Encyclopaedia of Architecture

FLASHING

In architecture flashing is pieces of metal, built into the joints of a wall, so as to lap over the edge of the gutters or to cover the edge of the roofing; the term is also applied to similar pieces used to cover the valleys of roofs of slate, shingles, or the like. By extension the term applies to the metal covering of ridges and hips of roofs.
Research Flashing