The upturned edge of a watertight membrane formed at a roof’s termination point by the extension of the felts vertically over the cant strip and up the wall for a varying distance, where they are secured with mechanical fasteners.
base flashing (Wikipedia)
Weatherproofing seam between a stone chimney and a tile roof on a building in Jersey, Channel Islands. The lead flashing is seen as light gray sheets at the base of the chimney.
Flashing refers to thin pieces of impervious material installed to prevent the passage of water into a structure from a joint or as part of a weather resistant barrier (WRB) system. In modern buildings, flashing is intended to decrease water penetration at objects such as chimneys, vent pipes, walls, windows and door openings to make buildings more durable and to reduce indoor mold problems. Metal flashing materials include lead, aluminium, copper,stainless steel, zinc alloy, and other materials.