Flashing is thin metal or other waterproof material installed at joints and transitions in building exteriors to prevent water infiltration. Common locations include roof valleys, chimney bases, window and door frames, wall-to-roof junctions, and anywhere different building materials meet. Flashing works by directing water away from vulnerable joints and overlapping in ways that prevent water from flowing underneath. Materials include aluminum, galvanized steel, copper, and modern synthetics. Proper flashing installation is critical to preventing water damage, rot, and mould—many building leaks result from missing, damaged, or improperly installed flashing. Building inspectors verify flashing during construction. For existing buildings, deteriorating flashing may require repair or replacement. Climate-appropriate flashing design considers local precipitation patterns, freeze-thaw cycles, and wind-driven rain. Quality flashing is a relatively inexpensive investment that prevents costly water damage repairs.