Roof Flashing

Flashing is a thin, impervious material, usually sheet metal, installed wherever the roof covering meets something else: a wall, a chimney, a skylight, a vent pipe, another roof plane or the roof edge. Roof coverings shed water well across open slopes, but joints and interruptions are where most leaks begin, and flashing bridges those gaps by lapping water outward and downward in the same way shingles or tiles do.

Materials

Traditional and modern flashing materials include lead, copper, zinc, aluminium, galvanised and coated steel, and stainless steel, along with flexible products such as rubberised asphalt, butyl and other membranes. Before manufactured sheet flashing, builders relied on careful shingle placement, mortar fillets and chimneys built at the ridge; lead flashing details appear in builders' handbooks by 1832.

US residential code generally requires metal flashing to be corrosion-resistant and at least 0.019 in (0.48 mm) thick, roughly 26-gauge galvanised steel.

Main types

• Step flashing (soakers): individual pieces interleaved with each course at a roof-to-sidewall junction.
• Counter-flashing (cap or cover flashing): fixed into or onto the wall to cover the top of base or step flashing.
• Apron (headwall) flashing: a continuous piece where the top of a roof slope meets a wall, or across the lower face of a chimney.
• Valley flashing: lines the internal angle where two slopes meet.
• Drip edge: formed metal at eaves and rakes.
• Kickout flashing: diverts water away from the wall at the bottom of a roof-wall junction.
• Pipe boots: collars around vent pipes and other round penetrations.
• Reglet: a groove cut into masonry to receive the top edge of a flashing.

Compatibility

Dissimilar metals can corrode each other. Copper and lead should not touch, or drain onto, aluminium, zinc or coated steel. Aluminium and zinc are attacked by contact with treated timber and by wet mortar. In coastal areas, salt spray speeds corrosion, so more resistant metals are commonly specified.