step flashing
Individual pieces of sheet metal material used to flash walls, around chimneys, dormers and such projections along the slope of a roof. Individual pieces are overlapped and stepped up the vertical wall.
step flashing: roof-to-wall seal that climbs the slope
Step flashing is a series of short rectangular metal pieces, typically 10 inches wide and 4 to 6 inches tall, installed in a staggered pattern where a roof meets a vertical surface like a chimney, dormer, or parapet wall. Each piece is bent at a right angle so half sits on the roof and half against the wall, and they are installed one above the other in overlapping rows that follow the roof slope upward.
The flashing is almost always aluminum or galvanized steel, both chosen for corrosion resistance and cost effectiveness. Aluminum is lighter and easier to form on site; galvanized steel costs less but requires more care to avoid fastener corrosion where dissimilar metals meet. The overlap between consecutive pieces is at least 4 inches, with the upper piece always lapping over the lower one so water sheds downward and outward.
Installation and material integration
Step flashing works in tandem with roof shingles or other roofing material on the sloped surface. The shingle course is installed under the horizontal flange of each step; the metal flange is then tucked behind the vertical siding or masonry on the wall. This lap sequence ensures water running down either surface is directed safely outward rather than trapped in the joint. Fasteners, typically galvanized nails or screws, hold the roof-side flange; the wall-side flange relies on the siding or mortar bed to hold it in place.
Failure occurs when the overlap is too shallow, allowing water to creep backward under the upper piece, or when the vertical wall component is driven too tightly against the surface, preventing proper water shedding. Expansion and contraction between different materials also creates movement; if fasteners are too rigid or spaced too close, the flashing can crack or pull loose. Poor integration with siding is a common problem: if the siding does not properly lap the flashing, water enters the wall cavity.
The term 'step' refers to the staggered, rising pattern of installation, not to the shape of individual pieces. It distinguishes this method from continuous flashing, which is a single unbroken piece, and from bent-on-site methods that are less reliable on steep pitches. Step flashing remains the standard on residential roofs because it accommodates settling, thermal movement, and the practical reality that roof and wall materials age at different rates.