Step flashing is a series of small, L-shaped metal pieces installed one at a time, woven in with each course of shingles, where a roof slope meets a vertical wall. Each piece overlaps the one below it and is tucked partly under the siding above, so water is stepped down the wall and onto the roof surface instead of behind the siding. It is the standard flashing method anywhere a roof runs into a wall, such as against a dormer, a chimney side, or an addition.
Wherever a sloped roof meets a wall running up from it, such as along a dormer or where a lower roof ties into a taller part of the house, that joint needs to be sealed a course at a time. Step flashing is built specifically for that job.
Instead of one long strip of metal, step flashing is a stack of short, bent pieces, each one set at the top of a single shingle course and tucked behind the wall covering above it.
As the next course of shingles goes on, it covers the bottom of that flashing piece, and the next flashing piece goes on above it, tucked under the wall covering again. The whole assembly steps up the wall in a shingled, overlapping pattern.
That overlap is the entire point. Water running down the wall lands on a flashing piece and is directed onto the shingle below it, never finding a straight path behind the siding or under the roof.
Step flashing has to be installed one piece per course, by hand, which makes it slower and more skill-dependent than the open field of a roof. It is also one of the most common places a rushed re-roof cuts corners, either by reusing old flashing or by caulking a seam instead of properly weaving new pieces.
For a homeowner, a wall-to-roof leak, especially a stain on an interior wall near a dormer or addition, points squarely at step flashing before anywhere else.
It is a targeted, well-understood repair once correctly diagnosed, and rarely requires touching the rest of the roof.
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