When installing step flashing along a wall intersection, each individual piece of metal must overlap the piece directly below it by a minimum of two inches, with three inches preferred for areas prone to heavy wind-driven rain. This overlapping margin ensures that water cascading down the roof slope cannot splash or capillary-action its way backward into the joints. Similarly, the horizontal leg resting on the roof deck must maintain proper shingle headlap coverage to maintain a continuous watertight seal.
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