How much overlap on step flashing is specified by building codes and shingle manufacturer installation instructions to ensure a watertight barrier, and proper overlap is critical for preventing leaks. Standard step flashing pieces are typically 7 to 10 inches long and 7 to 10 inches wide, bent into an L-shape. Each piece should overlap the piece below by at least 2 inches, with many professionals using 3 inches of overlap for extra protection, ensuring water running down cannot get behind the joint. The vertical leg that goes up the wall should be at least 4 to 6 inches high, and the horizontal leg that goes onto the roof deck should extend at least 4 inches onto the deck to be properly covered by shingles. When interwoven with shingles, each piece of step flashing should be installed so that it is covered by the shingle course above and covers the shingle course below, with the shingle overlapping the flashing by at least 1 inch. At the bottom of a wall, the first piece of step flashing should overlap the kick-out flashing that directs water into gutters. At the top of a wall, the last piece should be covered by counter flashing. Insufficient overlap, such as less than 2 inches, is a common cause of leaks, as is using continuous flashing instead of individual step pieces. Always follow manufacturer specifications for minimum overlap.
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