Yes, you can install step flashing on an existing roof as a repair or when adding a sidewall or chimney, but it requires removing surrounding shingles to properly integrate the flashing, as step flashing must be interwoven with shingle courses to be watertight. To install step flashing on an existing roof, you need to carefully remove shingles along the wall area using a flat pry bar to break sealant bonds and remove nails, exposing the roof deck and wall. Then install new L-shaped step flashing pieces starting at the bottom, with each piece overlapping the piece below by at least 2 to 3 inches, nailing the flashing to the roof deck only, not the wall, to allow for thermal movement. Each piece should have a vertical leg up the wall of 4 to 6 inches and horizontal leg onto deck of at least 4 inches. Then reinstall shingles over each flashing piece, with each shingle course covering the flashing except for the lower edge, ensuring proper overlap. For chimney applications, you also need to install counter flashing cut into mortar joints over the step flashing. While possible as a DIY project for handy homeowners on low-slope roofs, it is labor intensive and risks damaging surrounding shingles, especially if they are older and brittle. For steep roofs, chimneys, or if siding needs to be removed, professional installation is recommended to ensure watertight integration and preserve warranty coverage.
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