How it works

Step flashing is typically formed from 26-gauge galvanized steel, aluminum, or copper, bent into an L with one leg lying on the roof deck and the other rising against the chimney brick. Roofers install one piece per shingle course, weaving them so each piece sheds water onto the shingle below. The vertical leg is then covered by counterflashing that is embedded in a mortar joint above.

When step flashing fails, it is almost always because the metal has corroded, the shingle weave has been disturbed during a roof replacement, or the counterflashing has slipped out of its mortar joint. Improperly installed flashing is the single most common source of chimney-related interior leaks. Sweeps and roofers should jointly evaluate flashing during a CSIA Level 2 inspection">Level 2 inspection that includes interior moisture concerns.

DFW context

DFW’s hailstorms drive flashing replacement cycles every seven to ten years on average. The June 2025 storm season alone damaged over 100,000 roofs across Tarrant, Dallas, and Collin counties, and step flashing was frequently dented or torn loose. TSE coordinates with roofing partners to ensure replacement step flashing is woven correctly with new shingles and that counterflashing is reset into a clean mortar joint, not surface-caulked.

Related terms

• [Counterflashing](/glossary/counterflashing/)
• [Cricket flashing](/glossary/cricket-flashing/)
• [Saddle](/glossary/saddle/)
• [Spalling](/glossary/spalling/)

Sources

• IRC 2021, Section R905.2.8.3
• NFPA 211 (2024), Section 10.4.4
• ASTM D224 flashing standards