What is Counterflashing? | TSE Glossary

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Counterflashing is the second layer of metal that overlaps step flashing where a chimney meets the roof. It is bent into an inverted L with one edge embedded in a horizontal mortar joint of the chimney and the other edge draping over the vertical leg of the step flashing below. Together, step and counterflashing form a two-stage water barrier that NFPA 211 requires at every roof penetration.

Installers cut a reglet, or saw kerf, into a horizontal mortar joint roughly 6 to 8 inches above the roof line. The top edge of the counterflashing is folded into the kerf and locked with lead wedges or fresh mortar. The lower edge hangs free over the step flashing’s vertical leg, allowing the two metals to slide independently as the structure expands and contracts. Water that gets behind the counterflashing simply runs out over the step flashing.

Failed counterflashing usually shows as a metal lip pulled out of its mortar joint, often the result of homeowners or roofers caulking over the joint instead of cutting a fresh reglet. Surface caulk fails within three to five years in Texas UV exposure, while a properly tucked counterflashing lasts 30 years. Sweeps and roofers identify these failures during inspection, and the fix is to grind a clean joint and re-tuck the metal.

In DFW, counterflashing is most often the failure point on chimneys built before 2000 because earlier roofers commonly caulked rather than cutting reglets. Texas summer heat bakes asphaltic caulk to brittleness within a few seasons, and the next storm pulls it loose. TSE recommends cutting fresh reglets and using lead-wedge installation any time a chimney needs flashing rework after a hail event.

• [Step flashing](/glossary/step-flashing/)
• [Cricket flashing](/glossary/cricket-flashing/)
• [Saddle](/glossary/saddle/)
• [Tuckpointing](/glossary/tuckpointing/)

• IRC 2021, Section R905.2.8.3
• NFPA 211 (2024), Section 10.4.4
• SMACNA Architectural Sheet Metal Manual

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