CSLB C-39 Roofing ContractorSafety and RepairHard
A roofing contractor is repairing a persistent leak around a chimney on a residential shingle roof. The existing flashing consists of step flashing only, without counter flashing. What is the most critical reason for adding counter flashing to the chimney repair to prevent future leaks?
- ACounter flashing prevents water from migrating behind the vertical leg of the step flashing.
- BCounter flashing helps to divert snow and ice away from the chimney base.
- CCounter flashing adds structural reinforcement to the chimney-roof intersection.
- DCounter flashing provides a more aesthetically pleasing finish to the chimney base.
Show answer & explanationAnswer & explanation
Correct answer: A. Counter flashing prevents water from migrating behind the vertical leg of the step flashing.
The primary purpose of counter flashing is to cap and protect the top edge of the vertical leg of the step flashing. This prevents water from running down the chimney and getting behind the step flashing, which is a very common source of leaks.
Why the other options are wrong
- B. While flashing indirectly helps manage water, the specific role of counter flashing is to seal the top of the step flashing, not primarily to divert snow and ice, which is more a function of the overall flashing design and roof pitch.
- C. Flashing is designed for waterproofing, not structural reinforcement. Structural integrity is provided by the framing and masonry of the chimney itself.
- D. While aesthetics can be a secondary benefit, the primary purpose of counter flashing is functional leak prevention, not appearance.
Chimney Counter Flashing Function
Counter flashing caps the vertical leg of step flashing to prevent water from penetrating behind it, a critical component of leak-proof chimney flashing.
- Protects the exposed top edge of step flashing.
- Embedded into the chimney masonry.
- Directs water outwards over the step flashing.
Memory trick: Step and counter, work together, for a leak-free roof forever.