CSLB C-39 Roofing ContractorFlashing, Ventilation and InsulationMedium
At a roof-to-wall intersection, base flashing is installed integrated with the roofing membrane and turned up the wall surface. What is the primary function of the counter flashing installed above it?
- AIt serves as the primary waterproofing layer at the eave
- BIt replaces the need for base flashing entirely on the wall
- CIt overlaps the base flashing from the wall side to shed water over it and prevent moisture intrusion behind it
- DIt anchors the shingles directly to the roof deck
Show answer & explanationAnswer & explanation
Correct answer: C. It overlaps the base flashing from the wall side to shed water over it and prevent moisture intrusion behind it
Base flashing seals the roof-to-wall joint from the roofing side, while counter flashing is installed into or against the wall and laps down over the base flashing, shedding water outward and keeping it from migrating behind the base flashing into the wall assembly.
Why the other options are wrong
- A. Eave waterproofing is handled by drip edge/underlayment, not counter flashing.
- B. Counter flashing supplements, not replaces, base flashing — both are needed.
- D. Shingle anchoring is unrelated to flashing function.
Base vs. Counter Flashing
Base flashing integrates with the roof covering and turns up the wall; counter flashing is set into the wall and laps over the base flashing to complete a two-part weatherproof system.
- Base flashing = roof-side component
- Counter flashing = wall-side component, installed above
- Together they form a complete watertight roof-to-wall transition
Memory trick: Base rises from the roof, counter comes down from the wall to cap it.