Two pieces, one job
At a roof-to-wall joint, base flashing turns up the wall from the roof, and counterflashing laps down over it from the wall above.
Kept separate, counterflashing can be lifted and reset at reroofing without cutting the roof seal.
Sources
- AFS Division 07 technical notes - Architectural Flashing Supply
- Copper in Architecture Design Handbook - 9.2 Counterflashing - Copper Development Association