Think of counter-flashing as the lid on the flashing system. Step flashing seals the roof plane, but the top edge of its vertical leg would still let water in if it were left open against the wall. Counter-flashing covers that edge: on a chimney it's cut into a mortar joint (a groove called a reglet) and folded down over the step flashing; on a sided wall, the siding itself often does the job. The two layers overlap without being fastened together, so the roof and the masonry can move independently without tearing a seal.
Why it matters in Douglas County
Chimneys take the worst of our weather. They're exposed on all four sides, masonry soaks up water like a sponge, and wind-driven winter rain — the Umpqua Valley specialty from October to May — tests every joint around them. Proper two-piece flashing is the difference between a chimney that stays dry for decades and one that stains the ceiling every February.
When it fails, what it costs you
Mortar joints crumble with age, counter-flashing works loose, and the "fix" many homes get is a bead of caulk smeared along the brick. Caulk against masonry is a patch, not a system — it typically fails within a season or two, and the slow leak that follows soaks insulation and drywall around the chimney chase. Re-cutting the reglet and setting new counter-flashing is routine roof repair work, and far cheaper than interior restoration. It's exactly the kind of detail worth checking before the rains return.
Why does the caulk around my chimney flashing keep failing?
Because caulk is doing a job it was never designed for. Masonry moves, roofs move, and sun and rain break the bead down. The durable fix is metal counter-flashing set into the mortar joint — no sealant required to keep water out.