Counter-flashing is a second layer of metal flashing installed above step flashing or base flashing, usually embedded into a masonry joint on a chimney or wall, that laps down over the top edge of the flashing beneath it. It caps and seals that top edge so water cannot get behind the step flashing from above. Counter-flashing is most often seen wrapped around brick chimneys, where it is cut into the mortar joint and bent down over the roof's step flashing.
Step flashing does the work of channeling water down a roof-to-wall joint, but on its own it has an exposed top edge where it tucks behind the wall covering. On a masonry wall or chimney, that edge needs its own cap, and that cap is counter-flashing.
Counter-flashing is a separate strip of metal set into a cut groove in the mortar joint of a brick or block chimney, then bent to lap down over the top of the step flashing below it.
That overlap creates a second, independent barrier. Even if water gets behind the counter-flashing at the masonry joint, it runs down the face of the counter-flashing and onto the step flashing rather than straight into the wall.
Two separate, overlapping pieces of metal, rather than one, is what makes the whole assembly reliable over the many freeze-thaw and thermal cycles a chimney flashing has to survive.
Because it is set into a cut mortar joint, counter-flashing has to be installed carefully and resealed with the right masonry sealant. A common shortcut is smearing roofing cement over a gap instead of properly cutting the flashing into the joint, which fails much sooner.
For a homeowner, a chimney leak is often a counter-flashing problem rather than a step-flashing one, especially on an older chimney where the mortar has cracked or the flashing has pulled loose from the joint.
A repair that only reseals one half of the assembly rarely holds. Confirm that both the step flashing and the counter-flashing are being addressed.
A free, documented inspection across the Triangle. A real estimator within the hour.