Short Answer
Counterflashing works with base or step flashing to create a water-resistant transition, particularly around masonry.
Detailed Explanation
Counterflashing is the upper piece of a two-part flashing system used at masonry walls and chimneys. The step (or base) flashing is installed on the roof surface, going up the vertical wall. The counterflashing is installed in the masonry — typically embedded in mortar joints — and overlaps the top of the step flashing, covering the joint between the step flashing and the wall. This two-part system allows the base flashing to be replaced (during a roof replacement, for example) without disturbing the masonry, because the counterflashing covers and protects the top of the base flashing. If counterflashing is missing, damaged, or improperly installed, water can get behind the step flashing and enter the building.
Why It Matters
Counterflashing is essential for masonry-to-roof transitions. Without it, water enters behind the step flashing and leaks into the building.