Counter flashing is installed over the top edge of the base flashing to provide an additional layer of protection against water infiltration. The purpose of counter flashing is to direct water away from the vulnerable joint between the base flashing and the wall or roof surface, and to prevent moisture from seeping into the building through this joint. Counter flashing is commonly used in masonry construction, where it is installed along the top edge of brick or stone walls, but it can also be used in other types of construction where base flashing is needed.