Final answer:
Counter-flashing should be installed at brick and stone walls to prevent moisture intrusion behind the sidewall flashing. It is installed on top of the base flashing and provides a barrier against water intrusion.
Step-by-step explanation:
To prevent moisture intrusion behind the sidewall flashing, counter-flashing should be installed at brick and stone walls.
Counter-flashing is a technique used in construction to prevent water from entering the wall at the point where the flashing is installed.
It is installed on top of the base flashing and provides a barrier against water intrusion.