A secondary flashing that prevents water from entering a base flashing is called a ________ flashing.

Counter flashing is a critical component in roofing systems, especially where vertical surfaces meet the horizontal surfaces of the roof, such as around chimneys or vent stacks. It is installed above base flashing to direct water away from these vulnerable areas and back toward the roof surface, preventing water from penetrating beneath the base flashing.

The installation of counter flashing is essential because it works in conjunction with base flashing, creating a barrier that helps to mitigate water intrusion. When properly installed, the counter flashing overlaps the base flashing, ensuring that any water that runs down the vertical surface will flow over the counter flashing and onto the roof rather than into seams or gaps that could lead to leaks.

Other answers may refer to different types of flashing with specific functions, but only counter flashing specifically serves the purpose of preventing water from entering at the junctions between vertical walls and the roof, making it the most suitable option in this context.