What is the purpose of vent flashing installed at the point where the stack penetrates the roof?

Vent flashing serves a critical function in the plumbing and roofing systems by preventing rainwater from entering the building, which is why this choice is the correct one. When a vent stack penetrates the roof, it creates a potential pathway for water to seep into the structure if not properly sealed. Vent flashing is installed around this penetration to effectively channel water away from the stack and direct it over the roof's surface, thereby protecting the interior of the building from moisture-related issues like mold growth and structural damage.

While enhancing aesthetic appeal, providing additional insulation, and securing the stack in place are relevant considerations in construction and plumbing, they do not address the primary function of vent flashing. Aesthetic concerns typically relate to the overall look of the roof rather than the functionality of the vent itself. Insulation is more about maintaining temperature regulation within the building rather than preventing water intrusion. Similarly, while securing the stack is important, it is not the main role of the flashing; other methods are typically employed to ensure that the stack is properly bolted or anchored. Therefore, the primary and most critical purpose of vent flashing is to keep rainwater out of the building.