In the flashing of a chimney, the counter flashing should always be wide enough to cover how many inches of base flashing?

• A. 2 inches
• B. 3 inches
• C. 4 inches
• D. 6 inches

Answer: (C)

When flashing a chimney, the goal is to create a continuous, weatherproof barrier where the chimney meets the roof. The counter flashing acts as the outer layer that covers the seam formed by the base flashing, so it must extend downward far enough to overlap that base flashing and protect the joint from water entry. Four inches of overlap is the standard minimum, giving a solid shield against rain driven by wind and temperature movement. If the overlap is smaller, water can creep behind the flashing and cause leaks around the chimney. Six inches would be more than the common requirement and isn’t typically necessary, while two or three inches wouldn’t provide adequate coverage. In practice, four inches is the reliable, commonly used overlap.