What is the difference between an open valley and a closed valley, and how does that affect flashing?

• A. Open valleys reveal metal flashing in the valley; closed valleys are shingled over with flashing beneath; each requires different flashing details per instructions.
• B. Open valleys and closed valleys use the same flashing details.
• C. Valleys do not require flashing.
• D. Only closed valleys require flashing.

Answer: (A)

Open valleys expose the metal flashing in the valley, while closed valleys have the flashing hidden beneath the shingles. In an open valley, the flashing is typically installed first and runs down the center of the roof, with the shingles cut back to reveal it so water flows directly over the flashing and into the valley. In a closed valley, the flashing is installed beneath the shingles, and the shingles themselves cover the valley, so the flashing isn’t visible in the finished roof. Because the flashing sits in different positions and interacts with the shingles differently, the flashing details—how it’s installed, sealed, and integrated with the surrounding shingles—vary and must follow the manufacturer’s instructions. This matters for water control and leak prevention: open valleys provide direct, visible flashing that’s easier to inspect, while closed valleys rely on precise shingle alignment and proper under-flashing to keep water from penetrating. Both require flashing, but the approach changes with the valley style to ensure a proper seal and reliable drainage.