What is the minimum size width for metal flashing used in a valley with asphalt shingles?

The minimum width for metal flashing used in a valley with asphalt shingles is 24 inches. This width is necessary to ensure that the flashing effectively diverts water away from the valley area and onto the roofing surface, which helps prevent leaks and protect the underlying structure.

Adequate width is critical because valleys are often areas of high water flow, especially during rainstorms, and insufficient flashing can lead to water infiltration and damage. The 24-inch width provides a balance between functionality and material efficiency, ensuring that the flashing is wide enough to handle the runoff while also not being excessively wasteful.

The other options do not meet the recommended standards for valley flashing in terms of performance and protection, as narrower sizes would increase the risk of water penetration during heavy rainfall.