Which component is generally used in forming open valleys with two layers of roll roofing?

In roofing applications, particularly with roll roofing, the formation of open valleys requires careful layering to ensure effective water drainage and prevent leaks. The correct choice involves using a first layer that is 18 inches wide. This width provides sufficient coverage for the valley area while allowing for proper overlapping with the adjacent roofing materials.

An 18-inch width helps ensure that the valley has a substantial amount of material to handle rainwater runoff effectively, especially in areas where two roof sections meet. This is crucial because valleys are often the points where water accumulates and, if not properly installed, can lead to water penetration and damage.

The other choices do not provide the optimal width for this application. A single 36-inch layer would be too wide, potentially causing issues with proper overlap and sealing. A 24-inch layer is also not ideal since it does not fully cover the requirements of keeping water from running underneath. A double layer of 12 inches might not provide adequate coverage either, as the total width may be insufficient for effective water management and can compromise the valley's integrity.