Most shingle roofing systems contain valleys, which are the points where two flat areas of roofing meet. Valleys play a significant role in water drainage since they divert water runoff to the exterior edges of a roof. There are two primary methods of flashing the valley areas of a roofing system. These two methods are called open valley and closed valley sealing.
Continue reading to learn the difference between both types of methods, including some basics of their construction.
Before you can clearly understand the difference between an open and closed valley, it helps to first understand their means of construction.
In an asphalt roofing system, a layer of underlayment is installed on top of the roof deck during the primary stages of roof building. In certain areas, a heavier gauge underlayment, or a self-adhering water and ice shield, is installed in addition to the original layer of underlayment. These areas of self-adhering underlayment are generally the valley linings.
In a closed valley system, a roofer will install asphalt shingles on top of the roof deck and underlayment. When doing so, they will also extend the shingles completely over the valley area, “closing off” and covering the exposed underlayment. As a result, the shingles become the valley lining, diverting water off of the roof.
In an open valley system, a roofer will install a pre-bent metal valley lining on top of the self-adhering underlayment. Generally, a metal that is corrosion-resistant and durable against contaminants is often used to manufacture pre-bent valley lining. Although the shingles in an open valley system are installed to reach the valley area, they do not cover the valley. Instead, they are cut out of the valley area, which “opens” and exposes the surface of the lining to water run-off and the environment.