What is a feature common to all properly installed "open" valleys?

In roofing, an "open" valley is designed to allow water to flow into the gutter system without obstruction. A key feature of open valleys is that they typically include a lining to facilitate proper drainage and protect against leaks. This lining is often made of materials such as fiberglass or peel-and-stick membrane, which are placed in the valley before the shingles are installed. The presence of this lining helps prevent water from seeping through the joints of the roofing materials and ensures that the structural integrity of the roof is maintained.

While overhangs, metal flashing, and granulated surfaces are important aspects of roofing systems, they do not universally define the attributes of open valleys. For instance, overhang refers to the part of the roof that extends beyond the exterior wall, which is related to eaves rather than valleys specifically. Metal flashing can be a component of various roof types but is not a requirement for all open valleys, as they may function effectively without it. The granulated surface is more pertinent to shingles themselves rather than the valley installation specifics. Thus, the correct answer focuses on the integral lining feature that every properly installed open valley shares.