Valleys are created when two sloping roof areas intersect and are one of the critical detail points on a steep-slope roof system. They occur on primary roof areas and where projections, such as dormers, break the plane of the main roof. A valley's main function is to create runoff pathways to direct water flow from the roof planes into a valley trough. The volume of water varies with the valley's length and slope.
Because of their configurations, valleys are susceptible to debris accumulation from vegetation, foliage and windborne particulates. Valley function and service life are affected by climatic conditions such as snow, ice, rainfall frequency and intensity, humidity, temperature extremes and wind. Geographic factors influencing valley function and service life include building location, elevation and oriented exposure to solar load intensity.
All these issues determine which valley configuration is appropriate for the specific building and selected roof system; I will discuss valley configuration as it relates to asphalt shingles.
The three basic valley types for asphalt shingle roof systems are woven, open and closed-cut. In addition, there are recognized options for open and closed-cut valleys commonly referred to as "California open valleys" and "California closed-cut valleys."