The valley area of a roof—the straight lines formed when two different planes of the roof meet—is a critical area that handles a tremendous amount of water run-off. Valleys are typically found where gable dormer roofs meet the main roof, or when a house has angled wings or additions where two roof sections in different planes meet. A house with an L-shaped or T-shaped footprint, for example, always will have roof valleys where the two sections of the house meet. The opposing surfaces of the roof planes drain the water towards the valley area, where it is then channeled towards the edge of the roof.
One of the details that varies between homes is the method of flashing the valley areas of the roof. There are two common methods for completing the flashing of a valley in a shingle roof system: closed valleys or open valleys. To understand what makes a valley either closed or open, it is important to understand how a valley is constructed.
What is a Closed Valley Roof?
Before the installation of a shingle roof assembly, the roofing contractor installs an underlayment over the entire roof deck. Incorporated into the underlayment may be a self-adhering ice and water shield or another type of underlayment that is a heavier gauge than the underlayment that covers the remainder of the roof system. This self-adhering underlayment acts as the valley lining.
For a closed valley, the roofer will install the shingles onto the roof deck and extend the asphalt shingles through the area where opposing roof planes meet, so that the shingles cover, or “close,” the valley area. When finished, the valley will be covered with shingles so that the self-adhering underlayment is completely covered with the asphalt shingles. The asphalt shingles serve both as the valley lining and as the wear surface to protect against water.
What is an Open Valley Roof?
In comparison to a closed valley, an open valley adds another layer of lining into the valley. After the self-adhering underlayment is installed across the valley, a pre-bent metal valley lining is installed. This valley flashing material can be manufactured from any metal that will resist the effects of weather, acid rain, and other contaminants. Galvanized steel, aluminum, and copper can all be used.
As the asphalt shingles are installed over the roof deck, the shingles are extended into the valley. However, these shingles are not run entirely through the valley area, but rather the center of the metal valley is left exposed. In addition, it is important to avoid nailing the shingles through the valley metal. A chalk line is then used to strike a line from the top of the valley to the bottom of the valley. The shingles are then cut out of the valley area, “opening” the surface of the valley lining to water run-off and the environment. With an open valley, a strip of metal will be visible on the roof wherever two opposing roof planes meet.
When to Choose a Closed Valley Over an Open Valley
There are several reasons why contractors and homeowners might choose to install an open valley or closed valley:
- Aesthetics: Contractors, designers, and owners have preferences as to the appearance of one type of valley over the other. It is important to remember that a closed valley conceals the valley lining and allows the planes of the roof to blend together. Open valleys, on the other hand, leave the metal exposed, which may be more pleasing to the eye, especially if copper is used as the valley lining.
- Cost: Contractors may be able to reduce the cost of materials on roofing projects by eliminating the metal being installed in the valleys. This will reduce the installed cost of the project, especially if there is a significant number of valleys in the roof system being installed.
- Functionality: Installer's and designer's opinions differ as to the functionality of closed valleys versus open valleys. Closed valleys may have a tendency, if not constructed properly, to channel water under the shingles laid over the bottom layer of shingles. However, open valleys may be susceptible to water back-up along the top of the cut-off shingles in the valley if not properly trimmed, potentially causing roof leaks.
In short, both options should be considered when replacing the roof system on your home or building. If you are choosing to complete your roof replacement yourself, make sure that you consider these various factors before beginning your installation. If you have decided to have a professional roofing contractor complete your roof replacement, be sure to discuss with them how they will address the valley detail. Addressing the valley detail before work begins will allow you to consider the possibilities and ultimately get the appearance and functionality you want in your roof system.
Valley blends with rest of roof
Can be slightly less expensive
Can be difficult to install if valley angles are sharp
Exposed metal may improve appearance
Water less likely to back up under shingles
Simplifies shingle installation