Closed valley shingling refers to the practice of not using a strip of metal flashing to finish the valley between two adjoining slopes on a shingled roof. Instead, shingles are laid all the way to the joint so that they meet in the center.
The alternate method, called open valley shingling, uses a strip of metal flashing to finish the valleys. The open valley method has the advantage of providing an extra layer of protection against leakage at the joint, but the choice is usually an aesthetic one. Some homeowners like the appearance of the metal valley, while others prefer the appearance of a closed valley, which allows the planes of the roof to blend together visually.