Roof valley
The V-shaped channel where two roof slopes meet and shed water.
A roof valley is the internal angle formed where two sloping roof planes meet. Because valleys carry the combined runoff of both slopes, they handle more water than any other part of the roof — which makes them a frequent leak point when the valley flashing or shingles beneath them fail. Debris that collects in a valley dams water and forces it sideways under the shingles.