What's the difference between a hip and a valley on a roof?

The primary difference between a hip and a valley on a roof lies in their structural direction and their role in managing rainwater drainage. A hip is an external angle or ridge where two upward-sloping roof planes meet to form an outward-facing corner, creating a convex angle that naturally sheds water outward and away from the house. In contrast, a valley is an internal angle where two downward-sloping roof planes intersect to form a V-shaped or U-shaped trough, creating a concave channel that catches and directs massive volumes of rainwater and melting snow downward toward the gutters. While hips are primarily designed to withstand high winds and provide structural strength to complex roof geometries, valleys act as high-capacity drainage highways that require specialized waterproof underlayment and flashing to prevent the concentrated runoff from penetrating the home.