An awning is a seconary covering attached to the exterior wall of a building. It is typically composed of canvas woven of acrylic, cotton or polyester yarn, or vinyl laminated to polyester fabric that is stretched tightly over a light structure of aluminum, iron or steel or possibly wood. The configuration of this structure is something of a truss, space frame or planar frame. Awnings are also often constructed of aluminum understucture with aluminum sheeting. These aluminum awnings are often used when a fabric awning is not a practical application where snow load as well as wind loads may be a factor.