An awning or overhang is a secondary 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 aluminium, iron or steel, possibly wood or transparent material. The configuration of this structure is something of a truss, space frame or planar frame. Awnings are also often constructed of aluminium understructure with aluminium sheeting. These aluminium 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.