Abstract: A prefabricated roof structure for bay or bow type window assembly. The roof structure includes a cap of unitary construction which covers a rigid infrastructure or frame. The frame is provided with a support ledge which forms an acute angle with the frame body, and a support beam is provided beneath the ledge to form a shoulder which abuts and engages a window frame for securing the frame thereto. A tensioning element such as a support bar, guy wire or cable may be provided to securely anchor the roof structure and window assembly to the wall of a building on which the window assembly is to be installed. The roof structure may be inverted to for a bottom support and cover to securely fasten the window assembly to the wall.