A window awning mounting system suitable for a non-planar surface includes a housing containing a roll of awning. A forearm is provided and includes a header at one end thereof attached to a leading edge of the roll up awning. A bracket mounted on the non-planar surface is provided for securing and stabilizing one end of the forearm. A spring loaded tension arm having one end attached to the forearm is provided and an arm bracket, which is separate from the housing bracket, pivotally attaches both a second end of the forearm and a second end of the tension arm.