Abstract: A cover assembly for a roof mounted air conditioner unit which allows an increased output air volume to flow from a roof mounted air conditioner into a cooling volume; the assembly comprises a cover, return air grills, side diffuser assemblies, and a register having frame, directional control louvers, volume control louvers, a throttle and mounting screws; the register is substantially in alignment with the output blower of a roof mounted air conditioner and sized to allow a maximum output air volume to flow into the cooling volume. The register's volume control louvers and directional control louvers control the amount of air passing through the register and the direction of air flow respectively. Additionally, a method for retro-fitting existing covers with a register aligned with the output blower of a roof mounted air conditioner thereby producing the same effect of the improved cover assembly is shown.