Roof structures for the housing of elevators, stairways, tanks, ventilating fans or similar equipment required to operate and maintain the building, fire or parapet walls, skylights, towers, steeples, chimneys, smokestacks, radio and television antennas, water tanks, silos, gas containers, or similar structures may be erected above the height limits herein prescribed, but no roof structure or any space above the height limit shall be allowed for the purpose of providing additional floor space for residential or commercial use.