Residential penthouses or roof structures for the housing of elevators, stairways, tanks, ventilating fans, air-conditioning equipment and similar mechanical equipment required to operate and maintain the building and fire or parapet walls, skylights, towers, amateur radio towers, spires, cupolas, steeples, flagpoles, chimneys, smokestacks or similar structures may be erected above the height limits prescribed in this chapter, but in no case more than 25% more than the maximum height permitted in the district as long as they cover no more than 15% of the roof area.