No building shall be erected, converted, enlarged, reconstructed, or structurally altered to exceed the height limit hereinafter established for the district in which the building is located, except that penthouses or roof structures for the housing of elevators, stairways, tanks, ventilating fans, or similar equipment required to operate and maintain a building, and fire or parapet walls, skylights, towers, steeples, stage lofts and screens, flag poles, chimneys, smokestacks, radio and television aerials, wireless masts, water tanks, and similar structures may be erected above the maximum height limitations established herein. No such structure shall occupy more than 25 percent of the total roof area of a building to be used for residential purposes.