Penthouses or roof structures for the housing of elevators, stairways, tanks, ventilating fans, air conditioning equipment or similar equipment required to operate and maintain buildings, and fire and parapet walls, skylights, towers, spires, cupolas, road signs (where permitted), flagpoles, chimneys, smokestacks, television and radio masts or similar structures may be erected above the height limit, but shall not be allowed for the purpose of providing additional floor space.