Projections about height limits. The height limitations of this chapter shall not apply to projections of buildings which are usually carried above the roof level, such as flagpoles, church spires, belfries, cupolas and domes not used for human occupancy, nor to chimneys, ventilators, skylights, water tanks, bulkheads or similar features, radio and television antennas for the use of residents of dwellings in apartments, and necessary mechanical appurtenances. Such projections, however, shall be erected only to a height necessary to accomplish the purpose they are intended to serve but in no case more than 15 feet above the projection's lowest point of contact with the roof. The total area covered by such features shall not exceed in cross-sectional area 15% of the area of the section of roof upon which they are located. Such features as water tanks, cooling towers and bulkheads shall be enclosed within walls of material and design in harmony with the main walls of the building on which they are located.