The height limitations of this chapter shall not apply to belfries, bulkheads, chimneys, church spires, cupolas, domes, skylights, ventilators, water tanks and other necessary mechanical appurtenances usually carried above the roof level. Such features, however, shall be erected only to such heights as is necessary to accomplish the purpose they are required to serve and shall not occupy in the aggregate more than 25% of the roof area of the main building.