The height limitations provided for in this chapter shall not apply to spires, belfries, cupolas, penthouses and domes not used for human occupancy; nor to chimneys, ventilators, skylights, water tanks, bulkheads, silos, barns, amateur radio receiving and transmitting equipment, and necessary mechanical appurtenances usually carried above the roof level. Such features, however, shall be erected only to such height as is necessary to accomplish the purpose they are designed to serve and shall in no event exceed 50 feet in height without the prior approval of the Board of Appeals granted in accordance with this chapter.