Chimneys, spires, etc. The height limitations of this chapter shall not apply to belfries, church spires, cupolas, penthouses and domes which are not used for human occupancy, not to chimneys, ventilators, skylights, water tanks and necessary mechanical appurtenances usually carried above the roof level, not to flagpoles, monuments, transmission towers and cables, radio and television antenna or towers and similar structures. Such features, however, shall be erected only to such height as is necessary to accomplish the purpose for which they are intended.