The height limitations of this chapter shall not apply to farm structures, silos, belfries, church spires, cupolas, penthouses and domes which are not used for human occupancy; nor to chimney's, ventilators, skylights, water tanks and necessary mechanical appurtenances usually carried above the root level; nor to flagpoles, monuments, transmission towers and cables, radio and television antennas 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. No advertising device of any kind whatsoever shall be inscribed upon or attached to that part of any chimney, tower, tank or other structure which extends above the roof limitations.