Chimneys, spires, etc. The height limitations of this chapter shall not apply to belfries, church spires, cupolas and domes which are not used for human occupancy; nor to chimneys, ventilators, skylights, water tanks or other storage tanks/silos and necessary mechanical appurtenances usually carried above the roof level; nor to silos or structures used for agricultural purposes; 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, and are subject to Planning Board review and approval. 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.