Height limitations. The height limitations of this chapter shall not apply to chimneys, except as limited in the definition of "height of structure," silos, church spires, gables, cupolas, standpipes, flagpoles, monuments, television and radio antennas or towers, cables or water tanks and similar structures and necessary mechanical appurtenances for the zone in which the building is located, provided that no such exceptions, when connected to and a part of a principal building, shall cover at any level more than 10% of the area of the roof on which it is located. All such items in excess of the height regulations shall be set back from the property line at least a distance equal to their height.