The height limitations of this chapter shall not apply to chimneys, silos, church spires, cupolas, standpipes, flagpoles, residential television and radio antennas which are not mounted on or connected to a private radio tower, 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 height regulations shall be set back from the property line at least a distance equal to their height.