Height exceptions. District building height regulations shall not apply to flagpoles, radio or television antennas, transmission towers or cables, spires or cupolas, chimneys, elevator or stair bulkheads, parapets or railings, water tanks or cooling towers or any similar structures, provided that such structures in their aggregate coverage occupy no more than 10% of the roof area of the building. Barns and silos may exceed the height limitations of this chapter, provided that for each one foot each barn or silo exceeds the height of 35 feet, the minimum front yard requirements for barn and silo shall be increased by one foot, but in no case exceed 100 feet.