Spires, belfries, chimneys, skylights, water or cooling towers, parapets or railings, elevators, stair bulkheads, solar collectors, air-conditioning units or similar structures which in their aggregate coverage occupy no more than 120 square feet or 10% of the roof area of the building, whichever is smaller, of which they are a part. Such features shall be erected only to such minimum height as is necessary to accomplish the purpose for which they are intended.