Appurtenances Attached to Principal Buildings. The height limitations of this chapter shall not apply to antennas attached to buildings, nor to chimneys, ventilators, skylights, water tanks, bulkheads and the necessary mechanical appurtenances usually carried above the roof level.
Such features, however, shall not exceed in coverage 20% of the total roof area and shall not exceed a height such as is necessary to accomplish the purpose which they are intended to serve.