Chimneys, ventilators, skylights, water tanks, television and radio antennas and similar features, necessary mechanical appurtenances usually carried on and above the roof level and parapet walls or cornices used for ornamentation may exceed the height limitations of this chapter. Mechanical appurtenances, ventilators, skylights and similar features shall not exceed the top of the roof structure by more than five feet. Chimneys, water tanks and antennas shall not exceed the height limitations by not more than 15 feet, provided that they do not exceed 10% of the roof area. No part of the structure subject to this provision shall be used for occupancy.