The height limitations of this chapter shall not apply to church spires, belfries, cupolas or domes not used for human occupancy; nor to chimneys, ventilators, skylights, water tanks, bulkheads and necessary mechanical appurtenances usually carried above the roof level; nor to noncommercial radio and television antennas or transmission apparatus; except that such features, if erected on a roof, shall not exceed 20% of the total roof area. Penthouses or roof structures for housing mechanical equipment shall also be exempt, provided that said structure does not exceed 1/3 of the building height or 12 feet, whichever is less.