The maximum building heights at § 175-7.3, 175-7.4, or elsewhere in this chapter shall not apply to churches, church spires, belfries, flagpoles, water towers, fire towers, chimneys, public buildings, standard radio or television antennas, or to necessary mechanical appurtenances or to parapet walls, except as follows:
(a)
No parapet wall may extend more than four feet above the building roof;
(b)
The height of bulkheads, elevator enclosures or water tanks or other structures constructed upon the roof of a building and occupying in the aggregate less than 20% of the area of the roof upon which the same are located shall not be included as a part of the height of the building for the purpose of the regulations and restrictions of this chapter, but in no event may they exceed permitted building height by more than 10 feet.