The height limitations of this chapter shall not apply
to belfries, bulkheads, chimneys, church spires, cupolas, domes, skylights,
ventilators, water tanks and other necessary mechanical appurtenances
usually carried above the roof level. Such features, however, shall
be erected only to such heights as is necessary to accomplish the
purpose they are required to serve and shall not occupy in the aggregate
more than 25% of the roof area of the main building.