No building shall be erected, reconstructed or structurally altered to exceed in height the limit designated for the zone in which such building is located, except the height limitations of this chapter shall not apply to church spires, belfries, cupolas, antennas, domes not used for human occupancy; nor to chimneys, ventilators, skylights, water tanks, silos, bulkheads, and necessary mechanical appurtenances usually carried above the roof level, except where, in the opinion of the Building Inspector, such may be deemed to interfere with aerial navigation or constitute a fire hazard. Such features, however, shall not exceed in total coverage 20% of the total roof area and shall not exceed a reasonable height to be determined upon reference of all such cases to the Zoning Board of Appeals by the Building Inspector.