Nothing herein contained shall be construed so as to limit the height of any belfry, bulkhead, chimney, church spire, clock tower, cupola, dome, silo, skylight, ventilator, water tank and other necessary mechanical appurtenances usually carried above the roof level. Such features, however, shall be erected 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.