Height regulations. Main and accessory buildings in any industrial district may be erected to a height not exceeding the width of the side or rear yard adjoining any residential district, but in no case to a height exceeding 50 feet. Chimneys, spires, cupolas, domes, towers, flagpoles, water tanks, radio or television antennas, monuments and other mechanical appurtenances located upon or constituting an integral part of a main building shall not exceed a height of 100 feet above the finished grade.