The height limitations created hereunder shall apply to chimneys, ventilators, solar energy facilities, skylights, tanks, stair towers, elevator towers, and appurtenances usually carried above the roof line of a building, except that the same may exceed said height limitations by not more than 15 feet, except that skylights, heating and air-conditioning equipment and ventilators may exceed the height limitation by no more than 10 feet. Such features shall not exceed, in total coverage, 10% of the total roof area.