No building shall exceed five stories with a maximum height of 54 feet, except that the height limitation shall not apply to ventilators, skylights, water tanks, bulkheads or similar features and necessary mechanical appurtenances usually carried above the roof level. Such features shall only be erected to a height necessary to accomplish the purpose it is intended to serve, but in no case more than 14 feet above its lowest point of contact with the roof. The total area covered by such features exempted from the height limit shall not exceed, in cross-sectional area, 25% of the roof area on which they are located.