The height limitations of this chapter shall not apply to chimneys, church spires, belfries, standpipes, water towers, flag poles or monuments; or to elevators, radio or television aerials, skylights, ventilators, clock towers, cupolas, water tanks and similar structures and mechanical appurtenances which are part of a building; provided that the aggregate horizontal area of such structures or parts of buildings shall not exceed 25% of the ground area covered by the main building. No parapet wall or cornice may exceed the height limitations of this chapter. A railing on a flat roof may exceed the height limitations, provided that such railing has at least 50% of its vertical face open.