The "height" limitations of this article shall not apply to spires, cupolas and domes of churches or other places of worship, nor to chimneys, ventilators, skylights, water tanks, bulkheads or similar features and necessary mechanical appurtenances usually carried above the roof level. Such features, however, shall be erected only to such "height" as is necessary to accomplish the purposes they are intended to serve, but in no case exceeding eight feet above the roof line, and the total area covered by such features shall not exceed 10% of the area of the roof on which they are located.