Flagstaffs, chimney flues, elevator shafts, radio and televisions antennas and screened mechanical equipment designed to service the building may exceed the height of the roof by not more than 12 feet, provided that such structures do not exceed 10% of the ground area covered by the building and are not used for any other purpose. Structures to provide for safe access to the roof area are, however, exempt from the above requirements. The screening provided shall be of a material similar to the building facing or as approved by the Land Use Board as compatible and durable.