Flagstaffs, chimney flues, radio and television antennas and screened mechanical equipment designed to service the building may exceed the height limitation 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 the safe access to roof areas 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.