Rooftop bulkheads, elevator penthouses, water towers, fire towers, hose towers, cooling towers, air-conditioning or heating equipment, flagpoles, dish antennas, radio or television aerial, provided that such features shall not occupy, in the aggregate, more than 10% of the area of the roof of a building and are set back from the edge of the roof at least one foot for each one foot by which such features exceed the maximum height otherwise specified for the district in which they are located. All mechanical equipment located on the tops of buildings shall be visually screened. In any district that permits research and laboratory uses, up to 25% of the area of the roof of a building may be occupied by specialized air-handling equipment, lab venting systems and other rooftop equipment specifically related to the research and laboratory use. The same setbacks from the edge of the roof required for other rooftop equipment regulated by these provisions shall apply. Such equipment shall be screened so that it is not visible from off-site locations.