An outer frame is fixedly mounted on a ceiling to define a rectangular opening therethrough, which opening is normally closed by a lid having a panel supported by an inner frame so as to be flush with the ceiling. The lid is normally locked in its closed position by means including guides slidable along horizontal grooves formed on the confronting surfaces of the outer frame and by one or more turnable arms connecting the lid to the outer frame, such that when unlocked the lid is caused by gravity to turn to an open position with its free end moving substantially vertically downward as the aforesaid guides coupled to the opposite ends of the lid slide along the horizontal grooves of the outer frame. The apparatus serves either as an emergency smoke vent, in case the outer frame is coupled to a smoke duct, or as a smokeproof screen, in case the outer frame is closed permanently at its upper end.
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