220ASUNP Finish: Unplated Soss now offers a complete line of fire rated invisible hinges. Whatever the door or opening, there is a Soss hinge for you. Whether your doorway is made of wood or metal, rated 20 minutes or 3 hours, there now is a SOSS fire-rated hinge that will comply and still give you the "uniquely invisible" hinge that you expect from Soss. Features: -Matl. Thickness of 2", 90/180 min UL Fire Rating, 1 each. -Used on hollow metal, sheet metal, tin clad, steel composite or wood core swinging type fire doors. -Whichever calculation demands more hinges. -One hinge for each 60 lbs. -Frames with 20 minute ratings. -Use at least one fire-rated hinge for every 20" of door height.