- A door that carries a certified fire-rating issued by Underwriters' Laboratories, Inc. 3-hour fire doors (A) are used in walls separating buildings or dividing a single building into fire areas. 1-1/2-hour (B and D) fire doors are used in openings in 2-hour rated vertical enclosures such as stairs, elevators, etc., or in exterior walls subject to severe fire exposure from outside the building. 1-hour fire doors are for use in openings in 1-hour rated vertical enclosures. 3/4-hour fire doors (C and E) are for use in openings in corridor and room partitions or in exterior walls that are subject to moderate fire exposure from outside the building. 1/2-hour fire doorsand 1/3-hour fire doorsare used where smoke control is a primary consideration, and for the protection of openings between a habitable room and a corridor when the wall has a fire resistance rating of not more than one hour.