Fire doors are a type of door used to contain a fire in a certain part of a building. They form part of a passive fire protection system. The walls around fire doors are also often made to be fire-resistant. Fire doors and walls have hourly ratings, which express the time that they are able to stand up to fire in test conditions.
There are several materials which these doors can be made out of. These materials are not all fireproof, but rather are meant to delay the fire from spreading for as long as possible. Some doors are made of timber, others of steel, and still others are partly composed of natural minerals. The door frame is most often also fire-resistant, featuring strong construction and seals to stop the passage of smoke. The door hardware must also tested and found suitable for use on a fire door. The door, frame, and hardware together form a "doorset," which carries a given fire rating.
Fire ratings are assigned based on how long the door or doorset is able to withstand direct exposure to fire. Hourly ratings include three hours, 1.5 hours, one hour, 3/4 -hour, and 1/3- hour. A three hour fire door would be most commonly used in a wall that separates two buildings or two parts of a large building. The wall around the door in this case would be rated for an even longer time, such as four hours. 90-minute or 1.5-hour doors are commonly found in stairwells such as those in apartment buildings.
Generally, the smaller the areas that a door divides, the lower the fire rating of the door. Fire doors with ratings under 1.5 hours are used to divide occupancy areas of a building, with 1/3-hour or 20-minute doors being used mainly for smoke control in corridors and other interior areas. Many fire doors have internal windows which are also fire-rated. The window is there to allow a person to judge whether the door may safely be passed through in a fire. Wire mesh glass is common for use in these windows, as well as ceramic glass.
Fire doors are sometimes unable to provide the protection they were meant to, most often due to incorrect operation. Doorstops should never be placed to hold a fire door open, as this would render it useless in an emergency. Most fire doors are meant to be closed at all time, however some are held open by devices which release them during a fire to close on their own. Doors must be certified by a testing laboratory and bear the laboratory’s label, in order to be used as fire doors, according to federal regulations.