Steel fire doors are “rated” by time (in minutes or hours) that a door can withstand exposure
to fire test conditions. Hourly ratings include 1-1/2-hours, 1-hour, 3/4-hour, and 1/3-hour, with the maximum rating required of any swing-ing type fire door being three hours.
Three-hour (180 minute) doors
A door with a three-hour fire protection rating is usually found in walls that separate
buildings or that divide a large building into smaller fire areas. The wall rating is four hours.
1-1/2-hour (90 minute) doors
Doors rated for 1-1/2 hours are found in 2-hour rated walls. These doors are commonly located in stairwells, or other enclosures of vertical communication through a building. They also occur in boiler rooms and in exterior walls that have potential for severe fire exposure from the outside the building.
One-hour (60 minute) doors
One-hour rated doors are used in walls between rooms, which are also typically one-hour rated.
3/4-hour (45 minute) doors
Doors with 3/4-hour fire protection ratings are used in one-hour walls. A 3/4-hour rated door
is found in walls of corridors and room partitions. A door with this rating may also be located in the exterior wall of a building subject to moderate fire exposure from the outside of the building.
1/3-hour (20 minute) doors
One-third-hour or 20 minute doors are used in one-hour walls. These doors are used for cor-
ridor applications and in other applications where smoke and draft control is a primary concern.