Fire doors are a requirement in certain domestic situations, such as in flats, where a door leads into an integral garage, in any dwelling where there is a second floor 'habitable' room, such as houses with loft conversions or a 'room in the roof'.
All rooms in any of these situations are separated from other rooms, or compartmented, in order to:
• keep any fire in the compartment in which it starts
• protect the occupants (and contents) of other compartments
• provide a safe, protected route to allow the occupants to escape.
The walls, ceilings, entrances and exits are therefore designed to resist the fire for a specified period of time.
The simple purpose of a fire door in every day use is just as any other door. However, since a breakout of fire is never predictable, the fire door, unlike any other door, must then perform its prime purpose – to protect lives and offer protection to the remainder of the building and to other buildings.
The above information is taken from https://www.bwf.org.uk/choose-wood/fire-doors/