How do steel fire doors work?

Steel fire doors prevent the spread of fire for a specified time. They are constructed from materials that will withstand fire for either 20 minutes or 180 minutes, depending on the fire door rating. Fire doors are fitted with intumescent strips in a groove on every edge of the door or fire door frame. When a fire breaks out, the heat causes the intumescent strips to expand to fill the gap between the fire door and the frame. This seals the room and stops the spread of fire for a given time. A fire door will only work if it is closed when the fire breaks out, so you should always ensure that your fire door is fitted with an automatic door closer and a sign that identifies the door as a fire door.