Fire safety glossary
Fire door
Definition
A fire door is a door assembly — door leaf, frame, seals, and ironmongery — that has been tested and certified to resist the passage of fire and smoke for a specified period. It forms part of a building's passive fire protection, containing fire within the compartment of origin and protecting escape routes. A fire door must always be kept closed (or connected to an automatic hold-open device) to perform its function.
Fire resistance ratings
Understanding FD30, FD60, and the S suffix
Door assembly
The six components of a fire door assembly
Door leaf
The door itself — must be a certified fire-resisting construction, typically solid-core timber or steel.
Door frame
Must be part of the tested assembly. A fire-rated door in a non-rated frame loses its certification.
Intumescent seals
Fitted around the door edge — expand when heated to seal the gap between door and frame against flames and hot gases.
Smoke seals
Brush or fin seals that limit cold smoke passage. Required in FD30S / FD60S rated assemblies.
Self-closing device
A door closer that returns the door to the closed position automatically after use. Mandatory in fire doors.
Ironmongery
Hinges, handles, locks, and vision panels must all be part of the tested assembly or separately rated.
How a fire door protects occupants
Fire doors work through compartmentation — dividing a building into fire-resisting compartments so that a fire in one area cannot spread quickly to others. This is what allows people in other parts of a building to evacuate while the fire is still contained. Research from UL's Close Before You Doze study demonstrated that a closed door — even a standard interior door — dramatically reduces temperatures and toxic gas concentrations in adjacent spaces. A certified fire door provides far greater protection. See our flashover glossary entry for the fire development context.
The "always closed" rule
A fire door only works when it is closed. An open fire door is simply a gap in the compartmentation strategy — fire, smoke, and toxic gases travel freely through it. Wedging or propping fire doors open with furniture, rubber wedges, or cable ties is a common and serious violation found in fire safety audits. In UK workplaces and residential blocks, it is a legal breach of the RRO 2005. If a door must remain open for operational reasons, install an electromagnetic hold-open device linked to the fire alarm — it will release automatically when the alarm sounds.
Check the gap
The gap between the door leaf and frame (and floor) should be no more than 3 mm on all sides when closed. A larger gap allows fire and smoke to pass through before the intumescent seals have had time to expand. Test with a 3 mm coin — if it passes through easily, the door needs adjustment or replacement seals.
Fire door FAQ
Questions about fire doors, ratings, and compliance
FD30 and FD60 refer to the fire resistance rating of a fire door — specifically how long the door assembly (door, frame, and ironmongery) will contain fire and smoke under standard test conditions. FD30 provides 30 minutes of fire resistance; FD60 provides 60 minutes. The S suffix (FD30S, FD60S) indicates the door also has smoke seals fitted to limit smoke leakage. In UK residential buildings, FD30S is the most common specification for flat entrance doors and stairwell doors. Commercial and high-rise buildings may require FD60 or higher in certain locations.
Any modification to a fire door that affects its fire-resistance capability — including cutting holes for letterboxes, cat flaps, or additional glazing; changing the ironmongery; or painting over intumescent seals — can void its fire rating. All modifications must be assessed by a competent person and must not compromise the tested assembly. In the UK, the Fire Door Inspection Scheme (FDIS) and British Woodworking Federation (BWF) provide guidance on modifications. Letterboxes and cat flaps must be fire-rated assemblies themselves.
In the UK, the Fire Safety Act 2021 and amended Fire Safety (England) Regulations 2022 require responsible persons in multi-occupied residential buildings to carry out quarterly checks of fire doors in common areas, and annual checks of flat entrance doors. In non-domestic premises, fire door inspection should form part of the fire risk assessment review. For buildings of 11 metres or more in height (in England), the duty to check is prescribed by the 2022 regulations. NFPA 80 in the US requires annual inspection of fire doors.
Propping a fire door open with a wedge or other improvised device is a fire safety violation in UK workplaces and residential common areas. Under the RRO 2005, the responsible person must ensure fire doors operate correctly and are not wedged open. Prosecution and fines have resulted from fire door propping. If a door needs to be held open for operational reasons, an approved electromagnetic hold-open device linked to the fire alarm system must be fitted — it releases the door automatically when the alarm activates.
Legitimate fire doors carry a certification mark — typically a plug or label on the top edge of the door leaf, or on the frame or hinge area. Common UK marks include the BWF-CERTIFIRE scheme, CERTIFIRE, and the BM TRADA Q-Mark. The mark should include the fire rating (FD30, FD60), the manufacturer's reference, and the certification body. A fire door without a visible certification mark should be assumed non-compliant until inspected by a competent person.