Fire door gaps: how big is too big
The gap between a fire door and its frame decides whether smoke and flames can get round the door. You can check it in a minute with a simple gap card, and it is worth checking every time you look at the door.
What the gaps should be
Government guidance for England says the gap between the door and the frame should never be more than 4mm, except at the bottom of the door. At the top and sides, most fire door installers work to a gap of between 2mm and 4mm. Much tighter than 2mm and the door tends to rub or stop short of closing.
At the bottom, the guidance says the gap should be as small as practicable while making sure the door will not snag on the floor if it drops slightly on its hinges. Our own checklist uses 8mm as a working limit at the bottom, unless the certificate for that door says otherwise. Doors that also hold back cold smoke usually need a much smaller gap or a seal there.
Measuring it in a minute
- Close the door fully and let it latch
- Check the gap along the top and both sides at several points, using a gap card or feeler gauge
- Look for daylight through the gap, or a gap that is wide at one end and tight at the other
- Check the gap at the bottom with the door closed
- Photograph anywhere over 4mm at the top or sides, with the date showing
A gap that is wide at the top on the latch side and tight at the bottom usually means the door has dropped on its hinges.
What a wide gap usually means
Most wide gaps come from loose or worn hinges, a door that has dropped, or a frame that has moved. Tightening or replacing hinges and adjusting the door often brings the gap back. A door that has been trimmed too far or has shrunk may need its edges built back up by a competent joiner, within what the door's certificate allows, and a badly oversized gap can mean a new door.
Do not fill a gap with ordinary filler or sealant. It will not behave like the door's own seals in a fire.
What people ask about gaps
What is the maximum gap under a fire door?
Government guidance for England says the gap at the bottom should be as small as practicable without the door snagging on the floor. Our checklist uses 8mm as a working limit unless the door's certificate says otherwise, and smoke control doors usually need less.
Is a 5mm gap at the side of a fire door acceptable?
No. The guidance says the gap should never be more than 4mm except at the bottom, so a 5mm gap at the side needs the door adjusting or repairing.
Do I need a professional to check fire door gaps?
No. The guidance says these simple visual checks can be done by the responsible person or their staff. A full fire door inspection by a competent person is a separate thing.
Check your doors now
Door by Door is our free fire door check. Walk round with your phone, answer the checks at each door and add photos of anything wrong. You get a dated report by email to file, or to forward to whoever fixes it.
A plain summary, not legal advice
This guide follows the rules in England. Wales shares the Fire Safety Order but not the 2022 England regulations, and Scotland and Northern Ireland have their own fire safety law. Your fire risk assessment is the place that says what applies to your building, and a self check never replaces a fire door inspection by a competent person.
We put fire doors right across County Durham, Teesside and Tyne and Wear. If you are further away, a dated report from Door by Door is exactly what a local fire door specialist will want to see.
Tell us where to start
Ring Wayne Cator, our Managing Director, or send us the details and we will come back to you.