What gap under fire door

[Neil Ashdown MAFDI]

Where a threshold gap is more than 10mm this could cause the door to fail to provide its 'ultimate' fire resistance as tested to BS 476 part 22 or EN 1634-1. You say that smoke is not the issue in your case so we will not address that separate issue here.

The larger the threshold gap then the farther the door leaf can drop to the floor when the hinges fail to hold the door in position due to fire attack at those hinge positions. Therefore, a gap would open-up at the top edge of the door and the larger the gap, then the higher the risk of failure. 

So we are talking here about 'ultimate fire resistance performance' and this is the type of consideration that must be made by the fire risk assessor when deciding whether or not an existing fire door would provide adequate protection in a fire. 

 

[Neil Ashdown MAFDI]

Furthermore, to sustain a live fire oxygen is a necessary element. Therefore, the larger the threshold gap then the more oxygen can pass under the door to help sustain the fire. 

[Guest Steven]

I am wanting some advice on old existing fire doors that the bottom gap exceeds 10mm. I am aware of installation instructions stating what can can’t be done to new manufactured doors but this information is almost impossible to find without having originally documentation. A lot of time floor gaps are not consistent and installation of a threshold would cause a tripping hazard. Can a hard wood strip be installed to minimise the gap?

[Neil Ashdown MAFDI]

The issues with excessive (more than 10mm) threshold gaps are:

1) that the ultimate fire resistance performance for the door can be compromised because the door can drop further (in a fire attack scenario) to the floor thus opening up a large gap at the top of the door.

2) that the increased flow of air under the door can help to feed the fire thus making it potentially more dangerous

3) that an increased spread of smoke under the door could reach sleeping accommodation and escape routes with potentially highly dangerous consequences. The gap for restricted spread of cold smoke should be 3mm max or it should be closed with a flexible seal, refer to BS 8214: 2016.

To resolve these issues, you could fix a suitable hardwood lipping to the door bottom edge and/or fix a threshold plate to the floor. Fit threshold seals where necessary for smoke protection. But first survey each door to decide the most appropriate action.

[Guest Graham]

Hi Neil,

We have a number of FD30 Fire doors that have approximately 20-25mm gaps under the doors. I have been looking for a Surface Mounted Drop seal that would accommodate this distance but not having much luck. I noticed in one of your replies you posted 10September 2020 that you mentioned fixing a Hardwood strip to the bottom edge of the door and the same thickness as the door to address this problem before fitting the drop seal. Is this an acceptable way of getting over the issue.

 

 

 

 

[Neil Ashdown MAFDI]

The usual required threshold gap for a typical timber-based fire door is 10mm max.  It may be possible to reduce your 20mm to 25mm gaps by fixing a threshold plate to the floor or a hardwood lipping to the door bottom edge - you will need to survey each door and floor area to decide which (possibly both) is appropriate. 

A threshold plate may, of course, present a trip hazard. Assess the hazard.

Where restricted smoke spread is a requirement, it may be necessary to install a flexible seal to close the threshold gap in the door-closed position. 

[Guest Graham,]

Hi Neil 

Many thanks for your reply.

I have checked the offending doors and the largest Gap under these FD 30 Fire doors is actually 20mm but most are approximately 15mm.

I intend to machine a Hardwood strip and Bond and screw it to the bottom edge of the doors to reduce this Gap to approx 8mm.

If no smoke control is required should this satisfy the Fire Officer 

Regards 

Graham 

 

 

 

 

 

 

 

[Neil Ashdown MAFDI]

So long as the 're-lipping work' is done to a 'joinery' standard (the joint should be difficult to detect) this should be acceptable as a suitable repair. 

[Guest Meir]

Hi, 

I have a fire door that requires a smoke seal and it has a 20mm gap at the bottom, any advice on the easiest way to fix this?

 

[Mike North]

The maximum gap for a smoke door is 3mm.

It may be possible to fit a threshold seal, but I have never come across one that would bridge that gap

It may also be possible to lip the door, but again I have never come across one that would bridge that gap

You could put a 17mm hardwood threshold detail under the door fixed to the floor, but that may cause a trip hazard

[Neil ashdown]

On 03/11/2025 at 15:03, Guest Meir said:

Hi, 

I have a fire door that requires a smoke seal and it has a 20mm gap at the bottom, any advice on the easiest way to fix this?

 

Generally, the maximum threshold gap for a fire resisting door is 10mm. For a door that is required to restrict the spread of cold smoke its 3mm.

3mm is often difficult to acheive due to imperfect floor conditions, so a 10mm gap is acceptable so long as its sealed with a flexible seal. Ref. BS 8214: 2016 Timber-based Fire Door Assemblies Code of Practice. 

UK Gov fire door safety guidance reconises this difficulty.  https://assets.publishing.service.gov.uk/media/677fbe166f01ae28ab5c0554/A+guide+to+making+your+small+block+of+flats+safe+from+fire+2024.pdf  Page 18