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.