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.