The gaps around a fire door are not a finishing detail, they are part of what was tested. Too wide and the intumescent seal cannot bridge them when it expands; too tight and the door binds and stops closing. Both failures are common and both are measured in millimetres with a simple gauge.
The gap around the leaf
The gap at the head and down both stiles is specified by the door's certification, and the figures in common use across certified doorsets are a few millimetres with a small tolerance either way. The correct number for your door is the one in its certification documentation, not a figure from a website. Where the documentation is lost, an inspector will measure against the values the relevant scheme applies to a door of that type.
The gap under the door
The threshold gap is treated differently because floor finishes vary, and it is normally allowed to be larger than the gaps around the leaf. Where the door is required to restrict cold smoke, that larger gap has to be dealt with, usually with a drop down seal. A door with a generous threshold gap and no threshold seal is not automatically wrong; a smoke rated door with one is.
How to measure
A fire door gap gauge is a cheap moulded tool that fits into the gap and tells you which band it falls in. Measure at several points down each stile and across the head, because doors drop, frames move and a gap that is correct at the top is often not at the bottom. Record which door and which position, because the value of a measurement is being able to compare it next time.
Fixing a gap that is wrong
A gap that is too wide can sometimes be corrected by adjusting hinges or by fitting an approved intumescent gap filler or a compensating strip within the door's permitted alterations. Planing a door to fix a tight gap removes material from a tested assembly and can invalidate it if it goes beyond the permitted trim, which is usually very small. When in doubt the question is what the manufacturer permits, not what looks right.
Questions people ask about fire door gap regulations
What is the maximum gap under a fire door?
The threshold gap is allowed to be larger than the gaps around the leaf, and the permitted figure comes from the door's own certification. Where cold smoke performance is required, a threshold seal deals with it.
How do I measure a fire door gap?
With a fire door gap gauge, at several points down each stile and across the head, because doors drop and frames move. Record the door reference and the positions measured.
Can I plane a fire door to fit?
Only within the manufacturer's permitted trim, which is usually a very small amount. Removing more than that from a tested assembly can invalidate its certification.