I am hoping someone can give some advice here, as I can't find any anywhere else!

I am fully aware that gaps around modern fire doors should be 3mm (+/- 1mm) and this is because the intumescent strip and smoke seal are rated for gaps that size.

However, I conduct a lot of fire risk assessments for residential blocks of flats built in the 1970s and 1980s where the CP3 standard applied, requiring 25mm rebates. The requirement for that standard was the door to be flush against the rebate, to prevent smoke and fire from getting past.

In accordance with the LGA guidelines, and also BS 9794 (Clauses 9 and 10.4) I am happy to accept these doors in principle, including the two hinges, on the basis that they are fully self-closing.

However, my questions is about the gaps around, and what are the tolerances permitted, but nobody seems to know about this.

It seems to me, that the 3mm requirement should only apply to fire doors where there is a reliance on an intumescent strip and smoke seal, so what tolerance should I accept for these "notional" fire doors? Would say a 5mm gap be acceptable? Or maybe larger one, provided the door frame fits tightly against the rebate (ie up to 25mm)?

Thanks in advance for any advice

Before the use of intumescent edge seals fire door tests were carried out to early versions of the BS476 test. The test preceeding the 1987 Part 22 test, we still use today, was BS 476 Part 8 of 1972 and door leaf edge to door frame gaps were 3mm without using intumescent edge seals. There has never been an absolute requirement for a 3mm (or indeed 4mm max gap) but test results have been successful using that gap on a first principles basis. However, a very limited number of fire doors are permitted with larger gaps.

So two things to bear in mind:

1) Where smoke seals are necessary these are sometimes fitted to the edge of the door leaf and will usually be effective at 2mm to 4mm. But there would be a gap that could be exploited by cold smoke if the gap is larger. Where compression type smoke seals are used to seal between the rebate stop and the face of the door leaf, this larger gap should not cause a problem.

2) BS 8214: 2016 section 9.5.2, without making explicit reference to intumescent seals, states that a typlical gap to achieve good fire performance is between 2mm and 4mm. Therefore, it is possible that larger gaps may adversely affect fire performance. These gaps refer to test conditions though.