GUIDES · INSPECTION
What are the correct gaps around a fire door?
A guide to fire door gaps by Daniel Storr, Founder & Managing Director · Last reviewed 11 September 2026
As a rule of thumb, fire door gaps at the top and sides of the leaf should be a consistent 2–4mm. The threshold gap should suit the door’s smoke-sealing requirement. However, the only figures that truly count are the ones in that doorset’s fire test evidence and the manufacturer’s installation instructions. Gaps are the single most common fire door inspection failure we find.
Why fire door gaps matter so much
A fire door works by staying shut and sealing the opening. If the gaps around the leaf are too wide, fire and smoke get an early path through. Too tight, and the door can bind, jam or fail to close fully into the frame. The intumescent strips in the door edge are sized to expand and close a specific gap in a fire. So a gap outside tolerance can mean the seals never bridge it. It is a small measurement with a big consequence, which is exactly why it is the fault we find most often. The government’s fire door guidance for the 2022 Regulations lists excessive gaps among the faults a routine check should pick up.
Typical fire door gap tolerances
| Where | Commonly specified gap | What we look for |
|---|---|---|
| Head & sides (leaf to frame) | 2–4mm, consistent | An even gap all the way round — no wide spots, no binding |
| Meeting stiles (double doors) | 2–4mm, consistent | Leaves meet evenly; rebates and seals intact |
| Threshold (under the door) | Per test evidence — smaller where cold-smoke sealing is required | Correct for the door’s rating; drop-seal fitted where specified |
Figures shown are widely-used guide values, not a universal rule. Under BS 8214:2026 and the door manufacturer’s instructions, the binding tolerances are the ones proven in that individual doorset’s fire test evidence — always check the door’s own documentation.
The threshold gap — where it gets misunderstood
The gap under the door causes the most confusion. A door that only needs to resist fire may tolerate a larger threshold gap than one that also has to hold back cold smoke. The smoke-resisting door needs the gap sealing, often with a drop-down seal. There is no single “correct” under-door figure that applies to every door. It depends on what the door is there to do and what its evidence allows. If in doubt, treat a visible daylight gap under a fire door as something to get checked.
What happens at inspection
On a fire door inspection we measure the gaps around every door with a gauge, not by eye. We record them door by door with photographs. Where a gap is outside tolerance, we flag it as a defect with the measurement noted. Most gap failures are repairable: re-hanging or adjusting the leaf, easing and re-sealing, or fitting the correct seals. Replacing the door is rarely the answer to a gap problem.
Checking fire door gaps yourself
You can get a rough sense with a simple test. A £1 coin is around 3mm thick. If it slides into the gap around the leaf easily, but a stack of two jams, you are roughly in the right region. That is a useful sanity check, not a substitute for a proper inspection. The seals, closer, hinges and the door’s own evidence all have to be right too. Our five-step visual check covers the rest. If the gaps look uneven, or you can see daylight under the door, it is worth getting it looked at.
Gaps outside tolerance are the fault we find most. Book a fire door inspection in Bournemouth or Poole and we will measure every door. We inspect from £20 +VAT per door and give you a photographed, door-by-door report. See our fire door inspections page, or go straight to fire door inspections in Bournemouth or fire door inspections in Poole. Elsewhere on the south coast? See the areas we cover or call 01202 399399.
Common questions
Are fire door gaps the same on double doors?
Broadly, yes. The meeting stiles between the two leaves should sit in the same 2–4mm range as the head and sides, with the rebates and seals intact. Double doors also need to close in the right order, so a selector or co-ordinator is often part of the fix.
Can a wide gap be fixed without replacing the door?
Usually. Re-hanging the leaf, packing or adjusting the hinges, easing a binding edge, or fitting the correct seals resolves most gap faults. We only recommend replacement when the leaf or frame itself is beyond adjustment.
Is a 5mm gap a fail?
Outside the common 2–4mm guide range, so we would record it as a defect. Whether it is a fail depends on the door’s own test evidence, which is why we note the measurement rather than just a tick or a cross.