Most fire doors need a gap of between 2mm and 4mm around the head and the two jambs. Measure it with the door closed. Three millimetres is the figure most inspectors work to.

That answers the question. It will not, on its own, tell you whether your doors pass a fire door survey.

The gap under the door follows different rules. Double doors add a fourth gap to check. And a gap that is too tight fails just as easily as one that is too wide.

In this guide we explain the accepted tolerances and where the threshold gap sits. We also cover what sets the limit on your particular doors, and what to do when your gaps fall outside it.

Why Fire Door Gaps Matter

The gap around a fire door is not a cosmetic detail. It is the space the intumescent seal has to close.

Intumescent strips sit in a groove in the leaf or frame and stay dormant at normal temperatures. Once the temperature climbs into the low hundreds, they expand rapidly and fill the gap between leaf and frame.

A seal can only bridge the gap the manufacturer tested it against. Widen that gap and the strip may not close it in time. The door then fails well before its rated period.

This is why a door that looks perfectly serviceable can still count as a defect. The leaf can sit undamaged, the certification can read correctly, and the closer can work. The door still fails on a measurement of three or four millimetres.

Fire Door Gap Tolerances at the Head and Jambs

Industry guidance puts the tolerance at 2mm to 4mm along both long edges of the leaf and across the top. Measure with the door closed and latched.

The BWF Fire Door Alliance publishes this range. Behind it sits BS 8214:2026, the code of practice for fire-resisting and smoke control doors.

Three millimetres is the working target. At four millimetres, most inspectors record a defect.

Consistency matters as much as the maximum. Take a door holding a steady 3mm down both jambs. Now take one measuring 1mm at the top of the hinge side and 4mm at the bottom. Both sit inside the range. Only the first is stable.

An uneven gap usually means the leaf has dropped, the frame has moved or the hinges have worn. It normally gets worse.

The 2mm lower limit matters too. A door that sits too tightly in its frame binds. It catches on the jamb, or stops short of the latch. Timber also moves with humidity. A leaf that clears by a hair in January seizes in August.

Fire doors only perform when they close and latch. A door that will not latch has failed, whatever the gap measures.

Double Doors and the Meeting Stile

A double doorset gives you a fourth gap to check, where the two leaves meet. The same tolerance applies down its full height.

Both leaves also need to close fully and latch in the correct order. Where the doorset has a selector, check that it still sequences them. A meeting stile can measure perfectly while you hold the doors shut by hand. In practice it still fails.

The Gap Under the Door

No single legal figure governs the threshold gap, and this is where most of the confusion sits.

Where the door carries no cold smoke control requirement, inspectors commonly accept up to around 8mm. The door’s own test evidence has to support it. Fire and hot gases rise, so the underside of the door does not form the critical leakage path that the head and jambs do.

Smoke changes that. Cold smoke control covers FD30S and FD60S doorsets, and most flat entrance and corridor doors in residential blocks. On those doors you have to close the threshold. Use a drop-down seal in the leaf or a threshold seal in the floor.

Floor coverings cause a lot of these failures. New carpet raises the finished floor level and leaves the door binding. A carpet somebody lifted and never replaced does the opposite. It opens a threshold gap well beyond anything the door saw in test. We recently carried out a drop-down seal replacement at a Bournemouth property for that reason.

What Sets the Limit on Your Doors

The 2mm to 4mm range is the general rule. Your individual door answers to its own fire test evidence, its certification and the manufacturer’s installation instructions.

Some manufacturers certify doorsets outside the usual range, a small number at 5mm and others tighter. Standard joinery tolerances do not apply to fire doors.

This is one reason certification matters so much. Without a label, plug or traceable reference, you cannot establish what the manufacturer tested the door with. So you cannot establish what gap it may run at. Our guide to FD30 and FD60 fire door specifications covers what evidence you should expect to find.

Underneath this sits the Regulatory Reform (Fire Safety) Order 2005. Article 17 requires the responsible person to keep fire safety measures, fire doors included, in efficient working order and good repair. Gaps that drift outside the tested tolerance count as a maintenance failure. A fire and rescue authority will treat them as one.

How to Measure Fire Door Gaps

Close and latch the door before you measure it. Never hold it shut by hand, and use a gap gauge rather than a tape or a coin.

A £1 coin runs about 3mm thick, which gives you a rough sense of scale during a walk-round. But it measures nothing, and it should never appear in a record.

Take readings at several points rather than one:

• Three down each jamb
• Two or three across the head
• The threshold at both edges and the centre
• The meeting stile on double doors

A single measurement hides the drift that tells you what is happening to the door.

Record them against the individual door reference rather than as a building-wide pass. A door reading 3mm, 3mm, 4mm down the closing jamb tells you something a compliant tick never will.

Gaps also move. Buildings settle. Timber swells and shrinks with the seasons. Hinges wear and let the leaf drop. And contractors disturb frames during unrelated refurbishment work.

That is why your inspection frequency should reflect the real use and condition of your doors. A date in a diary is not enough. Routine checks catch the obvious problems, while a professional fire door survey measures every gap against the door’s certification.

What to Do If Your Gaps Fail

An oversized gap does not automatically mean a new door. You can usually adjust or replace worn hinges, correct the frame fixings and packing, or fit an approved lipping to the leaf.

Lipping has limits. It must be a compatible material, at a thickness the certification allows. Whoever fits it needs to know what that evidence permits. An unapproved repair invalidates the door as surely as the original defect did. The same applies in reverse, and you cannot simply plane a fire door leaf to fit.

Where somebody has damaged or heavily modified the leaf, replacement usually makes more sense. The same goes for a leaf carrying no traceable certification. Either way, record the work alongside the inspection that found it. Our fire door remedial works page sets out what we can correct in place and what we cannot.

Fire Door Gaps Frequently Asked Questions

What is the maximum gap allowed around a fire door?

Between 2mm and 4mm at the head and both jambs, with 3mm as the usual working target. At four millimetres, most inspectors record a defect. The door’s own certification overrides the general figure.

How big can the gap under a fire door be?

No single legal figure governs it. Inspectors commonly accept up to around 8mm where the door carries no cold smoke control requirement and its test evidence supports it. Where the door has to control cold smoke, close the threshold with a drop-down or threshold seal.

Does a gap that is too small fail an inspection?

Yes, if it stops the door working. A leaf that binds on the frame or the floor has failed. So has one that will not close and latch on its own. The gap measurement does not rescue either.

Can you repair an oversized gap without replacing the door?

Often, yes. Hinge adjustment, frame correction and approved lipping resolve many oversized gaps. The repair has to stay within what the door’s certification allows.

Need Your Fire Door Gaps Checked?

At CFP Carpentry & Fire Prevention, we survey and inspect fire doors for building owners, facilities managers and managing agents. We measure every gap against the door’s certification and test evidence, then report the findings door by door. Where gaps have drifted, we complete the remedial works. Where we cannot bring a door back into compliance, we supply and install a certified replacement.

In the South and South West we cover Bournemouth, Southampton, Poole, Exeter and Plymouth. Across London and the South East we cover London, Guildford, Slough and Croydon. We also work in Bristol, Swindon and Oxford.

Want to know whether your fire door gaps would stand up to inspection? Get in touch to arrange a survey.