Fire Door Gaps and the 3 mm Rule Explained

“The gaps should be 3 mm” is the piece of fire door guidance everyone has heard and few can explain. The gap between the door leaf and its frame is where fire and smoke get through first, and it is also the space the intumescent seal needs to expand into. Too wide and the seal cannot close the gap in time; too narrow and the door binds, the closer cannot latch it and the seal has nowhere to go. This article sets out the actual tolerances, how to measure them and what to do when a door is outside them.

The tolerances

BS 8214, the code of practice for timber fire door assemblies, and most manufacturers’ installation instructions specify a gap of 2 mm to 4 mm between the leaf and the frame at the head and both sides, with 3 mm as the target. At the threshold the gap to the finished floor should normally be no more than 10 mm where no smoke seal is required, and 3 mm or less where the door is a smoke door or has a drop seal fitted. Always check the manufacturer’s certificate: some doorsets are tested with different gaps and the certificate overrides the general rule.

Why 3 mm

Intumescent strips expand many times their volume when heated, but they need time to react and a defined space to fill. The test evidence for the door assumes gaps in the 2 to 4 mm range; a 6 mm gap may let hot gases through before the seal has expanded, and the pressure of a developing fire can push smoke through long before that. A gap under 2 mm often means the door is rubbing, so it may not close fully from the closer alone, and a door that is not closed is not a fire door at all.

How to measure

• Use a gap gauge or feeler gauges, not a tape measure or a coin
• Measure at the top corner, middle and bottom of each vertical edge and at three points across the head
• Measure with the door closed and latched, as it would be in a fire
• Record the largest and smallest readings, not an average
• Check the threshold gap to the finished floor, including any mat well or threshold strip

Common causes of gaps out of tolerance

Doors are planed down to clear new floor coverings, frames move as buildings settle, hinges wear and let the leaf drop, and leaves warp in heated corridors. Uneven gaps, wide at the top on the latch side and tight at the bottom, usually mean worn or loose hinges. A consistent wide gap all round suggests the leaf was undersized for the frame when it was fitted.

What can be done

Many out-of-tolerance doors can be corrected without replacement. Hinges can be adjusted or replaced, packers fitted behind the hinge leaf, and the frame stop or leaf edge lipped with a hardwood strip to the manufacturer’s permitted repair details. Where the gap is beyond what the certification allows to be repaired, or the leaf is damaged, replacement is the only compliant answer. Our fire door repairs team carries out these adjustments and records each door’s before and after gaps in OneTrace.

Gaps are one check of many

Gap measurement is the headline item in any fire door inspection but it sits alongside seals, closers, hinges, glazing, signage and certification. Our fire door inspections record all of them for every door, and five fire door defects you shouldn’t ignore covers the others.

Frequently asked questions

Is a 5 mm gap a fail?

Under BS 8214 and most certificates, yes. A gap over 4 mm at the head or sides is outside tolerance and should be recorded as a defect and corrected.

What is the maximum threshold gap on a fire door?

Normally 10 mm to the finished floor where no smoke control is required, and 3 mm where the door must restrict cold smoke, unless a drop seal is fitted and the certificate permits more.

Can I fit a bigger intumescent strip to cover a wide gap?

No. Seals are part of the tested assembly and changing them invalidates the test evidence. Correct the gap instead.