Fire Door Gap Tolerances UK
— What’s Acceptable?

Published 18 April 2026 • DE Fire Compliance • 5 min read

Gap tolerances are one of the most commonly failed elements on a fire door inspection. A gap that looks minor to the untrained eye can be enough to fail a door — and a failed door compromises the entire fire compartment it protects.

Here’s what the standards say, what inspectors measure, and what you need to do if gaps are out of tolerance.

The 3mm Rule — What It Means

The most widely cited standard for fire door gaps is BS 8214:2016 — the British Standard for fire door assemblies. Under BS 8214, the permitted gap between the door leaf and the door frame on the three sides (top and both sides) is:

No more than 3mm when the door is in the closed position, measured at any point around the rebate.

This applies to all three sides of the door — the hinge side, the latch side, and the top. A gap that exceeds 3mm at any point is a failure.

The 3mm tolerance exists because intumescent strips — the expanding seal fitted in the door frame or leaf — need a tight enough gap to function correctly when exposed to heat. Too large a gap and the intumescent cannot bridge it in time to prevent fire and hot gas from passing through.

What About the Threshold Gap?

The threshold gap — the gap between the bottom of the door and the floor — is treated differently. BS 8214 allows a larger threshold gap, because:

• The threshold must be large enough for the door to swing freely without dragging
• A cold smoke seal (brush seal or drop seal) is usually fitted to address smoke rather than fire penetration at this point

The generally accepted threshold gap is up to 10mm where a cold smoke brush seal is fitted. Without a smoke seal, any gap that would allow cold smoke to pass through is a failure. For doors without a cold smoke seal, the threshold gap should be no more than 3mm — the same as the other sides.

In practice, most compliant fire doors have a threshold brush seal fitted as standard. If yours don’t, this is a relatively inexpensive remediation.

How Are Gaps Measured?

An inspector measures gaps using feeler gauges — thin metal strips of a known thickness. A 3mm feeler gauge is inserted at multiple points around the door perimeter with the door closed. If the gauge passes through at any point, the gap is at or above tolerance and is recorded as a failure.

Inspectors typically check:

• Top of door (both corners and centre)
• Hinge side (top, middle, bottom)
• Latch side (top, middle, bottom)
• Threshold (across the full width)

A door must pass at all measured points. A single excessive gap — even in one corner — is recorded as a failure for that element.

What Causes Excessive Gaps?

Oversized gaps are almost always the result of one or more of the following:

• Door drop — worn or incorrect hinges cause the door to sag, creating a larger gap on the latch side and a smaller or binding gap on the hinge side
• Frame movement — building settlement or moisture movement in timber frames widens the rebate over time
• Incorrect installation — the door was never fitted to tolerance in the first place
• Door swelling or shrinkage — solid timber doors can move seasonally; engineered core doors are more stable
• Planing or adjustment — doors that have been planed to stop binding are often taken below minimum leaf thickness and out of gap tolerance

What Happens If Gaps Are Out of Tolerance?

A door with gaps exceeding 3mm will be recorded as a failure in the inspection report. The remediation options depend on the cause:

• Hinge adjustment — if the door has dropped due to loose or worn hinges, new fire-rated hinges may restore correct alignment
• Intumescent strip repositioning — in some cases, repositioning or replacing strips can compensate for minor gap variation
• Frame adjustment or rebuild — if the frame has moved significantly, a frame repair or replacement may be required
• Door replacement — if the door leaf has been planed or is structurally compromised, replacement is the correct solution

Most gap failures are fixable without full door replacement — but the fix must be carried out by a competent person and re-inspected to confirm compliance.

Can You Use Draught Seals to Fix Gap Failures?

No. Standard draught excluders and foam seals are not fire-rated and are not an acceptable remedy for a fire door gap failure. Only fire-rated intumescent strips and smoke seals — installed correctly — are acceptable under BS 8214.

Using non-rated products may make the door appear compliant visually, but it will not pass inspection and does not provide fire resistance.

Summary — Gap Tolerance Quick Reference

• Sides and top: Maximum 3mm
• Threshold (with smoke seal): Up to 10mm
• Threshold (without smoke seal): Maximum 3mm
• Measurement method: Feeler gauge at multiple points
• Single failure point: Fails the element — must be remediated and re-checked