Fire Door Gap Requirements: What Size Gaps Fail Inspection?
A fire door gap that's too wide is one of the most common — and most consequential — failures in quarterly inspections. The gap between the door edge and the frame is where fire and smoke travel first when a door is closed. Get the measurement wrong and the door that should give occupants 30 or 60 minutes of protection is already compromised.
The measurement itself is simple. What catches managing agents out is not the 4mm standard — it's knowing which gaps it applies to, what causes gaps to widen over time, and what the remediation options are when a door fails.
The 4mm Rule: What the Guidance Actually Says
The government's Fire Safety (England) Regulations 2022 fire door guidance states: "The industry standard is that the gap size should never be more than 4mm, except at the bottom."
This applies to the vertical edges (the lock side and hinge side) and the top of the door. The bottom of the door is handled separately — there the guidance says the gap "should be as small as practicable, while ensuring that the door is unlikely to snag on the floor even if the door drops slightly on the hinges." There's no fixed number for the bottom gap — "as small as practicable" is the test, and you're balancing fire protection against the door's ability to open and close without scraping.
What the guidance does not specify is a minimum gap. Fire doors need some clearance to operate — a gap of 1-2mm across the top and sides is typical for a well-fitted door. The compliance problem is at the other end of the scale.
Scope of this guide: These requirements apply to multi-occupied residential buildings in England above 11 metres in height, under regulation 10 of the Fire Safety (England) Regulations 2022. If your buildings are in Wales, Scotland, or Northern Ireland, or below the 11-metre threshold, the Regulatory Reform (Fire Safety) Order 2005 still applies — fire door adequacy is required — but the regulation 10 specific inspection frequency does not.
How to Measure a Fire Door Gap
The guidance notes that "simple 'gap tester' cards are available for this purpose" — these are thin plastic or card gauges, usually marked at 3mm and 4mm, designed to slide into door gaps during inspection. Most fire door suppliers stock them cheaply.
If you don't have a gap tester card, the approach used in the field is a £1 coin. The thickness of a UK £1 coin is approximately 2.8mm, and a 20p coin is approximately 1.7mm. Neither is a precise instrument, but they're useful for a quick assessment:
- £1 coin slides through with room to spare: the gap is likely wider than 3mm — investigate with a proper gauge
- £1 coin passes through snugly: gap is around 3mm — acceptable
- £1 coin won't fit: gap is likely under 3mm — well within compliance
For a quarterly check under regulation 10, the coin test is a reasonable first pass. For any gap that looks marginal, measure with a proper gauge before recording the result.
Where to measure: Check all four sides of the door — lock side, hinge side, top, and bottom. Gaps are often inconsistent: a door that's fine at the top may have a 6mm gap at the bottom if it has dropped on its hinges. The worst measurement is what matters.
Common source of confusion: The existing posts in this cluster (including our step-by-step checking guide) reference 3-4mm as the range to aim for. That's accurate — 3mm is typical for a well-fitted door, and 4mm is the maximum. Some secondary sources quote "3-4mm" as if it's a target band; the official guidance is clear that 4mm is the upper limit, not a midpoint target.
Why Gaps Widen Over Time
A fire door installed to perfect tolerances will gradually develop wider gaps. The main causes:
Hinge wear and drop. Every time a heavy fire door opens and closes, it places stress on the hinges. Over months and years, hinge screws loosen, hinge knuckles wear, and the door begins to drop on the hinge side. The visible result: a tight gap at the top-hinge corner and a widening gap at the bottom-lock corner. Often accompanied by the door dragging on the floor or carpet.
Building movement. Modern residential blocks move — thermal expansion, settlement, and vibration all shift door frames incrementally. A frame that's plumb when installed may rack slightly over years, creating an uneven gap around the door.
Furniture and objects. Residents prop fire doors open, hang bags on closers, push heavy furniture against doors. Each instance places stress on the frame, hinges, or door leaf.
Closer force. If the self-closing device is set too strong, the door slams hard into the frame repeatedly. This can both loosen the hinges and cause the door to bounce back slightly on impact — meaning the latch doesn't engage and the door sits ajar rather than closed.
The practical consequence for managing agents: a quarterly check that passed six months ago is not a guarantee that gaps are still within tolerance today. Check every time.
What to Record and When to Escalate
Under regulation 10, you must keep a record of the checks — not just a note that you "checked the doors," but a record sufficient to demonstrate compliance. For gaps, that means recording:
- Which door was checked (location, floor, unit number)
- The gap measurement or assessment at each side
- Whether the door passed or failed
- If failed: what action was taken or scheduled
Our free fire door inspection checklist includes a gap-check section for each door that gives you a structured record in a format suitable for a building file.
When to escalate: A gap that fails the 4mm standard needs remediation. The government guidance is clear that where an inspection identifies a fire door needing repair or replacement, the work "must be undertaken by a competent contractor as soon as reasonably practicable." For communal doors, you have more control over timing than for flat entrance doors. The urgency scales with the margin of failure — a 5mm gap on an otherwise sound door is different from a 10mm gap on a door with missing seals and a failed closer.
A door with multiple simultaneous defects (gap failure + missing seals + closer failure) is a higher-priority remediation than one with an isolated gap issue.
Gaps and Intumescent Seals: Why Both Matter
The 4mm gap standard exists partly because that's the width intumescent seals are designed to handle. An intumescent strip expands in heat to seal the gap between door and frame — but it can only seal a gap that's within the range it was certified to cover.
If the gap is 7mm and the seal expands to cover 6mm of that, there's still a 1mm opening for fire and smoke to travel through. The interaction between gap width and seal capacity is one of the reasons both checks appear in every inspection.
If you're seeing failed gaps alongside damaged or missing seals, those need to be addressed together — rehanging the door to restore gap tolerance doesn't help if the seal that was specified for the original gap is no longer adequate for the adjusted gap.
Our guide to fire door seals and intumescent strips covers the seal side of this in more detail.
The Broader Defects Context
Excessive gaps are the most frequently identified failure in FDIS inspection data — appearing in the vast majority of doors that fail. Our guide to the 8 most common fire door defects covers the full set, including how to triage when a door has multiple issues and what the remediation priority order looks like across a building portfolio.
This guide applies to multi-occupied residential buildings in England above 11 metres. The regulation 10 fact sheet covers the inspection obligations in full. This is practical guidance for managing agents carrying out their own checks — not legal advice. For buildings requiring formal compliance sign-off or Building Safety Case documentation, consult a qualified fire safety engineer.
Last reviewed: June 2026