BS 8214 Gap Measurements Explained
Fire door gap measurement guide. The working tolerances for standard and smoke doors, where those figures actually come from, how to measure, and what to do when gaps fail.
Gap measurements are one of the most critical aspects of fire door inspection. Get them wrong, and the door won’t perform in a fire. This guide gives you the figures to work to on site, and explains where they actually come from — because it is not where most people assume.
The short version: work to 2-4mm at the head and jambs. That is the industry best-practice figure and it is well supported. It is not, however, a figure stated in BS 8214:2026 — the 2026 revision sets no numeric gap tolerance at all and defers to each door’s own supporting evidence. Where a door has a datasheet or Field of Application, that document beats any general figure.
Looking for the BS 8214:2026 PDF? The full standard is copyrighted and sold by BSI, so there is no free official download. This guide is a free, plain-English summary of the gap measurement requirements you can use on site. For the gap tolerances at a glance, see our Fire Door Gap Measurement Guide.
Why Gaps Matter
Fire doors are designed to contain fire and smoke within compartments. When gaps are too large:
- Fire spreads faster through larger openings
- Smoke escapes into escape routes
- Intumescent seals can’t bridge the gap when they expand
- The door may fail before its rated time
Conversely, gaps that are too small can cause:
- Door binding
- Difficulty closing
- Excessive wear on hardware
Fire Door Gap Tolerances: The Working Figures
These are the figures UK surveyors work to day to day. They are drawn from industry best practice — the ASDMA Best Practice Guide to Timber Fire Doors (2015 edition, §13.11) and BS 4787-1:1980 Table 1 — and they are the sensible fallback when a door has no supporting evidence of its own.
Standard Fire Doors (FD30, FD60, FD90, FD120)
| Location | Gap Tolerance |
|---|---|
| Head (top) | 2-4mm |
| Jambs (sides) | 2-4mm |
| Threshold (bottom) | No general figure* |
| Meeting stile (double doors) | 2-4mm |
Why 2-4mm? The minimum 2mm gap helps the door operate freely without binding. The maximum 4mm helps maintain fire and smoke integrity — beyond that, an intumescent seal may not reliably bridge the gap when it expands. Gaps outside this range often indicate a problem worth investigating.
Where the figure comes from. Three independent sources land on effectively the same window, which is why 2-4mm is so widely accepted:
| Source | Head / jamb gap |
|---|---|
| BS 4787-1:1980 Table 1 (2mm, +1.0/−0.5) | 1.5–3.0mm |
| ASDMA Best Practice Guide 2015, §13.11 (2~3.5mm ± 0.5) | 1.5–4.0mm |
| Common working figure | 2.0–4.0mm |
One caveat worth knowing: BS 4787-1 is a manufacturing supply specification. Its §4.2 sizes apply “as at the time of handing over to the purchaser”, so it is not a fair yardstick for a door that has been in service for forty years. For doors in service, BS 8214:2026 §15.1 asks you to assess function and condition — does it close, does it latch, are the seals continuous, is there excessive bow, cup or twist.
Note also that the 2015 ASDMA guide revised these figures down from the 2002 edition’s 3-4mm ± 0.5. If you see 3-4mm quoted anywhere, that is the superseded 2002 number.
*There is no general published figure for the threshold (under-door) gap. BS 8214:2026 §13.5.3 defers it to the manufacturer’s installation instructions and the door’s supporting evidence, so check the door’s own documents. Where a drop-down seal is fitted, the seal’s own instructions apply.
Smoke Doors (FD30S, FD60S)
| Location | Gap Tolerance |
|---|---|
| Head (top) | 2-4mm |
| Jambs (sides) | 2-4mm |
| Threshold (bottom) | No general figure* |
| Meeting stile (double doors) | 2-4mm |
The ‘S’ designation means the door includes smoke seals at the head and jambs (brush or combined intumescent/smoke seals).
Threshold on smoke doors: check the door’s supporting evidence for both the threshold gap and how the threshold is sealed. Where a drop-down (automatic) seal is fitted, it drops as the door closes, and the seal’s own instructions set the gap it works with.
You may still see 3mm quoted as the maximum under-door gap for a smoke door with no threshold seal. That figure came from BS 8214:2016 (clause 12.3), which has been superseded. BS 8214:2026 gives no under-door figure and defers to the manufacturer’s installation instructions and the door’s supporting evidence. Other general threshold figures you may meet, such as 8-10mm, have no primary source in the current standards.
Note: Gap tolerances should always be verified against the specific door manufacturer’s fire test evidence, as requirements may vary. The ASDMA Best Practice Guide to Timber Fire Doors is the recognised industry reference, and ASDMA’s own position is clear: the overriding guidance for fire performance is to follow the supporting evidence provided by the door manufacturer or intumescent seal provider. The figures in this guide are general references; the door’s evidence takes precedence.
How to Measure Fire Door Gaps
Equipment Needed
- Gap gauge (leaf gauge or feeler gauge)
- Measuring tape
- Notebook or inspection app
- Torch (for poor lighting)
Measurement Process
- Close the door fully – ensure it’s latched
- Measure at multiple points – minimum 3 per edge
- Use the correct gauge – plastic leaf gauges or metal feeler gauges
- Record the largest gap – this is the critical measurement
- Check all four edges – head, both jambs, threshold
Measurement Points
For each edge, measure at:
- Top third
- Middle
- Bottom third
This identifies any warping or inconsistency in the gap.
Common Gap Problems
Excessive Head Gap
Cause: Door dropped, hinges worn, frame settling Solution: Adjust hinges, add packing, replace door if warped
Uneven Jamb Gaps
Cause: Door twisted, frame out of square, worn hinges Solution: Adjust hinges, plane door edge, refix frame
Large Threshold Gap
Cause: Floor settlement, door trimmed, incorrect installation Solution: Check the gap against the manufacturer’s instructions and the door’s supporting evidence. Where a remedy is needed, options include a threshold or drop-down seal fitted to its own instructions, or replacing the door
Meeting Stile Gap (Double Doors)
Cause: Both doors misaligned, worn rebate, incorrect closing sequence Solution: Adjust doors, fit coordinator, replace if damaged
When Gaps Exceed Tolerances
If gaps exceed the best-practice range and are not supported by the door’s own fire test evidence, the door would typically be assessed as non-compliant and require remedial action. Options include:
- Adjust the door – rehang on different hinges, plane edges
- Fit larger seals – 15mm or 20mm intumescent strips
- Replace the door – if warped beyond adjustment
- Replace the frame – if out of square beyond repair
Smoke Door Considerations
Smoke doors (FD30S, FD60S) require effective smoke sealing because:
- Smoke is a primary cause of fire-related fatalities
- Cold smoke travels along floors, making thresholds critical
- Smoke seals at head and jambs prevent smoke spread through these gaps
When inspecting smoke doors:
- Check for brush or combined intumescent/smoke seals at head and jambs
- Check the threshold arrangement against the door’s supporting evidence: there is no general published figure for the under-door gap, and where a drop-down seal is fitted, its own instructions apply
- Head and jamb gaps should be 2-4mm
- Always verify against the door’s specific certification
Using Gap Gauges
Leaf Gauges
- Plastic or metal leaves of known thickness
- Insert until slight resistance
- The highest gauge that fits = the gap size
Feeler Gauges
- Metal blades in a holder
- More precise than leaf gauges
- Better for smoke door measurements
Digital Tools
Modern inspection apps like IgnisTrack provide:
- Visual gap measurement interfaces
- Typical-practice hints (the 2-4mm fallback) for head, jamb and meeting-stile readings
- A threshold prompt that points you to the door’s evidence and seal, with no general figure
- The judgement stays with you: you record OK or Fault
Recording Gap Measurements
Good documentation includes:
- Date and time of inspection
- Door reference/location
- All gap measurements
- Pass/fail status
- Photos of problem areas
- Recommended remedial action
Digital record-keeping supports Golden Thread requirements for Higher-Risk Buildings and represents best practice for fire door management across all building types.
Conclusion
Gap measurements are fundamental to fire door compliance. The working figure of around 2-4mm for head, jamb and meeting-stile gaps comes from industry best practice, the threshold has no general published figure and is checked against the manufacturer’s instructions and the door’s evidence, and measuring correctly at multiple points is what makes an inspection defensible. Remember that BS 8214:2026 itself sets no numeric figure: where a door has a manufacturer datasheet or Field of Application, that evidence takes precedence over any general number.
Regular inspection and prompt remediation of excessive gaps keeps fire doors performing as designed, protecting building occupants and meeting legal requirements.
Frequently Asked Questions
Can I download BS 8214:2026 for free?
The full BS 8214:2026 standard is copyrighted and sold by BSI, so there is no free official PDF. You can buy it from the BSI Knowledge website. This guide is a free plain-English summary of the gap measurement requirements so you can apply them on site without needing the full document open.
What is the permitted gap between a fire door leaf and frame?
Industry best practice is 2-4mm at the head and both jambs (sides), and that is the figure most UK surveyors work to. It comes from the ASDMA Best Practice Guide and BS 4787-1, not from BS 8214:2026, which sets no numeric gap tolerance and instead defers to the door’s supporting evidence. There is no general published figure for the threshold (under-door) gap: check the manufacturer’s installation instructions and the door’s supporting evidence. Use 2-4mm as your working figure at head and jambs, but if the door has a datasheet or Field of Application, that document takes precedence.
Does BS 8214:2026 state a numeric gap tolerance?
No. BS 8214:2026 contains no numeric leaf-gap tolerance anywhere in the document. Clause 13.5.2 says the gap between the leaf edges and the frame should be in accordance with the supporting evidence, and clause 13.5.3 sends the under-door gap to the manufacturer’s installation instructions. The 2026 revision was deliberately made less prescriptive, focusing on what supporting evidence is needed rather than fixed figures. In practice you still need a number on site, so use the industry best-practice range of 2-4mm at head and jambs as your fallback.
Is the 3mm under-door gap still required for smoke doors?
No current standard sets it. The 3mm figure traces to BS 8214:2016 (clause 12.3), which has been superseded, and BS 8214:2026 gives no under-door figure: clause 13.5.3 defers to the manufacturer’s installation instructions and the door’s supporting evidence. On a smoke door, check the door’s evidence for the threshold gap and how the threshold is sealed; where a drop-down seal is fitted, its own instructions apply. A change in guidance does not by itself mean an existing door has to be upgraded.
How do you measure fire door gaps correctly?
Use a dedicated fire door gap gauge or feeler gauges, and measure at multiple points: three along the head, three on each jamb, and one at the threshold. Measuring at multiple points catches warping, frame distortion, and uneven gaps that a single reading would miss.
What happens if a fire door gap is too large?
A gap above tolerance lets fire and smoke pass and may stop the intumescent seal bridging the gap when it expands. The fix is to adjust the hinges, add packing, or refix the frame. If the door or frame is warped beyond adjustment, it should be replaced. Fitting oversized seals is not a compliant fix for an excessive gap.
Related Guides
- 11 Most Common Fire Door Defects: what inspections actually find.
- Fire Door Compliance UK: Complete Guide: the full regulatory picture.
- Fire Door Inspection Schedule (BS 8214): how often to inspect, and why.
This guide is provided for general information purposes only. The figures given are industry best practice drawn from the ASDMA Best Practice Guide to Timber Fire Doors and BS 4787-1; BS 8214:2026 sets no numeric leaf-gap tolerance and defers to each door’s supporting evidence. Gap tolerances should always be verified against the specific door manufacturer’s fire test evidence. Door assemblies tested to different standards may have different requirements, and the door’s supporting evidence takes precedence.
IgnisTrack shows the best-practice range alongside each gap measurement as a visual hint, clearly labelled as a fallback figure. It never decides for you: the inspector records the judgement, and the door’s own supporting evidence always takes precedence. Start your free trial to see how.
