BS 8214 Gap Measurements Explained: A Practical Guide

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 explains BS 8214:2016 gap requirements in practical terms.

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

BS 8214:2016 Gap Tolerances

Standard Fire Doors (FD30, FD60, FD90, FD120)

Location	Gap Tolerance
Head (top)	2-4mm
Jambs (sides)	2-4mm
Threshold (bottom)	8-10mm*
Meeting stile (double doors)	2-4mm

Why 2-4mm? The minimum 2mm gap is required for the door to operate freely without binding. The maximum 4mm ensures fire and smoke integrity. Gaps outside this range indicate a problem.

*The exact threshold tolerance depends on the specific door’s fire test evidence. Always verify against the manufacturer’s data.

Smoke Doors (FD30S, FD60S)

Location	Gap Tolerance
Head (top)	2-4mm
Jambs (sides)	2-4mm
Threshold (bottom)	3mm or 8-10mm*
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 gap options for smoke doors:

• 3mm maximum – when no threshold seal is fitted, the small gap itself limits cold smoke passage
• 8-10mm maximum – when a drop-down (automatic) smoke seal is fitted to the bottom of the door. The seal drops when the door closes, reducing the gap to effectively 0mm

There are no brush or blade seals fitted at the threshold – smoke control at the threshold is achieved either through a restricted gap or a drop-down seal mechanism.

Important: In most cases, threshold gaps should not exceed 8-10mm unless the specific fire test evidence for the door assembly permits a larger gap.

Note: Gap tolerances should always be verified against the specific door manufacturer’s fire test evidence, as requirements may vary.

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

1. Close the door fully – ensure it’s latched
2. Measure at multiple points – minimum 3 per edge
3. Use the correct gauge – plastic leaf gauges or metal feeler gauges
4. Record the largest gap – this is the critical measurement
5. 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: Fit threshold seal, add bottom seal, replace 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 BS 8214 recommended tolerances and are not supported by the door’s fire test evidence, the door would typically be assessed as non-compliant and require remedial action. Options include:

1. Adjust the door – rehang on different hinges, plane edges
2. Fit larger seals – 15mm or 20mm intumescent strips
3. Replace the door – if warped beyond adjustment
4. 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:
  • No threshold seal fitted: Maximum 3mm gap (the small gap limits smoke)
  • Drop-down seal fitted: Maximum 8-10mm gap (seal closes to 0mm when door shuts)
• Head and jamb gaps should be 2-4mm
• In most cases, threshold gaps should not exceed 8-10mm unless the fire test evidence specifically permits it
• 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
• Automatic pass/fail based on door type
• Traffic light indicators for each measurement
• Smoke door detection for stricter thresholds

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. Understanding BS 8214 tolerances – 2-4mm for head and jamb gaps, 8-10mm for standard door thresholds, and 3mm for smoke door thresholds (or 8-10mm with drop-down seals) – and measuring correctly at multiple points ensures accurate inspections.

Regular inspection and prompt remediation of excessive gaps keeps fire doors performing as designed, protecting building occupants and meeting legal requirements.

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This guide is provided for general information purposes only. Gap tolerances should always be verified against the specific door manufacturer’s fire test evidence and the current edition of BS 8214. Door assemblies tested to different standards may have different requirements.

IgnisTrack automatically compares recorded gap measurements against BS 8214 recommended tolerances and provides an initial compliant or non-compliant indication. Final assessment remains with the inspector, with full override and justification where professional judgement or fire test evidence applies. Start your free trial to see how.