Fire Door Gap Measurement Guide
Visual reference guide for measuring fire door gaps correctly. Includes the industry best-practice tolerances, a measurement points diagram, and pass/fail criteria.
Why Gap Measurements Matter
Fire doors are tested as complete assemblies with specific gap tolerances. Getting the gaps right is critical:
Gaps Too Large
Allow fire and smoke to bypass the door, compromising compartmentation and putting lives at risk.
Gaps Too Small
Cause the door to bind on the frame, preventing proper closure and latch engagement.
Work to 2-4mm at the head and jambs. That is the industry best-practice figure most UK surveyors use, and it comes from the ASDMA Best Practice Guide and BS 4787-1 — not from BS 8214:2026, which deliberately sets no numeric gap tolerance and instead defers to each door's supporting evidence. Where a door has a manufacturer datasheet or Field of Application, that document always takes precedence. This guide shows you where and how to measure.
Fire Door Gap Tolerances: The Working Figures
| Location | Standard Fire Door | Smoke Door (S-rated) |
|---|---|---|
| Head (top) | 2-4mm | 2-4mm |
| Hinge jamb (side) | 2-4mm | 2-4mm |
| Lock jamb (side) | 2-4mm | 2-4mm |
| Meeting stiles (double) | 2-4mm | 2-4mm |
| Threshold (bottom) | 8-10mm* | 8-10mm with drop seal 3mm max without |
* Threshold gap tolerance depends on the door's fire test evidence. Always check manufacturer documentation for the specific door.
10 Measurement Points
For a thorough inspection, measure at all 10 points shown below. This ensures you catch any warping, frame distortion, or uneven gaps.
The 2-4mm figures shown are industry best-practice values (ASDMA Best Practice Guide, BS 4787-1), not figures stated in BS 8214:2026. Always verify the actual tolerance against the specific door's manufacturer datasheet or Field of Application (FoA), which takes precedence.
Head Gap
3 points along top edge (2-4mm)
Hinge Jamb
3 points on hinge side (2-4mm)
Lock Jamb
3 points on lock side (2-4mm)
Threshold
Bottom of door (8-10mm per test evidence)
See full diagram in PDF
Pass/Fail Criteria
These are industry best-practice figures, not values stated in BS 8214:2026. Always verify the actual tolerance against the specific door's manufacturer datasheet or Field of Application (FoA), which takes precedence.
Pass
- Head/jamb gaps: 2-4mm at all measurement points
- Threshold gap: Within door's test evidence (typically 8-10mm)
- Smoke doors without drop seal: Max 3mm threshold
Marginal - Investigate
- Head/jamb gaps: 1-2mm (too tight) or 4-5mm (borderline)
- Threshold approaching limits - verify against test evidence
- May pass if supported by specific documentation
Fail - Action Required
- Head/jamb gaps: Less than 1mm or greater than 5mm
- Threshold gap exceeds door's tested limits
- Light visible through intumescent seal
- Door binds on frame
Recommended Tools
Gap Gauge Set
Dedicated fire door gauges with 2mm, 3mm, and 4mm blades. Quick and accurate.
RecommendedFeeler Gauges
Automotive feeler gauges provide precise measurements. Combine blades as needed.
Good AlternativeSteel Rule
Less accurate for narrow tolerances but useful for threshold gaps.
Basic OptionFire Door Gap FAQs
What is the maximum gap allowed around a fire door?
Head and jamb (side) gaps should be 2-4mm - the industry best-practice figure, drawn from the ASDMA Best Practice Guide and BS 4787-1 rather than from BS 8214:2026, which sets no numeric tolerance. The threshold (bottom) gap depends on the door's fire test evidence and is typically 8-10mm for a standard fire door, or a maximum of 3mm for a smoke door without a threshold seal. Always verify against the specific door's manufacturer documentation, which takes precedence.
What does BS 8214:2026 say about fire door gaps?
Less than most people expect. BS 8214:2026 sets 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 door's 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. In practice you still need a number on site, so use the industry best-practice range of 2-4mm at head and jambs (ASDMA Best Practice Guide, BS 4787-1) as your fallback, and treat the door's own datasheet as the authority where one exists.
How do you measure a fire door gap correctly?
Use a dedicated fire door gap gauge or feeler gauges. Measure at ten points: three along the head, three on the hinge jamb, three on the lock jamb, and one at the threshold. Measuring at multiple points catches warping and uneven gaps that a single measurement would miss.
What is the gap tolerance between double fire door leaves?
The meeting stiles gap between the two leaves of a double fire door should be 2-4mm, the same tolerance as head and jamb gaps, so that the intumescent seals on the meeting edges can close the gap under heat.
When is a fire door gap too large to repair?
If head or jamb gaps consistently exceed 4-5mm and cannot be brought back into tolerance by adjusting the hinges, packing, or refixing the frame, the door or frame is likely warped or distorted beyond adjustment and should be replaced. Fitting oversized intumescent seals is not a compliant fix for excessive gaps.
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Start 14-day free trialReferences: ASDMA Best Practice Guide to Timber Fire Doors (2015) §13.11 | BS 4787-1:1980 Table 1 | BS 8214:2026 - Code of practice for fire door assemblies (defers gap tolerances to supporting evidence) | Approved Document B - Fire Safety (Volume 2)
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