News — 21 Oct 2026
Fire Door Gap Tolerances: Inspection Guide 2026
Gap tolerance failures are the number one finding in fire door inspections across UK commercial and residential premises. A door that looks closed — and passes a visual check — may have gaps 3× the permitted tolerance. Here is how to measure correctly and what to do when a door fails.
Key fact
A fire door with a 6 mm gap at the stile allows smoke to pass through continuously from the moment a fire starts — before the intumescent strip has activated (intumescent strips activate at 150–200°C, which takes several minutes). The gap tolerance is therefore a smoke seal standard as much as a structural fire standard.
Maximum permitted gap tolerances — fire doors (UK)
| Location | Max permitted gap | Notes |
|---|---|---|
| Top of door (head) | 3 mm maximum | Larger gaps allow smoke passage even with intumescent strip fitted |
| Sides of door (stiles) | 3 mm maximum | Gaps at stile junctions with the frame are the most common failure point |
| Bottom of door (threshold) — no floor seal | 8 mm maximum | Without a threshold seal, a 8 mm gap provides minimal smoke protection at low level |
| Bottom of door — with threshold seal fitted | 3 mm to seal contact | A drop seal or automatic threshold seal reduces this gap effectively to zero when door is closed |
| Between door and doorstop | 2 mm maximum at intumescent strip | The intumescent strip must be in full contact with doorstop when closed |
Why gaps increase over time
Fire doors are subject to the same physical stresses as any door: building movement, humidity cycles, repeated use, and minor impacts. A timber fire door that was installed within tolerance will naturally drift as the building settles and the door and frame experience seasonal moisture changes. In high-use commercial premises, hinge wear alone can cause a door to sag and create over-tolerance gaps at the top corner within two to three years of installation. This is why regular inspection — not just installation quality — is critical.
2026 inspection requirements
Under the Fire Safety Act 2022, responsible persons for multi-occupied residential buildings must inspect flat entrance doors (which must be fire doors) as part of their fire risk assessment programme. The fire door inspection requirements for commercial premises are set by the fire risk assessment itself — but the National Fire Chiefs Council guidance recommends half-yearly inspections in high-use areas.
The six-point fire door inspection checklist — covering certification, gaps, seals, closers, hinges, and glazing — is set out in our fire door safety guide. For the compartmentation context, see our fire compartmentation entry and our fire door inspection update article.
Frequently asked questions
FAQ