Fire door gap requirements are a critical element of passive fire protection in buildings. Across UK commercial and residential premises, incorrect fire door gaps are one of the most common causes of inspection failures. The spaces between the fire door leaf and its frame directly affect how a door performs during fire and smoke conditions — restricting smoke movement, enabling intumescent seals to activate correctly, and supporting compliance with UK fire safety standards.
This article explains what fire door gap tolerances are typically required, why they matter, how they are measured and what happens when gaps are outside acceptable limits.
Contents
Why Fire Door Gaps Are Critical
Fire doors are tested as complete assemblies. Excessive gaps around or beneath a door can allow smoke and heat to bypass the door, significantly undermining its fire containment performance. Even small variations beyond specified tolerances can affect the speed and quantity of smoke spread during a fire.
Correct gaps also ensure that fire and smoke seals — including intumescent strips — can expand and create an effective barrier as intended under test conditions.
In higher-risk residential buildings in England, additional fire door inspection duties are outlined within the Fire Safety (England) Regulations 2022.
UK Fire Door Gap Tolerances (Standard Practice)
Gap tolerances should always be checked against the individual fire door manufacturer’s certification. However, the following measurements represent typical UK best practice for standard FD30 and FD60 doors.
1. Gaps Around the Top and Sides
In most UK buildings:
- The gap between the fire door leaf and frame at the head and sides should ideally be 2–4 mm.
- Gaps larger than this are generally considered non-compliant with best practice and may compromise performance.
- A consistent gap across all sides is important to allow even sealing during a fire.
A gap of approximately 3 mm is often regarded as the practical target for new installations.
2. Gap Beneath the Fire Door (Undercut)
The allowable gap beneath a fire door varies depending on the fire door’s intended performance:
- For fire doors that do not require additional smoke control, a common practice is a gap of up to 8 mm at the bottom.
- Some guidance accepts up to 10 mm, though checking the door manufacturer’s certificate is essential.
- Where a door is also required to limit cold smoke spread, the under-door gap should be smaller, often 3 mm or less, or achieved using a compliant drop seal.
3. Double Door and Meeting Stile Gaps
For pairs of fire doors (double doors):
- The gap between the meeting edges of the door leaves should generally be similar to the side gaps (around 3–4 mm) unless a specific tested solution dictates otherwise.
- Continuous intumescent and smoke sealing is important at the meeting stiles to prevent bypass.
Excessive meeting stile gaps are a frequent cause of non-compliance in communal residential blocks and commercial corridors.
Why There Are Gap Limits
Fire door gap tolerances are set to balance two needs:
- Operational clearance — ensuring the door opens and closes freely without binding or scraping.
- Fire and smoke performance — allowing intumescent and smoke seals to expand and maintain a continuous barrier under fire conditions.
Excessive gaps can enable smoke migration, while gaps that are too tight can prevent the door from closing correctly or damage seals.
How Gaps Are Checked
Fire door gap measurements should be taken with the door fully closed into the stop:
- Use a fire door gap gauge or precision measuring device rather than improvised tools.
- Check multiple points at the top, sides and bottom to ensure consistent spacing.
- Record measurements as part of inspection and maintenance documentation.
Installation and maintenance guidance for timber-based fire door assemblies is set out within BS 8214, which provides best practice for fitting and gap tolerances.
If gaps exceed acceptable limits, qualified fire door inspectors can recommend remedial actions — from adjusting the hanging and hardware to fitting compliant seals or drop seals.
These measurements are typically reviewed as part of structured fire door inspections and surveys carried out to assess ongoing compliance.
What Happens If Gaps Are Wrong
Incorrect gaps may result in:
- Reduced ability of the door to prevent smoke spread.
- Fire safety audit failures.
- Increased risk to building occupants in the event of fire.
In higher-risk residential buildings, failure to address excessive fire door gaps may conflict with duties placed on the Responsible Person under UK fire safety legislation.
It’s important to check gap requirements against the specific fire door manufacturer’s certification and installation instructions, as these may vary from general guidance.
Gap Tolerances Summary Table (Typical UK Practice)
| Location | Typical Tolerance |
|---|---|
| Sides & Top | 2–4 mm (≈3 mm ideal) |
| Undercut (fire only) | Up to 8 mm (sometimes up to 10 mm) |
| Undercut (smoke control) | 3 mm or less with compliant seals |
| Double door meeting gap | ~3–4 mm |
Practical Tips for Maintaining Correct Gaps
- During installation, verify frame squareness and confirm door leaf dimensions before final fitting.
- After installation, verify fire door gaps before adding seals.
- If floors are re-levelled or coverings changed, recheck bottom gaps.
- Document gap measurements as part of ongoing fire door maintenance.
Regular monitoring helps ensure fire doors continue to meet performance expectations throughout their service life.
Conclusion
Fire door gap requirements are a small detail with big safety implications. Keeping gaps within the recommended tolerances ensures that fire doors can restrict smoke movement and perform as tested — critical to maintaining passive fire protection in UK buildings.
While typical industry practice provides a useful benchmark, always check each fire door against its manufacturer’s certification and installation documentation for precise tolerances.
Understanding and managing fire door gap requirements is an important part of overall fire door compliance, particularly as excessive gaps are one of the most common defects identified during inspections.
Frequently Asked Questions
What is the correct gap around a fire door?
In most UK installations, the gap between the fire door leaf and the frame at the top and sides should typically be between 2mm and 4mm, with approximately 3mm considered ideal for consistent performance. Gaps should be uniform to allow intumescent and smoke seals to function correctly.
What is the maximum gap allowed under a fire door?
For fire-only doors (without additional smoke control), the gap beneath the door is commonly up to 8mm, although some manufacturers may permit up to 10mm. Where smoke control is required (e.g., FD30S doors), the gap is usually smaller and may require a compliant drop seal.
Always confirm the permitted undercut by checking the fire door manufacturer’s certification and test data.
Why are fire door gap tolerances important?
Correct gap tolerances ensure that intumescent seals expand properly during a fire and that smoke spread is restricted. Excessive gaps can allow smoke and heat to bypass the door, reducing its fire resistance performance.
How are fire door gaps measured?
Fire door gaps should be measured with the door fully closed using a calibrated gap gauge or precision measuring tool. Measurements should be taken at multiple points around the head, sides and bottom to confirm consistent spacing.
What happens if fire door gaps are too large?
If gaps exceed acceptable tolerances, the door may fail to prevent smoke and fire spread effectively. This can result in non-compliance during fire safety inspections and may require remedial adjustment or replacement.
Can fire door gaps be corrected?
In many cases, excessive gaps can be corrected by adjusting hinges, repositioning the door leaf, installing compliant seals, or fitting drop seals. However, major installation errors may require replacement of the door or frame.
