Fire door gap requirements: why are they important?

Fire door gaps are one of the most commonly found non-compliances on fire door inspections. Why? Well, the difference between a compliant and non-compliant fire door may be only 1 or 2 millimetres, but those small differences can have a significant effect on the ability of the door to contain fire and smoke during a fire situation.

Understanding the role fire door gaps have in the process of how a fire door works is essential for anyone responsible for building safety, whether that is a facilities manager, landlord, housing provider, contractor or responsible person under fire safety legislation.

Why do fire door gaps matter?

The primary purpose of a fire door is to slow down the spread of fire and smoke, protecting escape routes and giving occupants valuable time to evacuate safely.

A fire door is a tested assembly or set. The door leaf, frame, seals, hinges and hardware are all designed to work together to achieve a specific fire resistance rating, typically FD30 or FD60. If the gaps around the door leaf fall outside the tested tolerances, the fire performance of the entire assembly/ set can be compromised.

Excessive gaps can allow smoke and heat to pass through before intumescent seals have expanded. This can also cause seals to fail early in a fire situation. Gaps that are too small can prevent a door from closing properly, leaving it unable to perform its intended function in the event of a fire. This can also cause seals not to expand correctly in a fire situation.

For this reason, gap sizes are one of the most critical aspects of fire door installation, maintenance and inspection.

How do we know the correct gap sizes for each fire door?

There is no single piece of UK law that specifies exact gap dimensions. Instead, requirements are set by the manufacturer test evidence. BS 8214 2016 does recommend 2 to 4mm gaps between the fire door leaf and fire door frame. However, the 2026 version of BS 8214 does not mention this.

The Regulatory Reform (Fire Safety) Order 2005 places a duty on the responsible person to ensure fire doors are maintained in efficient working order and good repair.

The Fire Safety Act 2021 clarified that flat entrance doors in residential buildings fall within the scope of the Fire Safety Order, while the Fire Safety (England) Regulations 2022 introduced mandatory inspection requirements for certain blocks of flats.

Where a fire door manufacturer provides specific installation instructions or certification, those requirements always take precedence over the requirements set out in any version of BS 8214.

Guidance on fire door gap sizes

For many fire door installations, gaps may be set as follows depending on the manufacturer’s requirements:

*The 2026 version of BS 8214 advises that the threshold gap for a smoke-controlled door (i.e. FD30 s) should be “sealed with a suitable bottom of door sealing system”.

Any gap that consistently exceeds 4mm around the head or vertical edges is likely to be a non-compliance during a professional inspection. Check the manufacturer’s evidence of fire performance for the specific door to be certain.

How fire door gaps are measured

Accurate measurement is essential. Visual checks alone are not sufficient.

Using a calibrated fire door gap gauge is the best method to measure clearances at multiple points around the door. Measurements should always be taken with the door fully closed.

Inspectors will typically check:

• The gap at the head
• The gap on the hinge side
• The gap on the closing or lock side
• The threshold gap at the bottom of the door
• The condition and continuity of intumescent and smoke seals

Consistency is important. A door may appear compliant overall but will still be non-compliant if a section of the gap exceeds permitted tolerances.

Common causes of excessive gaps

There are various reasons for non-compliant gaps on fire doors. Many fire doors gaps can become excessive over time due to wear, building movement or changes in moisture content of the timber used for the fire door. Poor installation is also a major reason for non-compliant gaps.

Common issues include:

• Incorrect installation
• Building settlement affecting the door frame
• Damage to the frame or door leaf
• Flooring changes that alter the threshold gap
• Poor maintenance of fire door hardware

Regular inspections are therefore essential to identify problems before they affect fire safety.

Flat entrance doors and inspection requirements

Following recommendations arising from the Grenfell Tower Inquiry, inspection/ check requirements for residential buildings have increased significantly.

In England, responsible persons for residential buildings over 11 metres in height must carry out quarterly checks (approx. every 3 months) of fire doors in communal areas and use best endeavours to check flat entrance doors annually.

These checks should identify visible defects, including excessive gaps, damaged seals and doors that fail to close properly.

Where concerns are identified, a more detailed fire door inspection is likely to be required.

What happens if gaps are out of tolerance?

A non-compliant gap does not automatically mean a new fire door is required. In many cases, remedial work can restore compliance. Depending on the cause, solutions may include hinge replacement, frame adjustments, door realignment or the installation of suitable smoke control products.

However, simply replacing seals is rarely sufficient if the underlying gap size remains outside the required limits. Any remedial work should take account of the door manufacturer’s test evidence and installation requirements to ensure the fire resistance of the assembly/ set is maintained.

Professional inspection and training

Fire door gaps are only one aspect of compliance. Inspectors must also assess door closers, glazing, hinges, locks, seals, signage and the overall condition of the fire door assembly/ set.

Fire Doors Complete provide professional fire door inspections, and remedial advice for residential, commercial, healthcare and education buildings throughout the UK. Our reports clearly identify non compliances, explain the level of risk and outline practical recommendations for achieving compliance.

We also deliver fire door training courses for facilities managers, maintenance teams, contractors and anyone responsible for carrying out routine inspections. Training covers fire door gaps, inspection techniques, documentation requirements and current legal responsibilities.

Understanding the correct gap tolerances is a simple but important part of fire door safety. With regular inspections, proper maintenance and competent assessment, building owners can help ensure their fire doors perform as intended when they are needed most.

• For more information about Fire Doors Complete, contact us here.