Fire Door Gap Compliance

A Guide to Fire Door Gap Compliance

Understanding fire door gaps requires more than simply knowing the regulations. Inspectors, contractors and Responsible Persons also need to know how to test gaps correctly, why excessive gaps occur, and how they can be remediated in practice.

This guide forms part of Norseal’s fire door gap compliance series:

• How to Test Fire Door Gaps: a practical guide to measuring fire door gaps during inspection.
• Why Fire Door Threshold Gaps Fail: the most common causes of excessive bottom gaps.
• Fire Door Gap Remediation Solutions: practical ways to restore compliant performance without replacing the entire doorset.

Together these guides explain how to identify, diagnose and remediate fire door gap issues.

Excessive gaps in fire doors are among the most frequent causes of non-compliance identified during inspections and audits. While the fire door leaf itself, the door hardware and the frame may be fully certificated and correctly specified, excessive clearances around the perimeter of the door, and particularly at the threshold, can significantly compromise performance in the event of a fire.

In practice, issues relating to gaps around fire doors, inadequate door gap testing, and poorly controlled threshold gaps on fire doors continue to occur across social housing, healthcare, education, and commercial property portfolios. In many cases, these are not failures of intent but failures of maintenance, documentation, or post-installation alteration.

What Are the Regulations on Fire Door Gaps?

Fire doors are tested as complete assemblies: door blank, frame and hardware and performance is validated through fire resistance testing, typically to BS EN 1634, where the door leaf, frame, seals and hardware function together as a system.

What Gap Tolerances are Allowed?

A fire door needs a gap around it so it can work properly, and at the threshold gap so it can swing freely. Although the door’s individual certification determines exact limits, general guidance has, until recently, stated that the gap at the sides and head should be 3mm and up to 8mm at the threshold. 

Fire Door Inspection Regulations

The Fire Safety (England) Regulations 2022 outline how passive and active fire protection elements of buildings should be designed, installed, and maintained. It places a legal responsibility on building owners and operators  - the Responsible Person - to conduct risk assessments on the buildings they look after to prevent fires and reduce fire risk. In law, that Responsible Person needs to provide information to Fire and Rescue Services to help them plan for the event of fire and, if needed, to provide an effective operational response. This includes a requirement to:

• undertake quarterly checks of all fire doors (including self-closing devices) in the common parts
• undertake - on a best endeavour basis - annual checks of all flat entrance doors (including self-closing devices) that lead onto a building’s common parts

While the RRO only refers to buildings over 11m, the Responsible Persons for residential buildings below that height still have a duty to install and maintain general fire precautions, including making sure that all fire doors are working compliantly. 

How to Test Fire Door Gaps

Effective door gap testing should be systematic, repeatable and documented. A visual inspection alone is not enough, and proper measurement is a must.

1. Preliminary Checks

Because a door’s poor closing performance can create variable gaps around the door, some preliminary checks are needed: 

    Does the door close fully and engage the latch reliably?
    Are the hinges secure and free from excessive wear?
    Are the intumescents and smoke seals still present and undamaged
    Is there evidence of distortion or warping in the door or frame?

2. Measuring Perimeter Gaps

Use a calibrated gap gauge or feeler gauges to measure at multiple points along the hinge edge, the lock edge, and across the head.  Gaps should be consistent. Localised excessive gaps may indicate frame distortion or hinge issues. Record all measurements clearly.

3. Measuring the Threshold Gap on a Fire Door

Measuring the bottom gap requires particular care. Measure at several points along the width of the door. Do not assume uniformity. Where floor finishes have changed, for example, carpet removed and replaced with hard flooring, bottom gaps can exceed permitted tolerances. In smoke control doors, excessive gaps at threshold level can allow smoke to seep into fire compartments and escape routes.

4. Documentation

Making and keeping accurate records is an important part of compliance with current building safety legislation and contributes to the Golden Thread. These records need to include:

• Measured gap sizes
• Location of the door
• Door identification number
• Any remediation action required

This information forms part of the building’s fire safety record and contributes to the golden thread.

Why Threshold Gaps Fail Most Often

The threshold of the fire door is the most vulnerable area because of:

• Post-install changes: floor finishes have been changed after the door was installed. Even minor adjustments to the floor height can increase bottom gaps. 
• Door trimming: doors may have been trimmed or planed during installation or later maintenance.
• Building movement: any settlement or structural movement can change the geometry of the frame geometry
  
• Wear and impact: high-traffic levels can lead to hinge wear and door sag.
  

   

Unlike perimeter gaps, bottom gaps are sometimes missed because they are less visible during quick inspections, but in fire conditions, hot gases and smoke will exploit that opening.

How to Fix a Threshold Gap on a Fire Door

Where a threshold gap on a fire door exceeds certification limits, prompt remediation is vital. But full door replacement may not be necessary if the underlying door assembly remains sound. Tested fire door drop seals provide a compliant remediation option when selected and installed correctly.

What Is a Fire Door Drop Seal?

A drop seal is a mechanically activated sealing device installed into the bottom edge of a fire door. It retracts when the door opens, then drops automatically when the door closes. It makes contact with the floor, and this seal prevents smoke and fire from passing through the bottom gap.

When Should You Fit a Drop Seal?

Drop seals [link to Norseal drop seals] should be used where the threshold gap exceeds 8mm or where cold smoke control performance is required. They can be retrofitted without replacing the whole door and are particularly useful for remediation programmes on housing and public sector estates, saving money and reducing disruption for residents.

When selecting a fire door drop seal, it’s vital to:

• Confirm that the fire test evidence is correct for the door type [link to Norseal download section] so that the seal does not compromise the door's certified configuration
• Check if smoke performance is needed and that the seal supports this if it is required
• Ensure the seal is the right size for the door width and thickness
• Check the installation instructions

At Norseal, we have the UK’s only drop seal cutting machine, so you can be sure that the drop seal being fitted is exactly the right length for the door. [link to drop seal cutting].

Like-for-Like Replacement of Drop Seals

Substituting a drop seal without the correct test evidence could risk invalidating the door’s compliance and performance claims. So where seals are damaged or missing, the replacement should be like-for-like. There are many types of intumescent seals [link to Norseal blog about intumescents], so it is important to match any intumescents to the original spec, replicate any brush or blade smoke elements if they are needed, and maintain all the correct, tested configurations. 

The Golden Thread

The Building Safety Act emphasises the importance of providing accurate and accessible safety information throughout a building’s lifecycle. This ‘Golden Thread’ requires a building’s Responsible Person to keep accurate records of the design intent, installation, and ongoing maintenance of the building’s fire doors. A fire door inspection – including the threshold gap - and any remediation forms an important part of those records.

Where excessive threshold gaps have been identified, these should be recorded, assessed, remediated and verified. This process demonstrates both compliance and reasonable decision-making through the life of the building and the door. Threshold remediation is not just a maintenance adjustment; it is an intervention that will help preserve the whole building’s compartmentation strategy.

Practical Guidance for Facilities Teams and Contractors

To manage fire door gaps effectively, reduce long-term risk, and avoid larger capital expenditure.

• Carry out a planned and scheduled inspection regime
• Use the correct, calibrated measurement tools
• Compare any findings against certification data
• Address the excessive threshold gaps promptly
• Use tested remediation products [link Norseal to drop seal page]
• Maintain clear documentation

Proactive threshold gap management

Fire door gaps are small details that can have significant consequences. Excessive perimeter or threshold gaps can compromise the integrity of the fire door assembly and can allow smoke to spread.

Understanding the regulatory framework, conducting consistent door-gap testing, and selecting appropriate fire door drop seals for remediation will ensure continued compliance without unnecessary disruption. 

Where threshold gaps are identified on site, prompt and evidence-based remediation protects both occupants and Responsible Persons.

Next Steps

If you identify excessive threshold gaps on site, Norseal has a wide range of remediation solutions to help keep fire doors safe and compliant. Norseal’s technical team can help select the appropriate drop seal based on the door type and smoke requirements.

FAQs

What is the maximum threshold gap allowed on a fire door?

A gap of up to 8mm is allowed at the threshold, but always refer to the specific door’s certification data.

Why are excessive threshold gaps on fire doors important?

In the event of a fire, large gaps at the bottom of a fire door can allow smoke to spread quickly at floor level, undermining the fire door’s performance.

Q: How often should fire door gaps be checked?

Inspection frequency depends on building type and usage, but regular documented checks are required under fire safety legislation.

Can fire doors with excessive bottom gaps be compliant?

If the threshold gap exceeds the certification limits, remediation is always required. Tested drop seals may restore compliant performance.

Does installing a drop seal affect certification?

Only if inappropriate products are used. Tested and compatible fire door drop seals installed in accordance with instructions support compliant remediation.

Are smoke seals required on all fire doors?

Not all fire doors require smoke control, but many corridor and escape route doors do. The building fire strategy determines these requirements.