Fire Door Gap Regulations: What Size Should Fire Door Gaps Be?

Fire door gap regulations are an important part of ensuring a fire door can perform as intended in the event of a fire. As general guidance, gaps between the door leaf and frame are commonly expected to be between 2mm and 4mm, although the exact permitted gap depends on the fire door manufacturer’s test evidence and installation instructions.

Correct fire door gaps allow the door to close fully while helping intumescent strips and smoke seals perform effectively. Excessive gaps can compromise fire and smoke protection, while gaps that are too small can prevent the door from closing correctly.

For building owners and responsible persons, checking the condition and gaps around fire doors should therefore form part of wider fire door compliance procedures.

At Adaston, one of the most common questions we’re asked when inspecting fire doors is: “What should the gaps be around a fire door?”

The answer most people expect is 2mm to 4mm. That’s a useful starting point, but it isn’t the whole answer.

We don’t assess a fire door based on one measurement alone. A fire door is an assembly of components that have to work together. The leaf, frame, hinges, seals, closer, glazing, threshold and gaps can all influence how that door performs.

That’s why simply putting a gap gauge around a door and calling it compliant isn’t enough.

What Are the Fire Door Gap Regulations?

Fire door gap regulations in the UK are supported by several pieces of fire safety legislation, standards and manufacturer requirements. These include the Regulatory Reform (Fire Safety) Order 2005 in England and Wales, the Fire Safety (England) Regulations 2022 for certain residential buildings in England and guidance such as BS 8214.

BS 8214 provides guidance on the specification, installation and maintenance of timber-based fire door assemblies. However, a general gap size should never be considered in isolation.

The certification and installation instructions supplied for the particular fire door should always be checked. Different fire resisting doors and fire door assemblies can have different requirements depending on their design, components and fire test evidence.

As a general guide, gaps at the sides and head of a fire door are commonly expected to measure between 2mm and 4mm. A gap of around 3mm is therefore frequently used as a practical target when installing or checking a fire door.

The gaps should also be consistent around the door. A 3mm gap at one point and a significantly larger gap elsewhere could indicate problems with the door leaf, door frame, hinges or installation.

This is an important distinction. Fire doors aren’t all manufactured and tested in exactly the same way, so a generic measurement should never override the evidence for the actual doorset being inspected.

Why Are Correct Fire Door Gaps Important?

A fire door is designed to form part of a building’s passive fire protection system. When closed, it can help restrict the spread of fire and smoke between different areas of a building for a specified period.

Many fire doors provide at least 30 minutes of fire resistance, although higher-rated doors are available where the building design or fire strategy requires them.

The gap around the door is a critical part of this performance.

Intumescent strips are installed around many fire doors and expand when exposed to high temperatures. This helps seal the gap between the door and frame as a fire develops. Smoke seals may also be incorporated to help restrict the movement of cold smoke before temperatures become high enough to activate the intumescent material.

If fire door gaps are excessive, the fire and smoke barrier may not perform in the way the tested door assembly was designed to perform. Large gaps can provide a route for smoke, heat and flames to pass around the door edge.

This is particularly important because smoke can spread through a building before flames reach an escape route. Maintaining the correct gaps, seals and self closing devices can therefore make a significant contribution to overall fire safety.

But making the gaps as tight as possible isn’t the answer either. If they’re too small, the leaf can bind against the frame and prevent the self-closing device from closing the door properly.

What we’re looking for is the correct and consistent gap for that particular doorset.

A professional fire door survey can identify excessive gaps and other obvious defects that may affect the performance or compliance of existing doors.

What Is the Maximum Gap Allowed Around a Fire Door?

There is no single maximum gap that should automatically be applied to every fire door. The correct size ultimately depends on the fire door manufacturer’s test evidence, certification and installation instructions.

However, a 2mm to 4mm gap around the sides and top of a fire door is widely used as general guidance.

A gap larger than 4mm should not simply be assumed to be acceptable. If the maximum gap specified for the particular fire door assembly has been exceeded, further investigation or remedial work may be required.

The same applies when gaps are too narrow. A gap below the required tolerance could cause the door to bind against the frame, preventing the door from closing fully into its frame.

The aim is therefore not simply to make the door gap as small as possible. The objective is to achieve the correct gaps specified for the tested fire door assembly while ensuring the door operates and closes correctly.

What Should the Gap Be at the Top and Sides?

The head gaps and gaps along each vertical door edge are commonly expected to fall within the 2mm to 4mm range.

These measurements should remain reasonably consistent around the door. Uneven fire door gaps can indicate that the door has dropped, hinges have become worn or loose screws are allowing movement.

This is something we regularly see during surveys. You might have a 3mm gap at the top of the hanging edge but 6mm further down.

That immediately tells us there is more to investigate. Has the leaf dropped? Are the hinges worn? Are the hinge screws loose? Has the frame moved? Has the door been altered previously?

This is why our surveyors don’t just record a single measurement. We look at what’s happening around the complete perimeter of the leaf and, importantly, how the door operates.

An existing door can also change over time due to regular use, damage, alterations or movement within the frame. This is why a fire door that was compliant when originally installed may develop defects later.

A door should close fully into its frame under the action of its self-closing device without needing someone to give it a final push.

Correct fire door installation is important from the outset, but ongoing inspection and maintenance are required to ensure the door continues to perform as intended.

What Is the Maximum Gap at the Bottom of a Fire Door?

The threshold gap at the bottom of the door is treated differently from the gaps between the door leaf and frame.

You may see figures such as 8mm or 10mm quoted as a maximum gap for some fire-only doors. However, these figures should not be treated as a universal rule.

The permitted threshold gap depends on the particular doorset, its test evidence, whether smoke control is required and the type of threshold seal or other components being used.

Where a fire door also needs to restrict smoke, the permitted gap at floor level can be considerably smaller. The manufacturer’s evidence and specification should therefore always take priority over a generic maximum gap.

A threshold arrangement that may be acceptable for a fire-resisting door isn’t necessarily suitable for a door required to control both fire and smoke.

This is especially important where floor coverings have been changed after installation. Removing carpet, replacing flooring or altering thresholds can change the gap size at the bottom of the door and potentially affect its performance.

We regularly find that threshold gaps have changed since the door was originally installed. A door may originally have been installed correctly, only for a change in floor covering to create a much larger gap underneath it.

That’s why the current condition of the door matters just as much as how it was installed in the first place.

How Do Smoke Seals Affect Fire Door Gap Requirements?

Fire and smoke control are related, but they are not exactly the same thing.

An intumescent seal reacts to heat during a fire. A smoke seal is intended to restrict the passage of smoke at an earlier stage, before temperatures have risen sufficiently for the intumescent material to expand.

Because cold smoke can travel quickly through a building, damaged seals or excessive gaps can present a serious fire safety issue.

The required gap will depend on the specification of the fire and smoke door assembly. Some smoke-control doors may require tighter tolerances or specific threshold arrangements to achieve the required performance.

For this reason, simply measuring a gap and deciding that it “looks about right” is not enough. The inspector should consider the door, frame, seals, hinges, closer, certification and other components together.

The door’s ability to resist fire and smoke depends on the complete assembly functioning correctly, rather than any single component in isolation.

How to Measure Fire Door Gaps

Measuring gaps around a fire door is a relatively simple check, but accurate measurement is important. Visual checks alone may not identify whether a gap is within the tolerance specified for the door.

A gap gauge is the most suitable option for measuring fire door gaps. These are designed to provide a quick and precise measurement of the space between the door leaf and frame.

A £1 coin is approximately 3mm thick and is sometimes used as a rough visual comparison because 3mm sits within the commonly referenced 2mm to 4mm range.

As a very rough visual reference, it can help someone spot an obviously excessive gap. But that’s about as far as we’d take it.

A £1 coin isn’t a fire door inspection.

A coin is not an accurate measuring device and should not replace a proper gap gauge when assessing compliance.

When we’re carrying out a fire door inspection, we use appropriate measuring equipment and take measurements at several points around the door.

Checks should include both sides of the door, the head gap and the threshold gap at the bottom of the door.

The door should also be checked throughout its operation to confirm that it can close fully without catching against the frame or floor.

More importantly, those measurements are considered alongside everything else we can establish about the doorset.

A 3mm gap doesn’t automatically make a fire door compliant.

Anyone responsible for fitting a new fire door should follow the manufacturer’s installation instructions and test evidence. Our guide to installing a fire door correctly explains the wider installation process and the checks that should be carried out.

What Happens if Fire Door Gaps Are Too Large?

Excessive gaps can affect a fire door’s ability to act as an effective barrier against the spread of fire and smoke.

Intumescent strips are designed to expand when exposed to heat, helping to seal the space between the door and frame. If the gap is larger than the size for which the fire door assembly was tested, the seals may not be able to perform as intended.

Large gaps may also allow smoke to pass around the door before the intumescent material activates. Smoke inhalation is a major risk during a fire and smoke can quickly affect escape routes and other areas of a building.

The presence of excessive gaps does not automatically mean that the entire door needs to be replaced. The cause and extent of the problem should first be established.

Common causes can include worn hinges, loose screws, movement within the door frame, damage to the door edge or previous alterations to the door or surrounding floor.

Some defects can be addressed through appropriate maintenance or repair. Others can’t.

Any remedial work must maintain the certification and tested performance of the fire door assembly.

Can Fire Door Gaps Be Repaired?

Some incorrect fire door gaps can be addressed through suitable remedial work. Whether repair is possible depends on the condition of the existing door, the size and location of the gap and the requirements of the particular fire door manufacturer.

For example, hinge adjustment or replacement may help correct a door leaf that has dropped and created uneven gaps. Other defects may require more extensive work to the door or frame.

Damaged intumescent strips and damaged seals should also be assessed. Replacing these components may be necessary where they are missing, damaged or no longer suitable for the door.

However, repairs to fire resisting doors should not be improvised simply to reduce the visible gap.

Simply adding material to the edge of a door to make the gap smaller isn’t automatically an acceptable fire door repair. Adding unapproved materials, altering the door leaf or carrying out inappropriate modifications could affect the door’s certification and fire resistance.

The repair itself needs to be appropriate for the door and supported by the relevant evidence.

Professional fire door maintenance can identify defects and determine whether appropriate remedial work can restore the door or whether replacement should be considered.

When Does a Fire Door Need Replacing?

A new fire door may be required where an existing door is significantly damaged, unsuitable for its location or cannot be economically brought back to the required standard.

The decision should consider more than gap sizes alone.

Inspectors may also consider the condition of the door leaf and frame, glazing, hinges, intumescent strips, smoke seals, self closing devices and other essential components of the fire door assembly.

Obvious defects such as major damage, warping or unsuitable alterations may indicate that replacement is a more appropriate solution than repeated repairs.

The door should also close fully from any open position without assistance. A fire door that has to be manually pushed into the frame cannot be relied upon to provide the intended protection during a fire.

Where there is uncertainty over the suitability of an existing door, a competent fire door inspector can assess its condition against the relevant evidence and requirements.

How Often Should Fire Door Gaps Be Checked?

Fire doors are working components of a building.

They get opened, closed, knocked, held open, adjusted, damaged and, occasionally, altered without anyone considering the effect on their fire performance.

Their condition therefore changes over time.

Regular inspections help identify changes to fire door gaps before they develop into more significant compliance issues.

There is no single inspection frequency that applies to every fire door in every type of building. The appropriate frequency depends on factors including the building type, the use of the door, the level of traffic and the applicable fire safety requirements.

Doors in busy communal areas, for example, may experience considerably more wear than doors that are rarely used. Regular use can loosen components, damage seals or cause the door leaf to move within the frame.

BS 9999 recommends that fire doors are inspected at least every six months, with more frequent checks potentially appropriate for doors exposed to high levels of use.

There are also specific legal requirements for certain residential buildings in England under the Fire Safety (England) Regulations 2022.

Our guide explaining how often fire doors should be inspected provides further information on inspection frequencies and the factors responsible persons should consider.

Fire Door Gap Regulations for Flats and Communal Areas

The Fire Safety (England) Regulations 2022 came into force on 23 January 2023 and introduced additional requirements relating to fire doors in certain residential buildings in England.

Regulation 10 applies specific checking requirements in residential buildings containing two or more sets of domestic premises where the top storey is more than 11 metres above ground level.

Responsible persons must use their best endeavours to undertake checks of flat entrance doors at least every 12 months. These doors should include functioning self closing devices.

Fire doors in communal areas of these buildings must be checked at least every three months.

These simple checks are intended to identify obvious problems that could affect a door’s ability to perform correctly. The condition of the door, frame, seals and self closing device should all be considered.

These checks are about more than getting a tape measure out. Fire door gaps are one part of a wider inspection rather than the only measure of whether a door is compliant.

The Regulatory Reform (Fire Safety) Order 2005 also places duties on responsible persons to take appropriate fire precautions and maintain fire safety measures where required.

Building owners and responsible persons should understand which requirements apply to their particular premises rather than relying on annual checks as a universal inspection schedule.

Are Fire Door Gap Sizes Always the Same?

No. Although 2mm to 4mm is widely used as general guidance for gaps at the top and sides of a fire door, it should not override the evidence for the specific doorset.

Different fire door manufacturers may specify different tolerances depending on how their products were tested.

Factors such as the door construction, fire resistance rating, seals, frame, threshold arrangement and whether the door needs to control both fire and smoke can all influence the correct size.

This is why the manufacturer’s installation instructions and fire test certification are important when checking a new or existing door.

The same principle applies to the maximum gap at the bottom of the door. A threshold gap that may be permitted for one fire-only doorset may not be suitable for a smoke-control door.

If there’s one thing we’d like people to take away from this, it’s that there isn’t one magic measurement that makes a fire door compliant.

Simple Checks for Fire Door Gaps

Building owners and responsible persons can look for obvious defects as part of routine fire safety checks.

Look around the door edge to see whether the gaps appear consistent. Check whether the door closes fully into the frame and whether anything prevents the self closing device from operating.

Seals should be present where required and should not be painted over, damaged or missing. Hinges should be secure and the door should not visibly drop or scrape against the floor.

A gap gauge can provide a more accurate indication of gap size, but measurements should always be considered alongside the specification for the particular door.

If there are large gaps, damaged seals, problems with the closer or uncertainty over the condition of the door, further assessment may be required.

During an Adaston fire door survey, the gap is only one part of what we’re looking at. Depending on the door and the scope of the inspection, we’ll also consider the door leaf and frame, hinges and fixings, intumescent and smoke seals, self-closing device, glazing, ironmongery, threshold arrangements, signs of damage or alteration and certification where available.

It’s the combination of these things that gives us a meaningful picture of the condition of the fire door.

We’ve seen doors with good-looking gaps that have significant defects elsewhere, just as we’ve seen doors where an uneven gap has been the first indication of a relatively straightforward maintenance issue.

Keep Fire Door Gaps Compliant

Correct fire door gaps are a critical part of ensuring fire doors can perform as intended. While a gap of 2mm to 4mm at the top and sides is commonly used as guidance, the manufacturer’s test evidence and installation requirements should determine the correct gaps for each particular fire door.

Threshold gaps also need careful consideration, especially where smoke control is required. A maximum gap quoted for one type of door should not automatically be applied to every fire door in a building.

And don’t look at the gap in isolation.

Does the door close properly? Are the gaps consistent? Are the seals intact? Are the hinges secure? Has anything been altered or damaged?

Regular inspections can identify excessive gaps, wear, damaged seals and other defects before they compromise the performance of the door.

At Adaston, our teams survey, maintain and install fire doors across a wide range of buildings. Our approach is straightforward: understand the doorset, understand the evidence and make decisions based on what is actually in front of us rather than relying on a generic rule of thumb.

Adaston provides fire door inspection, maintenance and installation services to help building owners and responsible persons manage fire door compliance and maintain effective fire and smoke protection throughout their premises.

If you’re responsible for fire doors and aren’t sure whether the doors within your building are performing as they should, speak to the Adaston team. We can help establish their condition, identify defects and provide a clear route forward where remedial work is required.