Fire Door Gap Regulations: What Are the Gap Thresholds?

Contents

1. What Are the Maximum Gaps Around a Fire Door?
2. Why Do Fire Door Gaps Matter?
3. How to Measure Fire Door Gaps
4. What Should You Do If the Gaps Are Incorrect?
5. Is Your Fire Door Still Compliant?

A gap of just a few millimetres can affect how well a fire door contains fire and smoke: too large, and the seals may not work as intended; too small, and the door may stick or fail to close completely.

So, what do UK fire door gap regulations actually require? For many fire doors, the typical gap at the top and sides is 2-4 mm. The acceptable gap underneath the door varies depending on whether it provides fire resistance alone or also needs to control smoke.

However, these measurements aren’t universal legal limits. Ultimately, you must check the certification, installation instructions and supporting evidence for your specific fire door.

Note: This article is intended as general guidance at the time of writing – not legal or professional advice. Fire safety requirements can change, and the correct specification or action for your building may differ depending on its design, use and risk profile. Always consult a qualified fire safety professional or contact our team for tailored advice.

Fire safety legislation also differs across the UK. This article primarily discusses requirements and official guidance applicable in England, although the technical principles and British Standards referenced are widely used across the UK.

What Are the Maximum Gaps Around a Fire Door?

The following measurements provide a typical starting point when checking a fire door:

• Top of the door: 2-4 mm

• Sides of the door: 2-4 mm

• Between the leaves of double doors: Typically 2-4 mm

• Under a fire door without a smoke-control requirement: Often up to 8-10 mm, but only where permitted by the door’s test, assessment or installation evidence.

• Under a smoke-control door: No standalone gap dimension establishes compliance. The complete threshold arrangement, including any drop seal or fixed seal, must be supported by suitable smoke-leakage test or assessment evidence.

These figures reflect commonly used fire door tolerances; however, the correct measurement for an individual door depends on its tested or assessed configuration.

A fire door works as a complete assembly. The door leaf, frame, seals, hinges, closer and other components must work together in the same configuration that achieved the required fire or smoke performance.

What Is the Maximum Gap at the Top and Sides?

For many fire doors, the gap between the door leaf and frame should be 2-4 mm at the top and along both sides. The gap should also remain reasonably consistent around the door. For example, a 3 mm gap on the hinge side doesn’t compensate for a 6 mm gap in the top corner.

Correctly controlled gaps allow the door to open and close while keeping the leaf close enough to the frame for the seals to perform as intended. Intumescent seals expand when exposed to heat, helping close the space between the leaf and frame during a fire. Cold smoke seals restrict the passage of smoke before temperatures become high enough to activate the intumescent material.

A gap wider than 4 mm falls outside the typical tolerance for many doors and should be investigated. However, 4 mm isn’t a universal statutory maximum – the door’s supporting evidence may specify a different permitted range.

A gap can also be too narrow. If the leaf catches on the frame or compresses the seals excessively, the door may fail to close and latch under the power of its self-closing device. A fire door that remains partly open cannot provide its intended protection, regardless of how small its gaps appear.

What Is the Maximum Gap Under a Fire Door?

Threshold gaps – the space between the bottom of the door and the floor or threshold – require more careful consideration.

For a door providing fire resistance only, the supporting evidence may permit a larger threshold gap than at the head and jambs. Figures of approximately 8-10 mm are commonly quoted, but this isn’t an automatic allowance for every fire door.

Again, the maximum gap must match the door’s fire-test or assessment evidence. The installation instructions may specify a smaller limit or require a particular threshold configuration.

The requirements are even more demanding when the door also needs to control ambient-temperature smoke. These doors are commonly identified by an “S” suffix, such as FD30S or FD60S.

Older guidance and many inspection checklists use a maximum gap of 3 mm beneath a smoke-control door. However, BS 8214:2026 makes clear that the measured gap alone should not be relied upon to demonstrate adequate smoke control.

The smoke-leakage performance of the complete doorset or door assembly must be considered, including the threshold. Depending on the tested or assessed configuration, this may require a drop seal, fixed threshold seal or another supported sealing arrangement. Fundamentally, the correct solution must restrict smoke while still allowing the door to close reliably.

Why Do Fire Door Gaps Matter?

Fire doors are designed to restrict fire and, where required, smoke for a specified period. Their performance depends on the door being correctly installed, closed and maintained.

Excessive gaps can create a route around the door leaf, potentially allowing fire, hot gases or smoke to pass through sooner than intended. They may also be too wide for the installed seals to perform in the way demonstrated by the door’s supporting evidence.

Uneven gaps may also indicate a problem with the assembly. The door may have dropped on its hinges, the leaf or frame could be distorted or movement may have occurred around the opening.

Insufficient clearance creates a different risk: a door that rubs against the frame or catches on the floor may not close completely. It may stop before reaching the frame or fail to engage the latch.

For this reason, checking the gap alone isn’t enough. You should also confirm that:

• The door closes fully from any position

• The latch engages where one is fitted

• The leaf doesn’t catch on the frame or floor

• The gaps remain reasonably consistent

• The intumescent and smoke seals are present, continuous and undamaged

• The hinges, closer and other components remain securely fitted

Note that while a gap measurement may identify a potential defect, it doesn’t assess the performance of the complete fire door by itself.

How to Measure Fire Door Gaps

You can identify obvious problems using a purpose-made fire door gap gauge or a suitable set of feeler gauges.

1. Close the Door Fully

Allow the self-closing device to shut the door without pushing it into position. Check whether the door closes completely and engages the latch where one is fitted. If you have to pull or push the door into the frame, the closing action already requires attention.

2. Measure the Top and Both Sides

Insert the gauge between the door leaf and frame. Take several measurements along the head, hinge side and closing side rather than checking one convenient point. Measurements at different positions will reveal gaps that widen towards a corner or vary along the height of the door.

3. Check Double Doors

For double doors, measure the gap between the two leaves as well as the gaps around the frame. You should also check that the doors close in the correct order where a door coordinator is fitted. Both leaves must reach their intended closed positions.

4. Measure the Threshold

Measure the gap beneath the door at several points across its width. Floor finishes and thresholds may be uneven, so a single reading can miss the largest clearance.

Where smoke control is required, inspect the complete threshold sealing arrangement. A measurement of the open gap without checking the seal doesn’t establish whether the door will restrict smoke adequately.

5. Record What You Find

Record the door’s location, identification number and measurements. Note any uneven gaps, damaged seals or problems with closing and latching.

As a tip, a modern £1 coin is 2.8 mm thick, which makes it useful for a very rough visual comparison with a nominal 3 mm gap. However, please note that it cannot provide an accurate recorded measurement and shouldn’t replace a proper gap gauge.

In addition, make sure to review the fire door guidance provided under the Fire Safety Regulations, which requires routine checks for visibly excessive gaps alongside other obvious defects. More detailed inspections should be completed by someone with the appropriate competence.

What Should You Do If the Gaps Are Incorrect?

An incorrect gap is a symptom, not necessarily the underlying problem. Before attempting a repair, you need to determine why the door no longer fits its frame correctly.

Common causes include:

• Loose, worn or unsuitable hinges

• A door that has dropped out of alignment

• Warping or damage to the door leaf

• Movement or distortion in the frame

• Changes to carpets or other floor finishes

• Excessive layers of paint

• Previous trimming or alterations

• Incorrect installation

• Incompatible replacement seals

The appropriate response might involve adjusting or replacing the hinges, realigning the leaf, repairing the frame, installing a supported threshold sealing system or replacing part or all of the door assembly.

Any work must remain consistent with the door’s certification and supporting evidence, which can be especially tricky for older doorsets. Avoid improvised fixes such as filling the operating gap with ordinary sealant, installing a larger intumescent strip without evidence or planing the door simply to make it close.

Even a seemingly minor alteration can significantly impact the relationship between the leaf, frame and seals. If you can’t identify the door or establish its permitted tolerances, arrange an inspection before modifying it.

You should also check gaps following any work that could affect the door, which includes replacing floor coverings, adjusting hinges and painting a fire door.

Is Your Fire Door Still Compliant?

A gap outside the expected range can reveal that a fire door has dropped, warped or been installed incorrectly. A correction of the visible clearance without identifying the underlying cause may leave you with a door that still cannot provide its intended protection, resulting in non-compliance and threatening the safety of occupants.

Our fire door specialists can inspect the complete assembly, measure and document defects, and recommend appropriate remedial work. We’ll consider the door leaf, frame, seals, closer, ironmongery and available supporting evidence rather than assessing one measurement in isolation.

Get in touch today to arrange a professional fire door inspection and establish what action your building requires.