Fire doors are a fundamental component of a building’s passive fire protection strategy. Their role is to prevent or slow the spread of fire and smoke, thereby preserving escape routes and allowing additional time for evacuation and emergency services to intervene. However, their effectiveness relies on strict adherence to installation and maintenance standards. One of the most significant yet often overlooked aspects of fire door performance is the size and consistency of the gaps surrounding the door leaf, including the clearance at the threshold.
When fire door gaps fall outside the permissible range, the door’s fire-resisting capabilities can be compromised, putting both life and property at risk. This guide explains current regulations concerning fire door gaps in the UK, outlines accepted tolerances and provides guidance on inspection and rectification where gaps are found to be excessive.
Fire door gap regulations
UK fire door compliance is governed by several overlapping pieces of legislation and technical standards. The Regulatory Reform (Fire Safety) Order 2005, commonly referred to as the RRO, assigns responsibility for fire safety to the ‘responsible person’, who must ensure that fire protection systems, including doors, are installed and maintained in accordance with the law. This includes checking that fire doors are correctly fitted with suitable clearance around all edges.
Approved Document B of the Building Regulations provides further detail on fire safety in building design and use. Supplementary technical guidance is found in BS 8214:2016, which covers timber-based fire door assemblies, and BS 476-22:1987, which specifies fire resistance test methods. These documents provide essential benchmarks for manufacturers, installers and responsible persons to assess the suitability and compliance of door gaps.
If fire doors do not meet the required gap tolerances, they may fail prematurely in a fire, significantly reducing their ability to compartmentalise flames and smoke. It is, therefore, a legal and safety imperative to ensure all fire doors are installed and maintained in line with these standards.
Fire door gaps – what are the rules?
Fire door gaps are strictly defined for good reason. If the gap between the door leaf and its frame is too large, fire and smoke can pass through before the intumescent seals activate, thereby defeating the purpose of the door. The rules around permissible gaps aim to balance operational functionality with fire safety performance.
According to BS 8214:2016, the gap between the top and sides of a fire door and its frame should be 3 mm, with a tolerance of plus or minus 1 mm. This means an acceptable range of 2 mm to 4 mm. For the gap beneath the door, a maximum of 10 mm is allowed in most standard installations. However, in cases where smoke control is a requirement, the permitted clearance under the door is reduced to a maximum of 3 mm to prevent the ingress of smoke into protected areas.
These rules serve a dual purpose. While the door must swing freely and accommodate ventilation needs, any gap that exceeds these limits can allow the early passage of dangerous gases, which undermines the door’s ability to delay fire spread.
What size should a fire door gap be?
In practical terms, a fire door should have a clearance of between 2 mm and 4 mm along the top and sides when measured with the door closed. This narrow range is designed to allow the door to operate without friction while ensuring that intumescent seals can expand and fill the space effectively during a fire.
At the bottom of the door, a gap of between 8 mm and 10 mm is acceptable for general installations. However, if the door is required to provide smoke protection under Building Regulations, the threshold gap must not exceed 3 mm. It is important to confirm this with the manufacturer, as different fire door models and ratings may have specific requirements.
These measurements should be taken using calibrated tools such as feeler gauges, ensuring accuracy and consistency across all sides of the door. Inconsistent gaps, even if technically within limits in some areas, may indicate poor installation and should be investigated.
What is the maximum gap allowed around fire doors?
The maximum gap allowed between a fire door leaf and its frame is 4 mm. This applies to the top edge and both vertical sides. These tolerances are designed to ensure that, once the intumescent seals expand during a fire, the gaps are sealed quickly and effectively to maintain compartmentation.
For the bottom of the door, the situation varies depending on the function of the door and the fire strategy of the building. A typical threshold gap should not exceed 10 mm. However, where the fire door is also designated as a smoke control door, the allowable gap is reduced to a maximum of 3 mm. This is particularly relevant in escape routes, such as stairwells and corridors in multi-occupancy buildings.
Consistency in gap size is as important as the actual measurements. A single section of the door with an excessive gap can compromise the performance of the entire assembly, so care must be taken during both installation and maintenance.
Do I need to replace my fire door if the gaps between the leaf and frame are too big?
If a fire door has gaps that exceed the permitted tolerances, it may not need to be replaced outright. There are specialist products on the market that can be used to remediate non-compliant door gaps, thereby restoring the door’s performance and ensuring regulatory compliance.
One such solution is FrameFit, a retrofittable system comprising preformed fillets made from intumescent and charring materials. These fillets are applied to the frame to reduce the clearance between the door leaf and the frame. Upon exposure to heat, the material expands, sealing the gap and enabling the door to meet its fire-resistance rating. FrameFit is capable of remediating gaps of up to 15 mm, offering an effective alternative to door replacement in many cases.
For threshold gaps, the FrameFit Bottom DoorSeal is available and can be applied to the underside of FD30 (30 minute rated fire doors) and FD60 (60 minute rated fire doors) fire doors. This component provides protection for gaps of up to 25 mm and has been rigorously tested for fire and smoke resistance.
While these systems offer a practical and compliant solution, it is vital to ensure that any retrofit products are tested and certified for use with the specific door in question. If such products are unsuitable or unavailable, or if the door has other faults, replacement may still be the only viable option.
How to fix fire door gaps
Rectifying fire door gaps depends on the nature and extent of the non-compliance. For side and top gaps that are slightly too wide, intumescent sealing systems such as FrameFit can be added to the frame. These systems are specifically designed to activate during a fire and seal the opening, allowing the door to perform to its certified rating.
Where the bottom gap of a door is excessive, there are various drop-down or threshold seals available. Surface-mounted drop-down smoke seals can be fitted to the lower edge of the door. These seals include a mechanism that activates when the door closes, causing the seal to drop and fill the gap at floor level. This prevents smoke from passing underneath, which is critical in certain types of protected areas.
If the fire door has been incorrectly installed or has become warped, it may be necessary to rehang the door or carry out minor planing to bring the gaps within permitted tolerances. In such cases, care must be taken to avoid damaging the door’s fire-resistant core or certification label. Professional advice should always be sought before carrying out physical alterations.
In some situations, misalignment or deterioration of the frame may be the underlying issue. If the frame is the cause of excessive gaps, it might need to be adjusted or replaced. Again, the work should only be undertaken by a competent person, ideally someone with experience fitting and maintaining fire door assemblies.
In severe cases where gaps cannot be corrected through these means, the complete replacement of the door and frame may be required. This should always be accompanied by proper certification and third-party testing to ensure compliance.
Summary
Fire doors play a vital role in preserving life and property by compartmentalising fire and smoke. For them to function as intended, they must be installed and maintained in strict accordance with established standards. One of the most critical requirements is the maintenance of correct door gaps. These gaps allow for door movement and ventilation in normal conditions, but must be narrow enough to ensure effective sealing during a fire.
Ultimately, the responsibility for ensuring that fire door gaps meet legal and safety requirements lies with the designated responsible person. Regular inspections, correct installation and accurate measurement are essential to maintaining compliance and protecting occupants. By taking these responsibilities seriously, building owners and operators can significantly reduce the risks associated with fire and smoke spread.
