Fire doors are one of the most critical elements of a building’s passive fire protection system. They are designed to slow the spread of fire and smoke, protect escape routes, and ultimately save lives. However, even a certified fire door can fail if it is poorly fitted or if the gaps around it are incorrect.
At fireriskassessments.com, we regularly identify issues with fire door fitting and gaps during fire risk assessments. This blog explains why correct fire door fitting is so important, what the regulations say about gaps, and how to ensure your fire doors remain compliant.
Why Correct Fire Door Fitting Is Essential
A fire door only works as intended when it is installed correctly. Its performance relies on the door leaf, frame, hinges, seals, and surrounding structure all working together. Incorrect fitting can allow fire, smoke, and toxic gases to pass through far earlier than expected.
Poorly fitted fire doors can:
• Compromise compartmentation
• Allow smoke to spread rapidly through a building
• Reduce available evacuation time
• Put occupants and firefighters at greater risk
• Lead to non-compliance with fire safety legislation
Even small fitting errors can significantly reduce the effectiveness of a fire door.
Understanding Fire Door Gaps
Fire door gaps are the clearances between the door leaf and the frame or threshold. These gaps are tightly controlled because excessive gaps mean the smoke seals and intumescent strips may not work effectively and allow smoke and heat to bypass the door.
Recommended Fire Door Gap Sizes
Under UK guidance BS 8214 :
- Side and top gaps: Ideally 3mm and not exceeding 4mm when the door is closed
- Bottom gap (without smoke seal): Usually up to 10mm
- Bottom gap (with smoke seal): Often limited to 3 maximum 4mm, depending on the seal design
These tolerances ensure the door can close properly and the gap is not excessive for intumescent strips and smoke seals to work effectively in a fire.
Gaps consistently exceeding 4mm at the sides or top are one of the most common fire door failures identified during fire risk inspections.
The Role of Intumescent and Smoke Seals
Intumescent strips are fitted into the door leaf or frame and expand when exposed to heat, sealing the gaps around the door. Smoke seals help restrict the spread of cold smoke during the early stages of a fire.
If gaps are too large:
- Intumescent seals may not expand enough to close the gap
- Smoke seals may fail to make contact with the door leaf
- Smoke can spread before occupants are alerted
Correct fitting ensures these strips and seals can provide the protection they are designed to achieve.
Common Fire Door Fitting Issues
During fire risk assessments, several recurring fire door fitting problems are frequently identified:
- Excessive Gaps
Gaps larger than recommended, often caused by poor installation, building movement, or wear over time. Particular attention should be given to double doors where the two door leaves meet. - Warped or Damaged Door Leaves
Doors that are twisted or damaged may not sit squarely in the frame, creating uneven gaps. - Incorrect or Loose Hinges
Fire doors must be fitted with three suitable fire-rated hinges and screws. Loose or undersized hinges can cause doors to drop, increasing gaps. Fire doors are heavy and three hinges are essential to ensure that the door doesn’t warp during a fire and remains in the frame for the period of fire protection time it is manufactured to. - Incompatible Frames
Fire doors must be installed in compatible fire-rated frames. A certified door leaf fitted into a non-compliant frame may invalidate its performance. When the frame is fitted into the opening it must also be ensured that any gaps between the wall and the frame are correctly filled so the level of fire protection is maintained. - Unauthorised Alterations
Trimming a fire door beyond the manufacturer’s permitted limits or adding unapproved hardware can create excessive gaps and invalidate certification.
Legal Responsibilities and Fire Door Compliance
The Responsible Person must ensure that fire doors are:
- Suitable and sufficient for the building
- Correctly installed and maintained
- Regularly inspected and kept in good working order
Failure to maintain compliant fire doors can result in enforcement action, fines, or prosecution.
Fire door gaps are a key focus during fire door inspections and fire risk assessments, as they directly affect life safety.
Fire Door Inspections and Ongoing Maintenance
Fire doors should be inspected:
- At least every 6 months in most non-domestic buildings
- More frequently in high-traffic areas such as Student accommodation care homes, and HMOs
During an inspection, a competent person will check:
- Door alignment and closing action
- Gap sizes around the door
- Condition of seals and hinges
- Evidence of damage or unauthorised alterations
Identifying and correcting gap issues early can prevent costly replacements and improve overall fire safety.
How Fire Risk Assessments Can Help
A professional fire risk assessment will identify issues with fire door fitting and gaps as part of a wider review of fire safety measures. Clear, prioritised recommendations help Responsible Persons understand what action is required and why.
At fireriskassessments.com, our experienced assessors regularly identify non-compliant fire doors and provide practical, risk-based advice to help organisations meet their legal duties.
Key Takeaways
- Fire doors must be correctly fitted to perform as intended
- Side and top gaps should ideally be 3mm and not exceed 4mm
- Excessive gaps are a common cause of fire door failure
- Poor fitting can lead to non-compliance with fire safety legislation
- Regular inspections and fire risk assessments are essential
If you are unsure whether your fire doors are correctly fitted or compliant, arranging a professional fire risk assessment is a crucial first step in protecting people and property.