Excessive gaps around a fire door are one of the most common issues identified during inspections and Fire Risk Assessments (FRAs).
This page is for landlords, managing agents and facilities managers who need to understand how fire door gaps are typically assessed, what tolerances inspectors commonly expect, and why gap size matters in practice.
Although the exact acceptable gap may depend on the door design, manufacturer’s evidence and whether smoke control is involved, inspectors will usually expect gaps to fall within recognised tolerances and to be consistent across the door.
Why fire door gaps matter
Fire doors are intended to resist the spread of fire and, in many cases, smoke.
If the gaps around the door leaf are too large, this can reduce the effectiveness of:
This is why gap size is commonly measured during detailed fire door inspections.
Which gaps are usually checked?
Inspectors typically assess:
- the gap between the door leaf and frame at the sides
- the gap at the head of the door
- the threshold gap at the bottom of the door
These measurements are often checked using a gap gauge rather than by visual judgement alone.
Typical fire door gap tolerances in practice
In many routine inspection contexts, assessors commonly expect:
Side and head gaps
Often around 3mm to 4mm, and reasonably consistent around the door.
Threshold gap
The acceptable bottom gap may vary more depending on the door design, floor finish, threshold detail and whether smoke control is required.
Because of that, inspectors usually consider:
- whether the gap appears excessive
- whether seals and threshold detail remain effective
- whether the door still performs as intended
- whether the gap aligns with available product or installation guidance
In practice, the threshold gap often attracts particular scrutiny where smoke control is important.
Is there one fixed legal gap measurement?
Usually not.
Fire safety legislation generally focuses on whether fire precautions are maintained in effective working order, rather than prescribing a single universal door gap measurement for every situation.
In practice, assessors usually rely on:
- recognised guidance
- common inspection tolerances
- the design and intended performance of the specific door
- available manufacturer or certification information where relevant
That is why reports often refer to gaps exceeding acceptable tolerances rather than citing a single statutory figure.
How inspectors usually assess door gap problems
During inspections, assessors commonly look for issues such as:
- visibly uneven gaps
- gaps large enough to affect seal contact
- excessive threshold clearance
- rubbing or binding caused by distorted alignment
- signs that the frame, hinges or closer are no longer holding the door correctly
Reports may use wording such as:
- “Gaps exceed tolerances”
- “Excessive clearance to head and jambs”
- “Threshold gap appears excessive”
- “Door gaps require adjustment”
These are common inspection findings.
What causes excessive fire door gaps?
Common causes include:
- wear over time
- poor adjustment
- incorrect installation
- frame movement
- worn hinges
- unauthorised alteration
- changes to floor finishes
- previous repair work carried out without regard to fire door tolerances
In many buildings, excessive gaps develop gradually and are not noticed until a formal inspection takes place.
What happens if the gap is too large?
Where gaps are considered excessive, remedial action may involve:
- adjusting hinges or closers
- rehanging or realigning the door
- replacing seals where appropriate
- carrying out frame or door repairs
- seeking specialist advice where the defect is more significant
The correct solution depends on why the gap has developed and whether the door can still achieve its intended performance.
Can a fire door remain in use with excessive gaps?
Sometimes this depends on the severity of the defect, the location of the door and the level of risk.
Low-level issues may occasionally be managed while repair works are arranged, but more serious defects may require urgent attention, particularly where the door protects:
- a key escape route
- sleeping accommodation
- a higher-risk area
The Fire Risk Assessment should always guide prioritisation and any temporary measures.
FAQs
What are the acceptable gaps around a fire door?
In many inspection contexts, side and head gaps are commonly expected to be around 3mm to 4mm and reasonably consistent. Bottom gap expectations vary more depending on the door design and whether smoke control is required.
How are fire door gaps measured?
Assessors commonly use a fire door gap gauge to measure the gaps at the sides, head and threshold.
Is the bottom gap allowed to be bigger?
Often, yes, but not without limit. Inspectors will usually consider the threshold detail, the door design and whether smoke control is required.
Are uneven gaps a problem?
Yes. Even where a gap is not obviously large everywhere, inconsistent or uneven gaps may indicate alignment, hinge or frame problems.
