Why Fire Door Threshold Gaps Fail

Why Do Fire Door Threshold Gaps Fail?

Fire door threshold gaps often fail because of changes made after installation. Common causes include changes to floor finishes, door trimming during maintenance, hinge wear that causes the door to drop slightly, and even gradual building movement. These changes can widen the bottom gap beyond permitted tolerances, which is why regular door gap testing is essential.

During fire door inspections, one issue appears more often than almost any other: excessive threshold gaps. While the perimeter gaps around the sides and head of a fire door often remain within tolerance, the clearance at the bottom of the door is far more likely to drift outside permitted limits over time. When this happens, the door may no longer perform as the tested assembly intended.

Understanding why fire door threshold gaps fail is an important part of maintaining fire door compliance. In many cases, the door itself is not defective. Instead, gradual changes in the building or the door’s operation have altered the gap tolerances.

Recognising the causes makes it easier to identify problems early and take appropriate remedial action.

Why Threshold Gaps Are the Most Common Failure

A fire door requires a controlled clearance to operate correctly. The door must open and close freely without binding against the frame or floor surface. But the threshold gap is more vulnerable than the perimeter gaps because it is influenced by factors beyond the door itself. Changes to the surrounding building fabric, wear on the door hardware, or alterations made during refurbishment can all affect the clearance at the bottom of the door.

Unlike perimeter gaps, which tend to remain relatively stable once installed, the threshold gap often changes gradually over time. For this reason, it is one of the most common defects identified during fire door inspections.

Changes to Floor Finishes

One of the most frequent causes of excessive threshold gaps is a change in floor finishes.

During the life of a building, flooring is often replaced or upgraded. Carpet may be removed and replaced with vinyl or laminate flooring. In other cases, levelling compounds or new finishes may slightly alter the finished floor level.

Even relatively small changes can increase the gap between the bottom of the door and the floor surface. A door that originally met the permitted tolerance may suddenly have a much larger clearance once the new flooring is installed.

Because these changes often happen long after the door was installed, they can go unnoticed unless the door is inspected regularly.

Door Trimming During Installation or Maintenance

Another common cause of excessive fire door gaps is trimming. When doors are installed, installers may trim the bottom edge slightly to achieve correct clearance above the floor. This is often necessary where floor levels vary across a building.
However, doors are sometimes trimmed again during later maintenance or refurbishment. A door that sticks slightly against a new floor finish may be planed down to improve operation.

While this may solve the immediate issue of the door binding, excessive trimming can create a threshold gap that exceeds the limits specified in the door’s certification and over time, these incremental adjustments can lead to significant clearance problems.

Hinge Wear and Door Sag

Fire doors are often installed in high-traffic areas such as corridors, stairwells and communal entrances. Over time, the hinges supporting the door can experience wear.

As hinges loosen or the fixings begin to move slightly within the door leaf or frame, the door may drop marginally on its axis. Even a small amount of movement can change the clearances around the door.

This typically results in the threshold gap increasing while the head gap becomes tighter. In some cases, the door may also begin to rub slightly against the frame or floor surface. This type of movement tends to occur gradually, making it difficult to detect without proper, regular door gap testing.

Building Movement and Frame Settlement

Buildings themselves move over time. Minor settlement, structural movement or changes in temperature and humidity can alter the geometry of door frames.
Where frames shift slightly, the clearances around the door may change as well. In older buildings or large estates that have undergone multiple refurbishments, this can gradually lead to inconsistent or excessive door gaps.

These changes are rarely dramatic, but they can accumulate over time. A door that was originally installed with correct tolerances may slowly drift outside those limits.

Why Threshold Gap Problems Can Go Unnoticed

Threshold gaps are surprisingly easy to miss in a casual visual inspection. The bottom of the door is naturally less visible than the sides and head, particularly where lighting is poor or floor finishes are dark in colour. A door may appear to function normally even when the bottom clearance exceeds acceptable limits. So, without proper measurement, it can be difficult to judge whether the gap falls within the permitted tolerance.

This is why structured inspection programmes increasingly rely on door gap testing using calibrated gauges rather than relying solely on visual checks.

Why Early Detection Matters

Although the threshold gap may seem like a small detail, it can have a significant impact on a fire door's performance.

During a fire, smoke and hot gases naturally spread through the path of least resistance. Excessive clearance at the bottom of the door can allow smoke to pass into escape routes or adjoining compartments much earlier than intended.

Maintaining correct gap tolerances helps ensure the fire door performs as part of the building’s compartmentation strategy. Regular inspection and accurate measurement allows these issues to be identified before they become more serious compliance concerns.

What Happens When Excessive Threshold Gaps Are Found?

When inspections identify excessive fire door threshold gaps, the door will usually be recorded as requiring remedial action. However, this does not automatically mean the door must be replaced.

In many cases, the door leaf and frame remain structurally sound, and the issue is purely a matter of bottom clearance. Where this is the case, threshold gap remediation may restore compliant performance.

One common approach involves installing a fire door drop seal, which forms a seal against the floor when the door closes. This helps control smoke movement while allowing the door to operate normally.
Other issues, such as hinge wear or damaged seals, may also be addressed at the same time as part of the remediation process.

Maintaining Compliance Through Inspection

Understanding why fire door threshold gaps fail highlights the importance of regular inspection. Under the Building Safety Act 2022, Responsible Persons must ensure fire safety measures are maintained in effective working order.

Routine inspections that include accurate gap measurements help ensure that fire doors continue to perform as intended by the building’s fire strategy. By identifying threshold gap issues early, building managers and contractors can take practical steps to maintain compliance without unnecessary disruption.

In This Series: Fire Door Gap Compliance

This article forms part of Norseal’s practical guidance series on fire door gaps and compliance. You may also find these guides useful:

• Fire Door Gaps: Regulations, Testing and Remediation: an overview of fire door gap regulations and inspection requirements.
• Door Gap Testing: How to Know If a Fire Door Has the Correct Gaps: a practical guide to measuring fire door gaps during inspection.
• Fire Door Gap Remediation Solutions: how excessive gaps can be corrected without replacing the entire door set.

FAQs

Why are fire door threshold gaps important?

Threshold gaps control how smoke and hot gases move beneath a fire door. If the gap becomes too large, smoke can spread more quickly into adjoining compartments or escape routes.

What causes excessive gaps under fire doors?

Common causes include changes to floor finishes, trimming during installation or maintenance, hinge wear that causes the door to sag, and even gradual building movement.

How do inspectors check threshold gaps?

Inspectors typically measure the bottom clearance using calibrated gap gauges. Measurements are taken across the width of the door to confirm whether the gap falls within permitted tolerances.

Can excessive threshold gaps be fixed?

Yes. In many cases, the door itself remains structurally sound, and the issue can be resolved through remediation measures such as installing a compliant fire door drop seal.