Fire Door Threshold Seal Requirements (UK)

Fire door threshold seals are used to control smoke and, in some cases, fire at the base of a door. In UK buildings, they are typically required where excessive bottom gaps could allow smoke to spread, particularly on doors protecting escape routes or sleeping accommodation.

This page explains when threshold seals are usually expected, how they relate to fire door gap tolerances, and what inspectors typically look for during a Fire Risk Assessment.

Why threshold seals matter

Fire doors are designed to limit the spread of fire and, in many cases, smoke. While much attention is given to gaps around the sides and head of the door, the threshold gap at the bottom can also significantly affect performance.

Where gaps are too large, smoke may pass beneath the door before seals around the frame have a chance to activate. Threshold seals are used to help reduce this risk.

When are threshold seals typically required?

There is rarely a single prescriptive rule that states every fire door must have a threshold seal.

Instead, inspectors usually consider:

• the size of the threshold gap
• whether smoke control is required
• the location and function of the door
• the overall fire door assembly performance

Threshold seals are commonly expected where:

• the bottom gap is considered excessive
• the door protects a key escape route
• the door serves sleeping accommodation
• smoke control is an important part of the door’s function

In these situations, a large untreated gap may be recorded as a defect during an inspection.

Relationship between threshold gaps and seals

Threshold seals are closely linked to fire door gap tolerances.

Inspectors typically assess:

• whether the bottom gap appears larger than expected
• whether the gap allows visible light or air movement
• whether the door still provides effective smoke containment

In practice:

• smaller, controlled gaps may be acceptable without a seal
• larger gaps may require a threshold seal or other corrective action

For more detail on acceptable gaps, see our guidance on fire door gap size requirements.

Types of threshold seals

Several types of threshold sealing arrangements are used in practice.

Floor threshold seals

• fixed to the floor beneath the door
• create a raised barrier to reduce smoke movement
• often used where retrofitting is required

Surface-mounted drop-down seals

• fitted to the bottom edge of the door
• drop down when the door closes
• retract when the door opens
• commonly used where a flush floor finish is required

Rebated threshold seals

• integrated into the door frame or threshold detail
• less visible but dependent on door design

Inspectors are generally less concerned with the specific type and more concerned with whether the overall arrangement is effective.

What inspectors typically check

During a fire door inspection, threshold sealing is usually assessed alongside gaps and seals around the rest of the door.

Common checks include:

• whether the bottom gap appears excessive
• whether a seal is present where one would normally be expected
• whether the seal is continuous and undamaged
• whether the seal makes contact when the door is closed
• whether the door can still close fully and latch correctly

Reports may include wording such as:

• “Threshold gap appears excessive”
• “No threshold seal provided”
• “Smoke control at base of door may be inadequate”

These are common inspection findings.

Common threshold seal-related failures

Fire Risk Assessments frequently identify issues such as:

• excessive bottom gaps without any sealing arrangement
• damaged or worn threshold seals
• drop-down seals that do not deploy correctly
• poorly fitted surface seals that leave gaps
• seals that interfere with door closing or latching

Many of these are straightforward remedial items once identified.

How threshold seal issues are typically addressed

Depending on the issue, remedial action may involve:

• installing a suitable threshold seal
• adjusting or replacing a drop-down seal
• reducing the gap through door or frame adjustment
• confirming that the sealing arrangement is appropriate for the door

The correct approach depends on the door design and the findings of the Fire Risk Assessment.

Can a fire door be used without a threshold seal?

In some cases, yes.

Whether a threshold seal is required depends on:

• the level of risk
• the function of the door
• whether the existing gap is within acceptable limits

Low-level issues may sometimes be managed temporarily while works are arranged. However, where smoke control is important, missing or ineffective threshold sealing may be prioritised for correction.