Fire Door Threshold Seal Requirements (UK) – When Drop-Down Seals Are Needed

Fire door threshold seals are used where the gap beneath a fire door may allow smoke to pass under the door. They are commonly fitted as drop-down seals, surface-mounted seals or threshold sealing systems where smoke control is required at the base of the door.

This page explains when threshold seals are usually needed, how drop-down smoke seals work, and what inspectors typically look for when assessing the bottom of a fire door during a Fire Risk Assessment.

Quick answer: when is a fire door threshold seal required?

A fire door threshold seal may be required where the gap under the door is large enough to affect smoke control, or where the fire door is expected to restrict smoke as part of its intended performance.

Inspectors commonly look at whether the threshold gap is excessive, whether smoke can pass beneath the door, and whether a suitable seal is fitted and working correctly.

Where excessive gaps are identified, inspectors may also assess other components such as hinges, self-closers and intumescent seals as part of the wider fire door assembly.

Why smoke control at the threshold matters

Fire doors are designed to limit the spread of fire and, in many cases, smoke. This forms part of the wider fire door assembly, including seals, hinges, closers and other essential hardware. The bottom of the door is a common weak point because smoke can pass beneath the door if the threshold gap is too large or poorly controlled.

A threshold seal helps close or reduce this gap when the door is shut. On many existing doors, this is achieved using a surface-mounted drop-down seal fitted to the bottom edge of the door.

When are threshold seals typically required?

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

In practice, threshold seals are most commonly considered where:

• smoke control is required
• the threshold gap is larger than expected
• the door protects an escape route
• the door serves sleeping accommodation
• the existing floor or threshold detail does not provide effective sealing
• an inspection has recorded concern about smoke spread beneath the door

These issues are commonly raised during routine inspections and may lead to remedial works being prioritised depending on risk.

The key question is not simply whether a seal is present, but whether the door can still perform its intended fire and smoke resisting function.

Threshold seals vs fire door gap tolerances

Threshold seals are closely related to fire door gap tolerances, but they are not the same issue.

The fire door gap size page explains how gaps around the door are commonly assessed and measured. This page focuses specifically on how the gap at the bottom of the door may be controlled using a threshold seal or drop-down smoke seal.

Where the issue is the seal rather than the measurement, the focus moves from inspection tolerances to corrective action.

In simple terms:

• gap tolerance guidance helps identify whether the bottom gap may be excessive
• threshold seals help control smoke movement where the gap needs sealing
• drop-down seals are often used where a retrofit solution is needed without altering the threshold

Types of threshold seals

Several threshold sealing arrangements are used in practice. The right option depends on the door, floor finish, threshold gap and intended fire and smoke performance.

Surface-mounted drop-down seals

• fitted to the bottom edge of the door
• drop down automatically when the door closes
• retract when the door opens
• useful where a retrofit solution is needed
• avoid creating a raised floor threshold

Floor threshold seals

• fixed to the floor beneath the door
• create a raised barrier to reduce smoke movement
• may be suitable where a raised threshold is acceptable

Rebated threshold seals

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

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

How drop-down threshold seals work

A drop-down threshold seal is fitted to the bottom edge of the fire door. When the door closes, a plunger or activation mechanism lowers the sealing strip so it contacts the floor or threshold.

This helps reduce smoke movement beneath the door while allowing the door to open normally without dragging across the floor.

Inspectors may check that the seal drops evenly, makes contact across the width of the door and does not prevent the door from closing or latching correctly.

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 a drop-down seal deploys fully when the door closes
• whether the seal leaves gaps at either end of the door
• 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.