Component 04 of 10 · Sealing system

Smoke seal

Cold smoke seal, modelled here as a combined fire and smoke seal. The first line of defence.

Fire takes minutes. Smoke takes seconds. Brush or fin seals close the perimeter gap at room temperature, holding back cold smoke long before flames arrive. The seal modelled here is a combined fire and smoke seal: an intumescent core with a brush standing proud of the same housing, a common arrangement where smoke control is required. Smoke seals are also fitted on their own, commonly as a flexible fin in the frame rebate.

Key facts

Function: Restricts cold smoke leakage around the closed leaf
Specification: Brush or flexible-fin profile, either combined with the intumescent seal or fitted on its own in the rebate, per the manufacturer's evidence
Materials: Nylon brush or extruded elastomer fins in a carrier
Critical tolerances: Light, continuous contact with the leaf or frame around head and jambs; the threshold arrangement achieves the required smoke control per the evidence and guidance
Standards: The “S” suffix, as in FD30S, is the Approved Document B notation for smoke control, shown by test to BS 476-31.1. The European route is a classification to BS EN 13501-2, such as Sa or S200, drawn from a test to BS EN 1634-3. A test report on its own is not a classification
Role in fire: Protects escape routes in the early phase, before heat activates the intumescent
Where it sits: In the rebate on the frame stop, or combined into the intumescent carrier in the leaf edge

Compliance

On an FD30S or FD60S door, smoke seals are not an optional extra. Without them the fire and smoke door does not meet its designation, however good the intumescent. The threshold is dealt with separately: whatever is under the door, a tightened gap, a threshold seal or a drop seal, must achieve the required smoke-control performance and follow the fire door manufacturer's test evidence and specific guidance.

Did you know

A smoke seal works by touching. If the brush or fin is not in light contact with the leaf or the frame, smoke simply passes underneath it and the seal is decorative.

On the model

Brush pile: Must flex, never painted
Carrier strip: Fixed to stop or seal groove
Contact line: Light contact, or it seals nothing

What inspectors check

Work through the checklist with the door closed, then in motion. Every line here is a recordable outcome for a Fire Door Inspector.

Confirm smoke seals are present where an “S” rating applies
Close the door and check the brush or fin makes light contact all the way round
Check brushes are free of paint and spring back
Inspect fins for tears and permanent set
Look for missing sections, and for seals that do not match each other
Check the threshold arrangement matches the evidence, and that any drop seal deploys onto the floor
Watch for daylight through the closed perimeter

Common defects

Brush pile painted over and rigid
Fins flattened, torn or perished
Sections missing at head or jambs
Seal not making contact with the leaf
Wrong profile fitted after redecoration
Seal removed to ease a binding door
Threshold arrangement not as the evidence requires

Go deeper

Three kinds of seal

Intumescent seals and smoke seals do different jobs: one needs heat, the other works cold. They come in three arrangements, and which you are looking at changes what you check.

Intumescent seal on its own: A fire seal with no brush or fin. It does nothing until heat activates it, so a door fitted with this alone restricts fire but not cold smoke.
Smoke seal on its own: A brush or flexible fin in its own carrier with no intumescent in it, normally fitted into the rebate on the frame stop. A flexible fin of this kind is now very common. Used where the fire door carries its intumescent elsewhere, or where a door is being brought up to an S rating.
Combined fire and smoke seal: One housing holding both: the intumescent core for fire, and a brush or fin standing proud of it for cold smoke. This is what the model here shows, and it is a common arrangement because a single groove serves both purposes. The intumescent in a combined seal activates exactly as it would on its own.
Why it matters on an inspection: On a combined seal, a damaged brush and a compromised intumescent are the same defect on the same strip. On separate seals they are two items in two places, and the smoke seal in the rebate is the one people forget to look at.

Contact, and how it is judged

Unlike an intumescent seal, a smoke seal has to be working now, at room temperature, with nothing activated. That makes it one of the few things on a fire door you can assess properly just by closing the door.

Light contact, all the way round: The brush or fin must touch the leaf, or the frame, continuously. Not compressed hard, which drags the door and stops it latching, and not standing off it. If there is a visible line of daylight, smoke has the same route.A seal that is not touching does not restrict smoke at all. It is not a partial seal, it is no seal.
Sa and S200, on paperwork rather than on site: European classifications you meet on a Declaration of Performance. The declaration belongs to the complete door assembly, declared under BS EN 16034, the product standard a CE or UKCA marked fire door is declared against; it is not a document an individual seal carries. Smoke control is tested to BS EN 1634-3 and classified under BS EN 13501-2: Sa covers leakage at ambient temperature, the cold smoke case, and S200 covers leakage up to around 200 °C. On a classification report, thirty minutes of fire and smoke performance might read E 30 Sa or EI 30 Sa. On a survey you will still mostly be talking about FD30S and FD60S.
The UK “S” suffix: FD30S and FD60S are Approved Document B designations from the national BS 476 route. The S is about smoke control and is separate from the 30 or 60, which is fire resistance in minutes. On that route the smoke leakage is shown by test to BS 476-31.1; a door with European evidence shows it instead with an Sa classification. A door can hold fire for its full rating and still leak smoke.
The threshold: The gap under the door is part of the smoke-control performance, and it is dealt with as the fire door manufacturer's test evidence and specific guidance set out: a tightened gap, a threshold seal or a drop seal. Do not assume every smoke-control door takes the same arrangement; read the evidence for the door in front of you.

Fire Door Anatomy is an introduction to fire doors, not the whole subject. Always defer to the fire door’s certification, its test evidence and a competent inspection.