Guide · updated 9 October 2026
Intumescent strips and smoke seals
Short answer: Intumescent strips expand when heated to seal the gap around a fire door during a fire. Smoke seals, usually brushes or fins, block cold smoke before that happens. Both must be the type and size the door was tested with, continuous and free from paint.
- Intumescent = expands in heat
- Smoke seal = blocks cold smoke
- Never paint brush or fin seals
- Size must match the door's evidence
What is an intumescent strip?
An intumescent strip is a strip of material set into the edge of a fire door or its frame that swells when it gets hot, filling the gap between the leaf and frame and helping stop flames and hot gases passing through.
What is a smoke seal?
A smoke seal is a brush, fin or blade that closes the gap to cold smoke in the early stages of a fire, before the intumescent material reacts. Doors with an S rating, such as FD30S, rely on smoke seals.
Are they the same thing?
No, though many products combine both in one strip. A door that needs smoke control needs both functions.
Can seals be painted?
Brush and fin smoke seals must never be painted, because paint stops them flexing. The safest rule is to mask all seals before decorating and to replace any that have been painted.
What size should the seal be?
The size and position the door was tested with. Seals are designed for a particular gap; fitting a bigger seal to hide a large gap is not a reliable repair.
When do seals need replacing?
- Lengths missing, broken or loose
- Painted over
- Brushes flattened or fins torn
- Wrong type or size for the door
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Related questions
Can intumescent strips be painted?
Brush and fin smoke seals should never be painted, because paint stops them working. Some intumescent strips tolerate a light coat, but the safest rule is to mask all seals before decorating and replace any that have been painted.
What is the difference between an intumescent strip and a smoke seal?
An intumescent strip expands in heat to seal the gap during a fire. A smoke seal, often a brush or blade, blocks cold smoke at lower temperatures. Smoke control doors, marked with an S such as FD30S, need both.
What gap is allowed around a fire door?
Most guidance and manufacturers' evidence expect gaps of about 2 to 4 mm at the head and edges, with 3 mm as a practical target. The door's own certification or test evidence takes precedence if it states a different figure.
How big can the gap under a fire door be?
It depends on the door's evidence. Doors without smoke control often allow a larger threshold gap, sometimes up to around 8 to 10 mm, while smoke control doors usually need 3 mm or less or a tested drop seal.
Sources checked 9 October 2026: GOV.UK Fire Safety (England) Regulations 2022 fact sheets; BSI and FPA announcements on BS 8214:2026; FDIS published inspection data. This guide is general information, not legal advice.
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