Intumescent Strips & Smoke Seals: Fire Door Guide

Last reviewed: 2026-07-11 · Certified Fire Doorsets technical team · Sources cited below

In short

Intumescent strips expand under heat — typically activating around 180-200°C — to seal the gap between a fire door and its frame, while smoke seals (brush or fin) block cold smoke at everyday temperatures. Both must match the doorset's test evidence, stay unpainted and continuous, and be replaced like-for-like when damaged, compressed or missing.

Key facts

Intumescent seals typically activate at around 180-200°C and expand to many times their original size — commonly around 10 times or more — plugging the door-to-frame gap.
Typical intumescent seal sizes are 15mm x 4mm for FD30 doors and 20mm x 4mm, or two 10mm x 4mm strips, for FD60 doors.
Smoke seals — brush or flexible fin and blade types — block cold smoke and give a fire door its 's' suffix, as in FD30s.
Government guidance on Regulation 10 checks says seals should be undamaged, make contact with the door or frame, and must not be painted over.
Replacement should be like-for-like against the doorset's test evidence: changing seal type, size or position can undermine the door's fire performance.
Under article 17 of the Fire Safety Order 2005, the responsible person must keep fire doors — seals included — in efficient working order and good repair.

What are intumescent strips and how do they work?

An intumescent strip is a heat-reactive seal fitted into a groove in the edge of a fire door leaf or its frame. At everyday temperatures it sits flush and does nothing. In a fire, once the temperature at the door edge reaches the activation range — typically around 180-200°C — the strip expands to many times its original size, commonly cited as around 10 times or more depending on the formulation. The expanded material presses into the gap between leaf and frame and forms a soft, insulating char that blocks the passage of flame and hot gases around the door edge.

This matters because a fire door is only as good as its perimeter. A well-hung timber fire door has a gap of typically 2-4mm between the leaf and the frame at the head and jambs — see our guide to fire door gap tolerances. That gap is essential for the door to swing, but in a fire it is also the weak point. The intumescent seal exists to close it at exactly the moment it becomes dangerous.

The active materials vary between manufacturers — the BWF Fire Door Alliance notes formulations based on materials such as sodium silicate or graphite, which expand at different rates and pressures. That is one reason replacement must be like-for-like: seals from different chemistries behave differently under heat, and the door was only ever tested with one of them. It is also why the same type of seal should be used all the way around a door.

What types and sizes of intumescent seal are used?

Leaf-mounted vs frame-mounted

Intumescent seals can be housed in grooves machined into the door leaf edges or into the frame head and jambs — BWF guidance describes fitting seals into grooves in the two vertical sections and top edge of the frame where possible, with the door edge as the alternative. Functionally both close the same gap; what matters is that the position matches the doorset's test evidence or the manufacturer's installation instructions. Doorsets arrive with the seals already fitted in the tested position, which is one of their compliance advantages.

Typical sizes by rating

Door rating	Typical intumescent seal	Notes
FD30 / FD30s	One 15mm x 4mm strip per edge	Standard recommendation for most modern 30-minute doors; always defer to the door's own data sheet
FD60 / FD60s	One 20mm x 4mm strip, or two 10mm x 4mm strips, per edge	The doubled or wider section reflects the longer exposure the door must survive
Smoke-control variants ('s' suffix)	Combined intumescent and smoke seal	Intumescent carrier with an integral brush or fin; threshold gap also restricted to around 3mm

Seals are sold in 10mm, 15mm and 20mm widths and various thicknesses, so it is easy to buy the wrong one. The correct section for a specific door is whatever its test evidence and certification require — not simply what fits the existing groove. If you are unsure of a door's rating, start with our guide on how to identify a fire door.

Combined intumescent and smoke seals

Many modern doors use a combined seal: an intumescent strip with a brush or flexible fin bonded along its centre or edge. One product then does both jobs — the fin or brush blocks cold smoke from day one, and the intumescent body expands when the fire arrives. Combined seals are common in blocks of flats, HMOs, offices and schools, where cold smoke control is usually required on escape-route doors.

What do smoke seals do, and how are they different?

Smoke seals answer a different problem. Intumescent strips only work once the fire is hot enough to activate them — in the early stages of a fire, cold smoke can pass freely through the door gaps long before any seal has expanded. Smoke is toxic and disorienting well before a fire generates serious heat, so this early leakage matters on any escape route — our fire statistics page covers the casualty data.

A smoke seal is a flexible barrier that touches the opposing surface at all times, with no heat needed. There are two common types:

Brush seals — a dense row of fine filaments that sweeps against the door or frame. Tolerant of slightly uneven gaps and common in combined seals.
Fin or blade seals — one or more flexible rubber or elastomer fins that press against the opposing face. Low-friction designs are used where a brush would make the door hard to close.

A fire door with cold smoke control carries an 's' suffix: FD30s or FD60s under the traditional UK designations, or the Sa / S200 classifications under BS EN 13501-2 for doors tested by the European route — see BS 476 vs EN 1634 testing. Whether a particular door needs smoke seals is set by Building Regulations guidance and the fire risk assessment; flat entrance doors and doors onto protected escape routes typically do. Smoke control also tightens the threshold: around 3mm at the bottom edge, rather than the larger threshold gap a non-smoke door may be allowed.

When must fire door seals be replaced?

Seals have no fixed service life, but they must be replaced whenever they can no longer do their job. Government guidance for the Regulation 10 fire door checks is explicit: check that intumescent strips and smoke seals are undamaged, make contact with the door edge or frame, and have not been painted over. Replace a seal when you find any of the following:

Painted or varnished over — paint bridges the seal and can stop the intumescent expanding or glue a fin in place; see can you paint a fire door?
Missing sections — any break in the seal line leaves an unprotected length of gap; the whole damaged length should be renewed, not patched
Compressed, flattened or worn brushes and fins — a smoke seal that no longer touches the opposing surface is not sealing anything
Loose, sprung or fallen out of the groove — seals that stand proud can hold the door off its stops and defeat the self-closer
Cut, gouged or melted damage — often from trolleys, keys or previous heat exposure
Wrong seal fitted historically — for example a bare intumescent strip on a door that should be FD30s, or an undersized strip in an oversized groove

The legal driver is article 17 of the Regulatory Reform (Fire Safety) Order 2005: the responsible person must keep fire safety facilities — fire doors and their seals included — in an efficient state, in efficient working order and in good repair. A door with a painted-over or missing seal is not in efficient working order, however sound the leaf itself is. In residential buildings over 11 metres, the Fire Safety (England) Regulations 2022 add the fixed rhythm: communal fire doors checked at least every three months, flat entrance doors at least every 12 months on a best-endeavours basis.

Can you replace fire door seals yourself?

Usually yes, for a like-for-like swap. Replacing a damaged seal with an identical one — same type, same section, same position, pushed or pinned into the existing groove per the manufacturer's instructions — is routine maintenance rather than alteration, and it is one of the most cost-effective fire door repairs there is. Many facilities teams and competent landlords do it in-house.

Identify the existing seal: manufacturers often print the brand and type on the strip itself, and the door's data sheet or certificate lists the required specification.
Remove the entire damaged length cleanly from the groove, including old adhesive and paint.
Fit the matching replacement continuously, without stretching, and cut ends square so there are no gaps at the corners.
Check the door still closes fully into its stops from any angle — a proud seal that holds the door open is worse than a worn one.
Record the work in the building's fire safety records.

The caveats are real, though. If you cannot identify the original seal, if the groove is damaged or the wrong size, or if the door is under a third-party certification scheme whose terms require repairs by approved technicians, hand the job to a competent specialist — see who can install fire doors. And a seal swap only restores one component: if the door also has excessive gaps, a broken self-closer or leaf damage, it needs a proper fire door inspection, not just new seals.

How does the seal specification tie to test evidence?

A fire door earns its rating in a furnace test to BS 476-22 or BS EN 1634-1, and the seals fitted during that test are part of the passing specification. The test report and the resulting certificate or data sheet record the seal manufacturer or type, the section (for example 15mm x 4mm), and whether it sits in the leaf or the frame. Installation guidance in BS 8214, the code of practice for timber fire door assemblies, works on the same principle: gaps and seals must reflect what was tested.

In practice this means the paperwork, not the catalogue, defines the right seal. For newer buildings, the fire safety information handed over under Regulation 38 should include doorset specifications. For existing doors, look for the certification label or plug on the top edge, then trace the specification through the certifying body or manufacturer. Where no evidence survives, an inspector will assess the door as a whole — which is one more reason to keep door records intact.

How do you inspect fire door seals? Key checkpoints

Seal checks are deliberately simple — the government's Regulation 10 guidance stresses that these checks are basic and should not need a specialist. Work around each door edge and look for:

A continuous seal along both jambs and the head, with no missing lengths, open corner joints or loose sections
No paint, varnish or sealant over the intumescent strip or the smoke seal
Brushes and fins that are upright and in contact with the opposing surface when the door is closed
Perimeter gaps of typically 2-4mm so the expanded seal can actually fill them — oversized gaps defeat the seal
The door closing fully into its stops past the seals from any opening angle
On 's'-rated doors, a smoke seal present, not just a bare intumescent strip, and a threshold gap of around 3mm

Our free fire door inspection checklist covers seals alongside gaps, hinges, closers and signage, and the fire door compliance checker will tell you which check regime applies to your building. Terms you meet along the way are defined in the glossary.

The most common seal failures found in practice

The same defects appear again and again in fire door surveys: seals painted over during redecoration; brush and fin seals compressed flat by years of use or by a door slamming under an over-powered closer; sections missing where seals were torn out and never replaced; strips loose in oversized grooves; and mismatched fragments of different seal types spliced along one edge. All are cheap to fix and all are straightforward breaches of the article 17 maintenance duty if left.

Frequently asked questions

What temperature do intumescent strips activate at?

Typical intumescent seal formulations begin expanding at around 180-200°C, though the exact activation point varies by manufacturer and chemistry. Once triggered, the material expands to many times its original size — commonly around 10 times or more — filling the gap between the door leaf and frame and forming an insulating char.

What size intumescent strip does an FD30 door need?

The standard recommendation for most modern FD30 doors is a single 15mm x 4mm intumescent strip in each jamb and the head; FD60 doors typically need 20mm x 4mm, or two 10mm x 4mm strips. Always confirm against the specific door's test evidence or data sheet, because the tested specification overrides any rule of thumb.

Can I replace fire door seals myself?

Yes, like-for-like replacement is generally acceptable maintenance: remove the whole damaged length and fit an identical seal — same type, section and position — per the manufacturer's instructions. Use a competent specialist if you cannot identify the original seal, the groove is damaged, or the door is under a certification scheme requiring approved repairers.

What is the difference between an intumescent strip and a smoke seal?

An intumescent strip is heat-activated: it expands in a fire to block flame and hot gases, but does nothing at room temperature. A smoke seal — a brush or flexible fin — touches the frame or leaf at all times and blocks cold smoke from the earliest stage of a fire. Doors rated FD30s or FD60s need both functions.

Do fire doors always need smoke seals?

No. Smoke seals are required where the door must resist cold smoke — indicated by an 's' suffix such as FD30s, or Sa/S200 classifications under the European system. Building Regulations guidance and the fire risk assessment determine which doors need them; flat entrance doors and doors protecting escape routes typically do.

Is it OK to paint over fire door seals?

No. Paint or varnish over an intumescent strip can prevent it expanding properly, and paint can stiffen or glue down brush and fin smoke seals. Government guidance for Regulation 10 checks specifically requires confirming seals have not been painted over. Painted seals should be replaced like-for-like, not scraped or overpainted again.

How often should fire door seals be checked?

In England, multi-occupied residential buildings over 11 metres must have communal fire doors checked at least every three months and flat entrance doors at least every 12 months on a best-endeavours basis, under regulation 10 of the Fire Safety (England) Regulations 2022. Elsewhere, the responsible person's maintenance duty under the Fire Safety Order still applies.

Sources