How does an intumescent strip work?

An intumescent strip contains a material (typically graphite-based or a sodium silicate compound) that undergoes a dramatic volumetric expansion — typically 3–5 times its original thickness — when exposed to temperatures above approximately 150°C. This expansion fills the gap between the door leaf and frame, creating a physical barrier against the passage of flames and hot fire gases. The strip is passive — it requires no power, no activation signal, and no maintenance other than ensuring it remains undamaged and correctly fitted.

What is the difference between FD30 and FD30S, and what does the S mean?

FD30 means the door provides 30 minutes of fire integrity (resistance to flames and hot gases passing through). FD30S adds the S suffix which indicates smoke seal performance — the door also prevents the passage of cold smoke (at ambient temperature) for the same 30-minute period. The S suffix requires a combined intumescent and cold smoke seal strip. Most fire doors in residential and commercial buildings should be FD30S, as smoke is the primary cause of fire fatalities.

Can I fit an intumescent strip myself?

Physically, yes — most strips are supplied with adhesive backing and are cut to length. However, any modification to a certified fire door — including fitting new intumescent strips — must use products that are tested and certified for use with that specific door and frame combination. Fitting an uncertified strip, or a strip not specified in the door's third-party certification, could invalidate the fire door's certification. For certified fire doors, the fitting instructions from the door manufacturer or third-party certifier (e.g. BWF-Certifire) should be followed.

How often should intumescent strips be inspected?

Intumescent strips should be visually inspected at every fire door check — at minimum annually, and quarterly for fire doors in high-rise residential buildings (required under the Fire Safety (England) Regulations 2022). The inspection checks that the strip is continuous (no gaps), undamaged (no crushing, peeling, or missing sections), correctly seated in its groove, and not painted over. A painted strip may fail to expand correctly when needed.

Do intumescent strips have a service life?

Intumescent strips do not expire in normal conditions — they remain dormant until heat activates them. However, mechanical damage, painting over, being compressed over time (loss of resilience), or exposure to damp can all degrade performance. In practice, strips that show visible damage, that have been painted over, or that are no longer correctly seated should be replaced. Use only a product certified for the specific door assembly.

⚠ Emergency: If fire is in the building — close all fire doors and follow the escape route. Fire doors with intumescent strips buy you time.

Glossary — Passive Fire Protection

Intumescent Strip

An intumescent strip is a passive fire protection product fitted to fire doors that expands when heated to seal the gap between door and frame, preventing the passage of flames and fire gases. Required on all FD30S and FD60S fire doors. Works without power or activation — it responds directly to heat from a fire.

Fire doors Passive protection FD30S / FD60S

How it works

At ambient temperature, an intumescent strip is a thin, inert insert in the door edge or frame rebate. When a fire develops and temperatures at the door gap reach approximately 150–200°C, the intumescent material undergoes rapid expansion — typically tripling or quadrupling in thickness — to fill the gap completely. This physical seal prevents flames and hot gases from passing through the door edge gap, maintaining the door's fire resistance rating.

Intumescent strip types

Type	Expands in fire	Smoke-sealed	Use case	Location
Intumescent only	Yes (at ~150–200°C)	No	FD30 (fire integrity only) — no smoke seal required	Edge of door leaf or rebate of frame
Intumescent + cold smoke seal	Yes	Yes (at ambient temperature)	FD30S and FD60S — smoke control required	Combined product fits in single rebate groove
Intumescent threshold seal	Yes	Combined	Bottom of door where threshold gap exceeds 3 mm	Door bottom rail or threshold sill
Intumescent letter box seal	Yes	Yes (combined product)	Letter plate openings in flat entrance fire doors	Around letter plate aperture

Intumescent strips and fire door certification

A fire door is tested and certified as a complete assembly: door leaf, frame, hinges, hardware, seals, and glazing all tested together. Changing any component — including the intumescent strip — for one not specified in the door's certification can invalidate the third-party certification. The BWF-Certifire scheme and other third-party certification bodies specify which seals are acceptable for each certified door assembly.

See our fire door safety guide and our fire door glossary entry for the full six-point check and certification requirements.

Intumescent strips in fire stopping

Intumescent products are also used in fire stopping applications — sealing service penetrations through compartment walls and floors. Intumescent collars are fitted around plastic pipes passing through compartment walls; when the plastic pipe melts and collapses in a fire, the collar expands inward to seal the hole. This is a critical and frequently missed element of fire stopping during construction and renovation works.

Common inspection failures

Strip painted over — prevents expansion; appears intact but functionally compromised
Strip missing from one or more edges (particularly the top of the door)
Strip damaged or crushed from repeated closing force (door not hung correctly)
Cold smoke seal element missing or removed separately from the intumescent strip
Wrong product used — intumescent-only strip where combined smoke seal is required (FD30S)

These failures are flagged in fire door inspections under the fire door gap tolerances guide.

Frequently asked questions

FAQ

Intumescent Strip

Intumescent strip types

Intumescent strips and fire door certification

Intumescent strips in fire stopping

Common inspection failures

Related entries

Frequently asked questions

Frequently asked questions