What Are Intumescent Strips and Smoke Seals? (And Why Fire Doors Need Both)

What does an intumescent strip do?

It expands to many times its original size when it gets hot, sealing the gap between the door and the frame so flames and hot gases cannot get through.

Most strips are graphite or sodium silicate in a plastic casing. They sit quietly for years and only react in a fire. Most start to expand at around 200°C, though some products react at lower temperatures. As the door edge chars, the expanding strip keeps the gap closed for the rated time.

A strip is sized for the gap the door was fire tested with. That is why gaps of 2 to 4mm matter. Too wide a gap and even a good strip may not bridge it in time.

What is the difference between intumescent strips and smoke seals?

Intumescent strips only work once heat arrives. Smoke seals work from the moment the door closes, which matters because cold smoke spreads long before the heat does.

Smoke is the main cause of death in building fires, so the early minutes count. Many modern products combine both in one strip: an intumescent core with a brush or fin seal along its face.

Does every fire door need both?

Every fire door needs fire seals suited to its tested design, and smoke-control doors, marked FD30S or FD60S, also need smoke seals.

Approved Document B says which doors in a new building need smoke control. Flat entrance doors onto common corridors and doors on protected escape routes commonly do. The "S" means the door has been tested for smoke leakage.

For older blocks of flats, the government's fire door guidance (revised August 2025) says that if seals were never fitted to an existing door, this may be acceptable, subject to the fire risk assessment. Where seals are present, they should be undamaged, touch the door or frame, and not be painted over.

Where should intumescent strips and smoke seals be fitted?

On the top and both long edges of the door, never the bottom. Double doors also need seals down the meeting stiles.

The door's test evidence decides whether the seals go in the door edge or the frame, and what size they are. Under the door, smoke-control doors usually use a small gap of about 3mm or a drop seal instead. See our guide to fire door gap tolerances.

What seal faults fail a fire door inspection?

Seals were missing, damaged or painted over on 35% of doors in our inspections (448 itemised doors), the second most common fault after gaps.

Painting over is the most common cause. A coat of paint glues a brush seal flat and can affect how an intumescent strip expands. The full figures are on our fire door inspection statistics page, and our fire door inspection checklist shows what to look for.

• Seals painted over, on the door or the frame.
• Lengths missing, often at the top corners or where a hinge or lock sits.
• Brush or fin seals crushed, worn or pulled out.
• Gaps at the joins, so the seal is not continuous.
• Wrong size or type for the door, often after a DIY repair.
• Seals that no longer touch the door because the gap is too wide.

How long do seals last, and what does replacement cost?

The intumescent material can last as long as the door, but casings and brushes wear. A new set of intumescent and smoke seals typically costs £30 to £150 a door fitted + VAT.

Replace seals like for like, matching the size and type the door was tested with. After new seals go in, check the door still closes and latches, because fresh brush seals add friction. The closer may need adjusting. See fire door closers explained.

Seals are often done alongside other work on the same visit. Repairs usually cost £150 to £900 a door + VAT, depending on the fault and how many doors are done on one visit. See fire door repairs.

We inspect and repair fire doors across Bedfordshire, Hertfordshire, London, the Thames Valley and the East Midlands.

Sources

• GOV.UK: Fire Safety (England) Regulations 2022, fire door guidance (updated 8 August 2025)
• Approved Document B: Fire safety
• BS 8214 code of practice for fire doors (2026 edition, replacing the 2016 version), FPA report
• Regulatory Reform (Fire Safety) Order 2005
• DC Fire & Security: fire door inspection statistics

Updated October 2026 by Sebastian Grabowski, DC Fire & Security. We inspect and repair fire doors across Bedfordshire, Hertfordshire, London, the Thames Valley and the East Midlands. Our own figures: fire door inspection statistics 2026.