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

How intumescent strips work

The strip — typically graphite or sodium-silicate compound in a PVC casing, set into a routed groove — stays inert at normal temperatures for decades. In fire, from roughly 200°C, it chars and expands (intumesces) up to ten times its volume, ramming the door-to-frame gap shut and holding the leaf in the frame even as edges char. That expansion is engineered against the 2–4mm gap tolerance: the strip, the gap and the tested door are one system, which is why painted strips, missing sections, or oversized gaps each break the chain.

Why smoke seals matter just as much

Intumescent strips do nothing until serious heat arrives — but cold smoke moves through gaps from the first minutes, and smoke inhalation is the dominant killer in building fires. Smoke seals (brush pile or flexible fins, usually combined into the same strip housing) seal the gap at ambient temperature, keeping corridors and stairs tenable while people escape. The 'S' suffix in FD30S means tested smoke control: required for flat entrance doors and escape-route doors. A fire door without smoke seals in those positions protects the building better than the people.

Inspection and replacement basics

• Check: continuous strip in its groove on top and both sides (or frame equivalent), no paint, no gaps at corners, brush/fins springy not crushed
• Strips are consumables: UV, slamming and vandalism degrade them — replacement is routine and cheap relative to any other fire door work
• Replacement is like-for-like in size and specification: 15×4mm graphite is not interchangeable with whatever's on the van
• Combined intumescent + brush seals are the standard retrofit for doors needing the S rating
• Drop seals handle the under-door gap where smoke control meets uneven floors
• After replacement: re-check gaps and closing — new seals change door friction