A fire door does most of its job at the edges. The leaf and frame matter, but it is the seals running around the perimeter that turn a closed door into a barrier against fire and smoke. They are easy to overlook, easy to damage, and one of the most common reasons a door fails an inspection.
This article explains the two kinds of seal you will find on a fire door, what each one does, and the faults that stop them working.
An intumescent seal is a strip, usually set into a groove in the leaf edge or the frame, that stays inert at normal temperatures. When it gets hot, typically a few hundred degrees, it expands many times its original size. That expansion fills the gap between the leaf and the frame, sealing the joint just as fire begins to find its way through.
Without that seal, even a perfectly hung fire door has a continuous gap around its edge for fire to pass through. The intumescent strip is what closes that gap when it counts.
A smoke seal, sometimes called a cold smoke seal, does a different job. It is usually a brush or a flexible fin that makes contact between the leaf and frame and restricts the movement of smoke at ordinary temperatures, before the fire is hot enough to trigger the intumescent strip.
This is what the S in a rating like FD30S refers to. Smoke is what harms people first in most fires, often well before flame reaches them, so on doors protecting escape routes the smoke seal is as important as the fire resistance. Many products combine both functions in a single strip, with an intumescent core and a brush or fin for smoke.
Seals fail in a small number of predictable ways, and an inspector checks for all of them:
Seals and gaps are two halves of the same story. An intumescent seal is designed to bridge a gap within a certain range. If the gap is too wide, the expanded seal may not reach across it; if the door is binding, the seal may be doing nothing because the door will not close. This is why an inspector looks at the seals and the clearances together, against a standard such as BS 8214.
It also ties back to the door's rating. A door labelled FD30S with painted-over or missing seals is no longer the assembly its label describes, and cannot be relied on to deliver the smoke control the rating claims.
Seals are checked on every inspection in DoorTRACE, with the result and a photo recorded against the specific door. A failed seal becomes a tracked defect with a priority and an owner, and it stays visible until the repair is done and verified. See the inspection checklist it sits within, or book a demo.
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