What is an intumescent strip?
An intumescent strip is a seal fitted into the edge of a fire door or its frame that expands when heated, closing the gap around the door and holding back fire and smoke.
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<img src="https://fireoptix.co.uk/glossary/intumescent-strip/diagram.svg" alt="intumescent strip — diagram from FireOptix" width="880" height="620" loading="lazy">
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<a href="https://fireoptix.co.uk/glossary/intumescent-strip">What is an intumescent strip?</a> — diagram by
<a href="https://fireoptix.co.uk">FireOptix</a>
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</figure>At normal temperatures it does nothing and looks like a plastic insert. In a fire it swells to many times its thickness, filling the gap the door needs for clearance in everyday use.
Many strips combine the intumescent element with a brush or fin that resists cold smoke before the heat arrives, because smoke moves ahead of fire and is usually what makes an escape route unusable first.
It is easily disabled by ordinary building maintenance — painted over, or damaged while adjusting a door that binds — which is why door inspections look at the strip specifically rather than at the door as a whole.
In practice
A strip that is present, continuous and full of gloss paint.
The door was redecorated with the strip in place, which is quicker than masking it and looks neater.
The material works by expanding, and anything filling or coating it interferes with that in a way nobody can assess visually.
Decorators are not briefed on fire doors, and this is the single most common avoidable defect on a redecorated corridor.
What goes wrong in practice
- Strips painted over during redecoration, restricting how they expand.
- Damaged or missing sections left in place because the door still closes and looks normal.