Intumescent Strips on Fire Doors
— What They Are & Why They Matter

Published 18 April 2026 • DE Fire Compliance • 5 min read

Intumescent strips are one of the most important — and most frequently failed — components on a fire door. They’re also widely misunderstood. Many building managers don’t know what they are until an inspector points one out during a survey.

Here’s everything you need to know about intumescent strips: what they do, how they fail, and what good remediation looks like.

What Is an Intumescent Strip?

An intumescent strip is a seal fitted into a groove in the door leaf or door frame (sometimes both). It is made from a material — typically graphite-based — that expands dramatically when exposed to heat. When a fire occurs, the strip expands to fill the gap between the door and the frame, preventing fire and hot gases from passing through for the door’s rated period (typically 30 or 60 minutes).

In normal use, the strip is barely visible — it sits flush within a rebate on the door or frame. You would not know it was there unless you looked for it. But its presence, condition, and correct installation are critical to the fire door performing as rated.

An intumescent strip in poor condition can expand incompletely or fail to seal gaps, reducing the door’s rated fire resistance from 60 minutes to a fraction of that time.

Intumescent Strips vs Cold Smoke Seals

These two seals are often confused, but they serve different functions:

• Intumescent strip — heat-activated; expands to seal against fire and hot gases. Does not provide cold smoke protection on its own.
• Cold smoke seal — a brush or blade seal that provides a physical barrier against cold (ambient temperature) smoke passing under or around the door. It operates at all times, not just in a fire.

Many modern fire doors use a combined intumescent and cold smoke seal in a single strip — this is often described as an “intumescent strip with a cold smoke seal.” You can identify these by a small brush or wiper element attached to or incorporated within the intumescent strip.

Where doors are installed in high-risk smoke environments — corridors, stairwells, escape routes — both intumescent and cold smoke protection are typically required.

Where Are the Strips Fitted?

Intumescent strips are fitted around three sides of the fire door assembly — the top and both upright sides. They are not fitted to the threshold (bottom) because this is addressed by a separate threshold seal or drop seal.

The strips can be located in:

• A groove in the door leaf itself (most common in modern doors)
• A groove in the door frame (more common in older installations)
• Both the leaf and the frame (used in higher-rated assemblies)

The location matters when inspecting — an inspector checks both the leaf and the frame, and the strip must be continuous, undamaged, and correctly seated throughout its length.

What Fails Inspection?

The most commonly recorded strip failures during fire door inspections are:

• Missing sections — strips ripped out, particularly at corners, by regular door use or impact
• Gaps or joins — a strip that has been cut short or has pulled away at a corner, leaving an unprotected gap
• Swelling or pre-activation — strips that have been exposed to minor heat (steam, sunlight) and partially expanded, compromising their future activation
• Incorrect strip type — a non-rated or wrong-width strip fitted as a repair that does not match the door’s test specification
• Paint over the strip — decorators who paint over intumescent strips seal them in place, preventing activation
• Loose or poorly seated strips — strips that have come loose from their groove and are not securely fixed

Can Intumescent Strips Be Replaced?

Yes — replacing intumescent strips is one of the most common and cost-effective fire door remediation tasks. However, it must be done correctly:

• The replacement strip must be the same type and specification as the original, or as specified in the door’s test evidence
• Strips must be continuous with no joins along straight runs — corner pieces must be mitre-cut correctly
• The strip must be firmly seated in its groove with adhesive backing or mechanical fixings as specified
• After replacement, the door must close fully and the gap tolerances must be maintained

Using a generic replacement strip that is not matched to the door’s specification is a common mistake. If the door holds a third-party certification (e.g. a BWF Certifire mark), the replacement must be compatible with that certification to maintain compliance.

What Does Replacement Cost?

Intumescent strip replacement is typically one of the lower-cost fire door remedials:

• Strip replacement only: £40–£80 per door, materials and labour, for a straightforward swap
• Plus cold smoke seal: Add £20–£40 where a combined or separate smoke seal is also needed

Costs vary by door type, access, and volume. For large portfolios, unit costs reduce significantly. See our full fire door remedial works cost guide for a complete breakdown.

Summary

Intumescent strips are a critical but often overlooked component of a fire door assembly. They must be present, continuous, undamaged, correctly specified, and free from paint or obstructions to function as intended in a fire. Their failure is one of the most common reasons fire doors fail inspection — and fortunately, one of the most straightforward to fix.