Intumescent seals play a vital role in passive fire protection, particularly around fire doors. These seals are designed to expand under heat, filling gaps to prevent the spread of smoke and flames. However, many building owners, caretakers, and decorators don’t realise that something as simple as a fresh coat of paint could render them ineffective.
Whether it’s part of a refurbishment or a routine repainting job, covering intumescent strips with the wrong materials can be a serious fire safety mistake. In this article, we’ll explain what intumescent seals do, why painting them can cause problems, and how to avoid compromising your building’s fire integrity.
What Are Intumescent Seals?
Intumescent seals are narrow strips typically found embedded in the edges of fire doors or in the door frame. When exposed to high temperatures, they expand, often up to 10 or 20 times their original size, to seal gaps around the door.
There are two main types:
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Intumescent-only strips – activate in heat, but do not block smoke until then.
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Combined intumescent and smoke seals – also include brushes or blades to prevent cold smoke from leaking through gaps in early fire stages.
They are a key component of a fire door’s certification and are required by UK Building Regulations and standards such as BS 476-22 and BS EN 1634-1.
Why Painting Intumescent Seals Is a Problem
Intumescent material is sensitive by design. Its performance relies on direct exposure to heat, so anything that creates a barrier between the material and fire can stop it from activating correctly.
Common risks of painting over intumescent seals include:
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Delayed activation – paint layers can act as insulation, slowing the expansion.
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Failure to expand fully – thick or hard-drying paint can restrict the material’s movement.
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Bonding and sticking – paint can cause the seal to adhere to the door leaf or frame, damaging it during operation.
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Unknown compatibility – many paints contain solvents, plastics or oils that can chemically alter the seal over time.
In short, painting intumescent seals can completely undermine their fire-stopping function, which could invalidate the door’s certification and put lives at risk.
Is It Ever Safe to Paint Fire Doors?
Yes, but with caution.
Fire doors can be painted, provided:
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The paint is compatible with fire-rated components.
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Intumescent seals are masked off and left untouched.
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The finish is not excessively thick (e.g. no heavy gloss layers or texture coatings).
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You’re not using intumescent paint unless it’s specifically specified as part of the door’s fire performance.
Always follow the manufacturer’s guidance for painting fire doors. If the original test evidence for the door included a painted finish, it may be acceptable, but only with the same type of paint used in the test.
What If Seals Have Already Been Painted?
If you’ve inherited a building or taken over maintenance and suspect seals have been painted:
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Inspect closely – Look for paint bridging onto or over the strip. If the seal is no longer flexible or appears cracked, sticky, or sealed over, it may have been affected.
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Don’t assume performance – Once compromised, the seal cannot be relied upon to function in a fire.
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Replace if in doubt – It’s often safer and more cost-effective to replace painted seals with fresh, correctly installed strips.
If you’re unsure, ask a fire door inspector or passive fire protection specialist to assess the condition of your doors and seals.
Best Practices for Decorators and Facilities Managers
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Mask seals carefully when painting fire doors or frames.
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Train staff and contractors on what not to paint during maintenance.
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Include seal condition in fire door checks—look for painted, brittle, or damaged seals.
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Keep a record of when doors were repainted and by whom.
Many breaches of fire compartmentation happen not through major alterations, but through minor, routine works like decorating. Make sure your maintenance processes preserve, not compromise, your fire safety.
Conclusion
Painting over intumescent seals might seem like a minor oversight, but it can have serious consequences. These seals are small, often overlooked components of fire protection, but they perform a critical job in the event of a fire.
To stay compliant and keep occupants safe, always inspect seals during maintenance, avoid painting them under any circumstances, and educate those responsible for refurbishment or decoration.
For expert advice on fire protection and prevention measures, contact Martyn Young Fireproofing Consultancy on 07585 896648