Fire Door Seals and Intumescent Strips: A Managing Agent's Guide
Seals are the part of a fire door that most managing agents understand least — and most inspectors find problems with. FDIS data shows that smoke sealing issues account for 34% of fire door failures — one of the most common failure types, alongside excessive gaps and care and maintenance issues.
The gap between "seals are fitted" and "seals are functioning" is wider than most people expect. Paint, age, flattening, and incorrect specification all produce seals that look fine on a quick walk-past but would fail in a fire. This guide covers what the two seal types do, how to assess them during quarterly checks under regulation 10, and what each failure mode means for your buildings.
Scope: This guide applies to multi-occupied residential buildings in England above 11 metres, under the Fire Safety (England) Regulations 2022. Requirements in Wales, Scotland, and Northern Ireland differ.
The Two Seal Types and What They Do
Fire doors typically carry two types of seals — intumescent strips and cold smoke seals — doing different jobs at different points in a fire.
Intumescent strips are heat-activated. The material — usually graphite-based — expands dramatically when exposed to temperatures above roughly 150-200°C, swelling to several times its original width. This expansion seals the gap between the door edge and the frame, stopping fire from passing through. Without the intumescent strip (or with it blocked by paint), the gap remains open as the fire reaches the door.
Cold smoke seals are mechanical, not heat-activated. They look similar to draught excluders — usually a brush strip or soft rubber gasket fitted alongside the intumescent strip. Their job is to prevent cold smoke from passing around the door before the fire is hot enough to activate the intumescent. In a fire, smoke is often the first and primary hazard — the cold smoke seal is the first line of defence while the room is still filling with smoke at relatively lower temperatures.
Not every fire door has both. The government guidance notes that seals are required only "if present" — which reflects the reality that some certified fire doors were designed without seals, relying on other means of compliance. But in practice, most fire doors in residential blocks carry both types, and the absence of seals on a door that should have them is itself a defect.
What the Regulation 10 Check Requires
The government fire door guidance is specific about what you're looking for: intumescent strips and smoke seals, "if present, are undamaged, make contact with the door edge or frame, and have not been painted over during decoration activities."
Three distinct checks:
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Undamaged. The seal should be continuous, with no cracks, missing sections, or crumbling. A seal that's deteriorated won't expand properly — or won't seal the gap mechanically.
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Making contact with the door edge or frame. A brush seal that's lost its spring and no longer touches the door face when closed isn't providing a seal. A rubber gasket that's hardened and pulled away from the door edge has the same problem. "Contact" means active contact in the closed position, not just physical presence.
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Not painted over. This is the most common defect found in older residential blocks. A decorator paints the corridor walls and doors during a refurbishment cycle and coats the intumescent strips in emulsion. The paint locks the seal — preventing it from expanding when exposed to heat. The strip looks intact but is effectively non-functional. Once painted over, simply removing the paint may not be sufficient if the strip has been compressed or deformed by the paint layer.
How to Spot Each Failure Mode
Painted-over seal: The strip will match the surrounding door surface colour if painted over — no slight colour or texture change where the strip meets the edge. In older buildings, look for successive layers of paint filling the groove where the strip sits. In severe cases the strip is flush with the door face rather than slightly proud of it.
Missing sections: Close the door and look along the door edge at eye level. The intumescent strip should be a continuous run around the three sides (both verticals and the top). Any gap in the strip is a defect. Also check the corresponding rebate on the door frame — some fire door assemblies carry the strip in the frame rather than the door, and this is easy to miss.
Flattened brush seal: Open the door and look at the brush or rubber seal. It should spring back when pressed with your finger. If the brush is flat and doesn't recover, or the rubber is rigid and cracked, it's no longer providing a seal. In high-traffic corridors this wears faster — doors that are opened and closed hundreds of times a day compress the brush faster than doors in lower-traffic areas.
Damaged or cracked intumescent strip: Graphite-based intumescent material can crack, particularly in older buildings, as it ages. A cracked strip will still partially activate in a fire but won't expand as fully or as uniformly as an intact strip. Minor cracking is lower priority than missing sections but should be logged and monitored.
The Paint Problem: What to Do
Painted-over intumescent strips are one of the most common findings in older residential blocks, and they're a common source of disagreement between managing agents and decorators.
The typical sequence: a managing agent contracts a decorator to refresh the corridors. The decorator paints over everything, including the fire door seals, because they don't know the strips are functional components — they look like decorative trim. The agent signs off the finished work.
Some remediation options:
- Carefully remove the paint. If the strip is still sound underneath and hasn't been compressed or distorted, removing the paint may restore function. This requires care not to damage the strip while removing the paint. A specialist fire door contractor can assess whether removal is sufficient or replacement is needed.
- Replace the strip. If the strip has been distorted by successive paint layers, or the paint removal risks damaging the substrate, replacement is more reliable. The replacement strip must match the original specification — same material, same dimensions, same expansion ratio — because the fire door certification is based on the original assembly.
- Add this to contractor briefings. The most effective fix is upstream: brief decorators before they start that fire door seals must not be painted, and include this as a condition in any maintenance contracts. A note on the fire door schedule works as a physical reminder.
When Seals Were Never Fitted
Some managing agents discover during a check that fire doors in their buildings have no visible seals at all. This doesn't automatically mean the doors are non-compliant — as noted above, some certified door assemblies don't require intumescent strips.
The question to ask is: were seals part of the original fire door certification? The certification label or plug on the door should identify the manufacturer and type. A fire door specialist can cross-reference the door type against its original certification to confirm whether seals were required.
If seals were required and are missing — for example, they were removed during maintenance and not replaced — that's a defect that needs remediation. If the doors were never certified to carry seals, this should be documented as part of the building safety case information.
Gaps and Seals: Why Both Matter
The seal check and the gap check from our guide to fire door gap requirements are related. Intumescent strips are certified to bridge a specific gap range — they expand to a defined width. If the gap has widened beyond the range the strip was designed for (because the door has dropped on its hinges), the strip's expansion may not fully bridge the gap.
In practice: a door with both excessive gaps and damaged seals needs both problems addressed together, not in sequence. Rehinging the door to correct the gap without replacing damaged seals leaves one defect in place. Replacing seals without addressing the gap means the new seals are working outside their rated range.
The 8 most common fire door defects guide covers how to triage when a door has multiple overlapping issues — including when to escalate to a specialist rather than attempting in-house remediation.
What Records to Keep
For the quarterly and annual regulation 10 checks, you need to record whether seals were assessed and what you found. This doesn't require detailed technical analysis at the regulation 10 level — a pass/fail assessment with a description of any defect found is appropriate for the routine check.
For the annual inspection (flat entrance doors) and for any remediation decision, a more detailed record is useful: which type of seal was assessed, what condition it was found in, and what action was taken or recommended.
Our free fire door inspection checklist includes a seal assessment section with fields for each of the three regulation 10 checks (undamaged, making contact, not painted). The checklist is a printable PDF designed for use on site during quarterly checks.
This is practical guidance for managing agents carrying out regulation 10 checks — not legal advice. Decisions about seal replacement, fire door certification, and compliance remediation should involve a competent contractor. For buildings requiring formal Building Safety Case documentation, consult a qualified fire safety engineer.
Last reviewed: June 2026