Last reviewed: October 2026
A Fire Door can look perfectly fine and still be failing its job. Most of that failure happens at the edges, in a thin strip of material most people never think about until a survey flags it.
We're talking about the seals set into the edge of the door or the frame. They're small, they're easy to paint over and they do almost all of the work a Fire Door is certified to do. If you're a facilities manager, landlord or caretaker doing your own checks, here's what each seal type does, how to spot one that's failed and why you can't just swap in whatever strip looks like a close match.
Quick answer: Yes - a Fire Door intumescent strip is part of the certified doorset, not an optional extra. It's heat-activated and inert until a fire starts; the separate Fire Door smoke seal handles the smoke risk from the moment the door closes, and any Fire Door seals replacement has to match the exact type, profile and manufacturer the door was tested with.
An intumescent strip is a heat-activated seal that sits quietly in a groove along the door or frame edge of an FD30 or FD60-rated doorset. At room temperature it does nothing at all. It's inert.
Once a fire reaches a certain temperature, the strip's chemistry changes. It foams and expands, swelling to many times its original thickness. That expansion fills the gap between the door and the frame, blocking the path flame and hot gases would otherwise take.
The exact temperature at which a strip starts to activate varies by product and formulation and manufacturers don't all agree on a single figure. What matters for your purposes isn't the precise number - it's that the strip is designed to work with a specific door, tested at that temperature and nothing else.
Intumescent strips wait for heat. Cold smoke seals aren't heat-activated at all - they're already working the moment the door shuts. That's the core difference between the two seal types fitted to a Fire Door.
A cold smoke seal - often a brush or wiper-pile strip - sits in continuous contact with the door or frame from the moment the door closes. It's a physical barrier, not a chemical one. Its job is to stop cold smoke slipping through the gap long before any heat-activated response could kick in.
This is why the two seal types aren't interchangeable. A door with only an intumescent strip and no smoke seal will still let smoke through in the early minutes of a fire, even once the strip eventually expands.
Many Fire Doors now use a combined strip that carries both functions in a single profile - a smoke-seal brush running alongside an intumescent core. One product, fitted once, does the cold-smoke job immediately and the heat-activated job later.
Combined strips are common enough on FD30 and FD60-rated commercial doors that it's worth checking which type is fitted on your doors before you assume a strip you can see is doing both jobs.
| Seal type | Trigger | Job |
|---|---|---|
| Intumescent strip | Heat (exact threshold varies by product) | Expands to block flame and hot gases once activated |
| Cold smoke seal / brush seal | Door closing (continuous contact) | Physical barrier against cold smoke from the outset |
| Combined strip | Both of the above, in one profile | Cold-smoke barrier immediately, heat-activated expansion later |
Fire Door failure is far more common than most people expect. Three in four Fire Doors inspected failed to meet the required standard, with gaps, smoke-sealing problems and poor maintenance named among the most common causes (Fire Door Inspection Scheme data via Guild of Architectural Ironmongers, 2021).
On a routine walk-round, look for:
Any of these is a reason to get the door looked at properly. Our guide to quick checks you can make on your Fire Doors covers the wider doorset and our piece on what to expect from a Fire Door inspection walks through what a formal survey involves.
This is the part people most often get wrong: assuming a seal that looks the right size is good enough, when the certificate says otherwise.
A Fire Door isn't tested as a leaf on its own. It's tested and certified, as a complete doorset - the leaf, the frame, the hardware and the seals, all together, to a recognised standard such as BS 8214:2026, the current code of practice covering fire-resisting and smoke-control doors (it replaced the timber-only BS 8214:2016 in March 2026). The test evidence is for that exact combination.
That means a replacement seal has to match what the door was tested with - same type, same profile, same manufacturer's specification - not any strip that merely looks the right size. The obvious shortcut - picking up whatever strip looks close enough from a builders' merchant - is exactly what BS 8214:2026 recommends against and exactly what takes the door out of its tested configuration. Fit a different seal and, strictly, you no longer have the tested configuration the door's certificate describes, even if the leaf itself is undamaged.
That gap tends to surface at the worst possible time - an insurer's post-incident investigation, a Fire Risk Assessment review or a tenant dispute - rather than during a routine walk-round. A like-for-like replacement is designed to close that gap before anyone has to find it the hard way.
This article covers the law as it applies in England. BS 8214:2026 recommends this like-for-like approach as best practice; it doesn't carry the force of law. The legal duty sits elsewhere, in the Regulatory Reform (Fire Safety) Order 2005. Article 17 requires the Responsible Person to keep Fire Safety facilities and equipment "in an efficient state, in efficient working order and in good repair" - a general maintenance duty, not a fire-door-specific one. It doesn't name Fire Doors, intumescent strips or smoke seals. That's general regulatory context, not legal advice: your own Fire Risk Assessment or a solicitor, is the right place to confirm what applies to your specific building.
Seal replacement isn't a job for whoever's nearest with a tape measure. It needs someone who knows which seal the door was tested with and who can source and fit the matching replacement.
We're a division of Blake Contractors Ltd, which is BM TRADA Q-Mark certified for Fire Door installation and maintenance - you can check that listing yourself on the certified-company search. It sounds like a small distinction - "the same seal" against "a similar one" - but that wording is exactly what the certificate is built around. It's why we always source the matching product, not the nearest equivalent.
That range runs from a 250-door estate survey to a single front door. When we carried out a Fire Door survey of over 250 communal and flat front doors across two blocks at The Maltings in Sawbridgeworth, the freeholder told Zone Property Management's Fire Safety & Compliance Officer: "The Freeholder mentioned that your company is fantastic and he was really impressed with the service and knowledge of your team on site."
At the other end of the scale, we've fitted a single replacement Fire Door to help one Southend resident look after her disabled brother. Ysanna wrote to us afterwards: "Thank you so much for the certificate and for fitting the new Fire Door. It is a great help to my disabled brother and his support staff. I cannot remember the name of the gentleman who fitted it, however please pass on my compliments as he is so good at his job and a professional gentleman." Big enough to cope with a 250-door block survey, small enough to care about one door for one family - that's the range we work across. You don't need a 250-door estate to justify getting the seal right; it matters exactly as much on the one door protecting a single flat.
Yes - an intumescent strip is part of what makes a Fire Door's certification valid in the first place. A Fire Door isn't tested as a leaf on its own; it's tested and certified as a complete doorset, with the leaf, frame, hardware and seals assessed together against a standard such as BS 8214:2026. The intumescent strip is the heat-activated element in that doorset: it sits inert at room temperature, then foams and expands once a fire reaches the strip's activation temperature, sealing the gap between door and frame. Without it fitted as tested, the doorset no longer matches the configuration its certificate describes, whatever condition the leaf itself is in.
There's no single figure - manufacturers don't all agree on one exact activation temperature, because it depends on the product's specific chemistry and formulation. What matters for a building owner isn't memorising a number; it's that the strip is designed and tested to activate with one particular door, not as a generic component. That's why a replacement has to be the same type and specification the door was originally tested with - a close-looking substitute may activate at the wrong temperature or not perform as the test evidence assumes.
They do different jobs on different timescales. An intumescent strip is chemical and heat-activated - it does nothing until a fire reaches activation temperature, then expands to block the gap. A cold smoke seal, often a brush or wiper-pile strip, is a physical barrier in continuous contact with the door from the moment it closes, stopping cold smoke long before any heat-activated response could kick in. A door fitted with only an intumescent strip and no smoke seal will still let smoke through in a fire's early minutes. Many commercial doors now use a combined strip carrying both functions in one profile - worth checking which type is actually fitted before assuming a visible strip covers both jobs.
No. A Fire Door's test evidence covers an exact combination of leaf, frame, hardware and seal - not a generic "fire-rated" category. BS 8214:2026, the current code of practice for fire-resisting and smoke-control doors, recommends against substituting a similar-looking strip for exactly this reason: fit a different seal and you no longer have the tested configuration the certificate describes, even if the door leaf is undamaged. A proper replacement has to match what the door was tested with - same type, same profile, same manufacturer's specification. That gap between "looks right" and "is tested right" tends to surface during an insurer's post-incident investigation or a Fire Risk Assessment review, not on a routine walk-round.
The legal duty sits in the Regulatory Reform (Fire Safety) Order 2005, not in BS 8214:2026 itself. Article 17 requires the Responsible Person to keep Fire Safety facilities and equipment "in an efficient state, in efficient working order and in good repair" - but this is a general maintenance duty and it doesn't name Fire Doors, intumescent strips or smoke seals specifically. BS 8214:2026 is best-practice guidance on how to meet that kind of duty for doorsets, not a legal requirement in itself. For what applies to your specific building, your own Fire Risk Assessment - or a solicitor - is the right place to confirm the detail, not a general guide like this one.
Look for sections of seal missing, cut short or not running continuously around the door edge; seals that are compressed, perished or crumbling rather than springy to the touch; heavy paint buildup clogging or stiffening the strip beyond the manufacturer's guidance; and gaps at the corners where the seal doesn't meet cleanly or has been damaged by sanding or paint stripper. Three in four Fire Doors inspected failed to meet the required standard, with gaps and smoke-sealing problems among the most common causes (Fire Door Inspection Scheme data via Guild of Architectural Ironmongers, 2021) - so these issues are common, not exceptional. Any of them is reason enough to get the door looked at properly rather than wait for a formal inspection.
If you've spotted a painted-over strip, a gap at the corner or a seal that's gone missing altogether, don't wait for a formal inspection to tell you what you can already see. Get in touch with our contact page and we'll talk you through what a proper check involves and where a like-for-like replacement is needed, it's designed to bring the door back in line with the certificate it was issued with - not a guess at what might fit, whether it's one door or two hundred and fifty. That's the standard we've held from our Southend HQ, family-run since 1979, serving Essex, London and the South East today.
