Understanding Fire Seals for Fire-Rated Doors
Fire-rated doors are one of the most critical elements in any building’s fire protection strategy — but their effectiveness depends on more than just the door leaf and frame. The fire seals fitted within and around the door assembly play an equally vital role.
At SDS London, we’ve supplied architectural ironmongery and fire-rated door hardware for over 39 years, working closely with joiners, contractors, and specifiers to ensure every component performs as intended. In this guide, we explain exactly what fire and smoke seals do, why they’re required, and how to install and maintain them correctly for full compliance.
What Are Fire and Smoke Seals?
Fire and smoke seals are passive fire protection components designed to prevent the spread of fire, smoke, and hot gases through the gaps between a door and its frame. These gaps are essential for door operation but, without proper sealing, can quickly become pathways for smoke and flames during a fire.
The two main types of door seals
Intumescent Fire Seals
Expand under heat to close gaps and contain flames.
Installed within the door edge or frame.
Usually supplied as a strip with an adhesive backing or fitted into a routed groove.
Smoke Seals
Prevent the passage of cold smoke — before the fire has generated enough heat to activate an intumescent seal.
Typically include flexible brush or rubber fins attached to the intumescent strip.
Essential for life safety, as most fire-related deaths result from smoke inhalation rather than direct contact with flames.
Together, these seals help compartmentalise a building during a fire, giving occupants valuable time to evacuate and reducing the risk of smoke damage.
How Intumescent Seals Work
The word intumescent comes from the Latin “intumescere,” meaning “to swell.” This property is exactly what makes these seals so effective.
Activation process
When exposed to high temperatures — typically around 180–200°C — the intumescent material expands up to 20 times its original size, forming an insulating char that seals the gap around the door. This expansion delays heat transfer and prevents flames and toxic gases from passing through.
Types of intumescent material
Graphite-based seals: Expand rapidly and uniformly, commonly used in modern fire door assemblies.
Sodium silicate or ammonium phosphate: Older materials that also expand but may be more brittle.
Combined intumescent and smoke seals: Feature both expansion material and a brush or fin for complete protection.
Typical locations
Perimeter of door leaf or frame: The most common placement.
Meeting stiles of double doors: Prevents fire spread between leaves.
Around glazing apertures: In fire-rated vision panels.
Key benefit: Once activated, the expanded seal insulates and reinforces the edges of the door, helping to maintain structural integrity for the duration of its fire rating — often 30, 60, or 120 minutes.
Fire Door Regulations and Compliance
UK Building Regulations and the BS 476 / BS EN 1634 standards specify the requirements for fire-resisting doors and their components. Fire and smoke seals form a mandatory part of these assemblies.
Are fire seals mandatory on fire doors?
Yes. All fire-rated doors must be fitted with intumescent seals tested and approved for use with that specific door set. Where doors are also required to control smoke (FD30S, FD60S), a combined fire and smoke seal must be used.
Relevant standards and guidance
BS 476-22 and BS EN 1634-1: Test methods for fire resistance of door assemblies.
BS 476-31.1: Tests for smoke control.
Approved Document B (Fire Safety): Outlines minimum building regulation requirements.
BS 8214:2016: Code of practice for the installation of timber-based fire-resisting doorsets.
BM TRADA Q-Mark / CERTIFIRE: Third-party certification schemes verifying compliance.
Important: Fire doors and their seals must always be installed and maintained as tested — substituting seal types, sizes, or positions can invalidate certification.
Common classifications
FD30 / FD60 / FD90 / FD120: Indicates the number of minutes the door and its seals can resist fire.
FD30S / FD60S: Indicates additional cold-smoke protection through combined seals.
Installing and Maintaining Fire Seals
Proper installation is crucial. Even the highest-quality fire seal cannot perform effectively if it’s incorrectly fitted or damaged.
1. Preparation
Always check the manufacturer’s data sheet for compatibility with the door’s fire rating.
Confirm the correct seal size (commonly 10 x 4 mm or 15 x 4 mm).
Ensure the groove depth and width match the seal specification.
2. Installation methods
Self-adhesive seals: The simplest option for retrofitting — ensure surfaces are clean and dust-free.
Routed seals: Installed into pre-cut grooves for a flush finish, often preferred for new fire door assemblies.
Combined brush seals: Ensure the brush faces the correct direction to maintain contact with the frame.
3. Positioning
Fit seals continuously around the perimeter of the door leaf or frame (excluding the hinge side on some models).
For double doors, include seals at meeting stiles and any rebated sections.
Where fitted with vision panels, use tested intumescent glazing kits to maintain integrity.
4. Maintenance and inspection
Fire and smoke seals should be checked every six months in commercial and communal buildings (or at least annually in domestic settings). Look for:
Damage from wear, shrinkage, or paint build-up.
Gaps or loose sections.
Discoloration or missing brushes/fins.
If any defects are found, seals must be replaced with like-for-like approved components.
Tip: Never paint over intumescent or smoke seals — doing so can prevent them from activating properly during a fire.
Common Issues and Inspection Tips
Even minor damage to fire seals can compromise an entire door’s performance. Knowing what to look for during inspections is essential for compliance and safety.
Common problems
Painted-over seals: A leading cause of failure; the intumescent cannot expand.
Incorrect size or material: Using seals not tested with the specific door leaf invalidates certification.
Gaps or discontinuities: Breaks in the seal line allow smoke to leak through.
Worn brushes: Over time, brushes can fray or detach, reducing smoke protection.
Improper fitting after maintenance work: When hinges or locks are replaced, seals are sometimes damaged or omitted.
Inspection checklist
Are the seals continuous and securely fixed?
Do the brushes or fins make full contact with the frame?
Is the door closing properly against the seals without sticking or resistance?
Are any seals missing, painted, or deteriorated?
Is there a visible certification label on the door and frame?
If any element fails inspection, the fire door should be reported and repaired by a qualified installer.
Maintaining Compliance and Safety
Fire doors are only as reliable as their weakest component. Regular maintenance ensures both legal compliance and long-term building safety.
Schedule routine inspections: Include seals, hinges, closers, and latches.
Train responsible persons: Ensure staff or maintenance teams understand how to identify damaged seals.
Document findings: Keep a log of all inspections, replacements, and remedial works.
Source certified products: Only purchase from recognised suppliers with third-party tested fire protection ranges.
Frequently Asked Questions
What do fire seals do?
Fire seals expand under heat to close the gaps between a fire door and its frame, stopping flames and hot gases from spreading and allowing safe evacuation.
What is the difference between smoke and intumescent seals?
Intumescent seals activate under high heat, while smoke seals (usually brush or fin types) block cold smoke from escaping before temperatures rise. Combined seals provide both levels of protection.
Are fire seals mandatory on fire doors?
Yes. Fire doors without correctly tested and fitted seals will not achieve their rated fire performance and will fail building regulations.
Where to Buy Certified Fire and Smoke Seals
SDS London supplies a wide range of fire-rated intumescent and smoke seals tested for FD30, FD60, and higher-rated door assemblies. Our products meet BS 476 and EN 1634 standards and are available in a variety of finishes to complement architectural hardware.
Whether you’re a joiner, contractor, or building manager, our experienced team can provide guidance on selection, installation, and ongoing maintenance.
