Fire doors are one of the most critical passive fire protection features in buildings. They save lives by containing fire and smoke, protecting escape routes, and buying time for evacuation. Understanding how fire doors work and what makes them effective is essential for building safety.
Do you understand how your fire doors work?
Check your fire door knowledge.
What makes a fire door different?
A fire door is not simply a normal door painted red. It's a carefully engineered assembly designed and tested to resist fire for a specific period.
The complete door assembly
A fire door is a system of components working together:
The door leaf:
- Core filled with fire-resistant materials
- Often includes steel reinforcement
- Typically heavier than standard doors
- Constructed to prevent fire penetration
The frame:
- Must be fire-resistant and compatible with door
- Properly fixed to the building structure
- Forms crucial seal with door leaf
- Often includes integral fire seals
Intumescent strips:
- Chemical strips that expand when heated
- Seal gaps between door and frame
- Prevent flames and hot gases passing through
- Usually fitted in grooves, not surface-mounted
Smoke seals:
- Brush or blade seals
- Prevent cold smoke penetration
- Work before intumescent strips activate
- Critical for FD30s and FD60s ratings
Self-closing device:
- Ensures door closes after use
- Maintains fire protection continuously
- Can be overhead closer, concealed closer, or rising butt hinges
- Must close door fully and latch it shut
Hinges:
- Minimum three hinges (more for heavy doors)
- Steel construction
- Sufficient strength to support door without sagging
- All screw holes filled
Latch:
- Holds door closed when latched
- Minimum 12mm engagement into keep
- Essential for door to remain shut
- Prevents door drifting open
Hardware and fittings:
- Letter plates, door viewers, handles must be fire-rated
- Size restrictions apply
- Must not compromise fire resistance
- Certification required for fittings
Every component matters. A fire door with a missing self-closer, damaged seals, or excessive gaps is no longer a fire door — it's a hole in your fire protection strategy. The system only works when all parts function together.
How fire doors work
Fire doors use multiple mechanisms to resist fire:
Physical barrier
The door leaf itself:
- Dense core materials resist heat penetration
- Multiple layers prevent burn-through
- Thickness and mass provide insulation
- Steel or mineral core stops flames
Typical construction:
- Outer faces: timber veneer or laminate
- Core: chipboard, mineral wool, or steel
- Total thickness: typically 44-54mm
- Weight: significantly heavier than standard doors
Active fire protection: intumescent strips
How intumescent strips work:
- Normal conditions: Thin strip in groove, no effect on door operation
- Heat activation: Around 200°C, chemicals react and strip expands
- Expansion: Strip swells to many times original size (can expand 10-20x)
- Sealing: Expanded material fills gaps between door and frame
- Pressure: Creates positive pressure seal against flames and hot gases
Why they're critical:
- Without intumescent strips, flames and hot gases pass through gaps even if door leaf intact
- Gaps as small as 3mm allow significant fire spread without sealing
- Damaged or missing strips can reduce fire resistance from 30 minutes to just a few minutes
Painted-over intumescent strips don't work properly. Paint can prevent strips from expanding correctly. Never paint over intumescent strips. If they've been painted, they should be replaced.
Cold smoke sealing
How smoke seals work:
Before fire reaches the door:
- Smoke spreads faster than flames
- Cold smoke can kill before fire arrives
- Smoke seals prevent smoke passing through gaps
- Work at normal temperatures (unlike intumescent strips)
Types of smoke seal:
- Brush seals: Flexible brush filaments create seal
- Blade seals: Flexible blade presses against door
- Combined seals: Intumescent and smoke seal in one product
Why smoke control matters:
- Smoke inhalation is the leading cause of fire deaths
- Smoke reduces visibility, hampering escape
- Toxic fumes can incapacitate in seconds
- Smoke-free escape routes save lives
Self-closing mechanism
Why fire doors must close:
A fire door only works when closed. The best fire door in the world provides zero protection if it's open.
How self-closers work:
Overhead closer:
- Hydraulic or mechanical device
- Mounted on door top or frame
- Adjustable closing speed and latching force
- Most reliable type
Concealed closer:
- Hidden in door or frame
- Neater appearance
- Harder to maintain and adjust
- Still effective when working properly
Rising butt hinges:
- Special hinges lift door as it opens
- Gravity pulls door closed
- Less reliable than overhead closers
- Unsuitable for heavy doors
Hold-open devices:
- Electromagnetic devices linked to fire alarm
- Hold door open for convenience
- Release when alarm activates
- Legal way to keep fire doors open
Never prop fire doors open with wedges, fire extinguishers, or any other object. If doors need to be held open, fit proper electromagnetic hold-open devices linked to the fire alarm system. Propping fire doors open is one of the most dangerous and common fire safety breaches.
Fire door ratings explained
Fire doors are rated by how long they resist fire in standardized testing:
FD30 — 30-minute fire door
Fire resistance:
- Resists fire for 30 minutes in standard test
- Prevents flames penetrating through door
- Limits temperature rise on non-fire side
Where used:
- Most common rating for general use
- Internal doors in commercial buildings
- Flat entrance doors (minimum acceptable, FD30s preferred)
- Corridor subdivision doors
- Doors to protected stairways
Real-world meaning:
- Provides 30 minutes for people to escape
- Allows fire service time to arrive and set up
- Prevents fire spreading between compartments
FD60 — 60-minute fire door
Fire resistance:
- Resists fire for 60 minutes in standard test
- Higher level of protection
- Thicker construction and enhanced sealing
Where used:
- Basement access doors
- Plant room doors
- High-risk storage areas
- Separation between different occupancies
- Some doors in complex or tall buildings
Real-world meaning:
- Enhanced protection where higher risk exists
- Longer time for evacuation of large buildings
- Protection of critical infrastructure (plant rooms, service risers)
FD30s and FD60s — with smoke control
The "s" suffix:
- Indicates smoke seals fitted in addition to intumescent strips
- Provides cold smoke control as well as fire resistance
- Preferred for residential premises
Performance:
- FD30s: 30-minute fire door with smoke control
- FD60s: 60-minute fire door with smoke control
- Prevents smoke spread even before fire reaches door
When required:
- Flat entrance doors (strongly recommended/required)
- Doors in sleeping accommodation
- Any location where smoke control is critical
FD30 vs FD30s Fire Doors
FD30 (without smoke seals)
- •30-minute fire resistance
- •Intumescent strips only
- •Prevents flames and hot gases
- •Acceptable for some applications
- •Slightly lower cost
- •Cold smoke can pass through gaps before heat activates intumescent strips
FD30s (with smoke seals)
Recommended- •30-minute fire resistance
- •Intumescent strips plus smoke seals
- •Prevents flames, hot gases, and cold smoke
- •Preferred for residential premises
- •Essential for flat entrance doors
- •Better protection, especially in early stages of fire
Bottom line: For flat entrance doors and residential premises, FD30s (with smoke seals) is strongly preferred and often required. The modest extra cost is justified by significantly better smoke protection, which saves lives.
How fire doors are tested
BS 476-22:1987 standard test:
- Test setup: Door assembly installed in test furnace
- Fire exposure: Subjected to standardized fire with controlled temperature rise
- Integrity test: Door must prevent flames and hot gases penetrating through
- Insulation test: (Where required) Limits temperature rise on non-fire side
- Duration: Test continues for rated period (30 or 60 minutes)
- Pass criteria: No integrity failure during rated period
What's tested:
- The complete door assembly (leaf, frame, seals, hardware)
- As installed, not just the door leaf
- Specific combination of components
Certification:
- Passing test results in certification
- Certification label or plug on door
- Proves door meets stated fire resistance rating
Essential fire door components in detail
Intumescent strips specifications
Size and specification:
- Width: typically 10-15mm
- Thickness: 2-4mm
- Expansion ratio: 10-20 times original thickness
- Activation temperature: typically 200-250°C
Installation:
- Fitted in routed grooves (not surface-mounted)
- Groove depth: typically 8-10mm
- Located on all three closing edges (top, sides)
- Bottom edge usually excluded unless specified
Materials:
- Sodium silicate based (most common)
- Graphite based
- Various proprietary formulations
- Must match door certification
Smoke seal specifications
Types and specifications:
- Brush seals: nylon or polypropylene filaments
- Blade seals: flexible silicone or rubber
- Combined: intumescent and smoke in one product
Installation locations:
- Same locations as intumescent strips
- Often combined in single groove
- Creates seal at normal temperatures
Performance:
- Prevents smoke leakage at standard pressures
- Maintains flexibility over door lifespan
- Doesn't impede door closing
Self-closer specifications
Force requirements:
- Must close door from any angle
- Sufficient force to latch door shut
- Not so strong that vulnerable persons can't open door
- Adjustable closing speed
Types and selection:
Overhead closers (recommended):
- Visible on door top or frame
- Easy to maintain and adjust
- Most reliable performance
- Various sizes for different door weights
Concealed closers:
- Hidden in door edge or frame
- Neater appearance
- Harder to adjust and maintain
- Must be specified at manufacture
Rising butt hinges:
- Special hinges cause door to self-close
- Less reliable than overhead closers
- Can stick or seize with age
- Only suitable for lighter doors
Hinge requirements
Minimum three hinges:
- Standard fire doors: 3 hinges minimum
- Heavy or tall doors: 4+ hinges
- Spacing: evenly distributed
Hinge specifications:
- Steel hinges (not brass or aluminum)
- Typically 100mm grade 13 or higher
- All screw holes must be filled with screws
- Fixed with screws of appropriate length
Why adequate hinges matter:
- Prevent door sagging and dropping
- Maintain correct gap tolerances
- Ensure even seal compression
- Support door weight reliably
Latch requirements
Minimum engagement:
- Latch must enter keep by at least 12mm
- Provides positive hold keeping door closed
- Prevents door drifting open
Types acceptable:
- Mortice latch
- Mortice lock (with latch function)
- Tubular latch
Not acceptable:
- Cylinder lock alone (without latch)
- Magnetic catches
- Roller catches
Glazing requirements
If door has vision panel:
Glass specification:
- Must be fire-resistant glass (not standard glass)
- Certified to same rating as door (FD30/FD60)
- Wired glass or intumescent gel sandwich typically
Size restrictions:
- Usually maximum 25% of door area
- Specific size limits in certification
- Larger panels need higher specification glass
Installation:
- Intumescent glazing tape around perimeter
- Appropriate beading system
- Must follow manufacturer's instructions exactly
Certification:
- Glass should have visible certification marks
- Proves glass meets fire resistance rating
Never replace fire door glass with standard glass. Standard glass shatters quickly in heat, creating a large opening for fire and smoke. This completely defeats the fire door's purpose. Always use certified fire-resistant glazing.
Gap tolerances: why they matter
The 3mm rule
Maximum gap tolerances:
- Top and sides: 3mm maximum
- Bottom: 8mm maximum (often 3mm for smoke control)
Why 3mm?
- Small enough for intumescent strips to seal effectively
- Prevents significant smoke and gas leakage before activation
- Large enough to allow smooth door operation
- Can check with £1 coin (approximately 3mm thick)
Consequences of excessive gaps
Gaps too large (over 3mm):
- Smoke and gases escape before intumescent strips activate
- Reduces effective fire resistance time
- May allow smoke through even after strips activate
- Invalidates fire resistance rating
In fire conditions:
- 4-5mm gap: significant performance reduction
- 6-8mm gap: fire resistance reduced by 50% or more
- 10mm+ gap: door may fail within minutes
Causes of excessive gaps
Door has dropped:
- Worn or inadequate hinges
- Too few hinges
- Door too heavy for hinges
- Solution: replace or add hinges, rehang door
Frame has moved:
- Poor initial fixing
- Building movement
- Solution: refix frame, pack as necessary
Door has warped:
- Moisture damage
- Poor quality door
- Age and use
- Solution: replace door if beyond adjustment
Incorrect installation:
- Poor initial fitting
- Wrong size door for opening
- Solution: rehang or replace door
Checking gap tolerances
Simple £1 coin test:
- Close and latch door
- Try to slide £1 coin (3mm) into gap at various points
- If coin slides in easily, gap may be too large
- Check all around door perimeter
Professional measurement:
- Gap gauges provide accurate measurement
- Professional surveys measure at multiple points
- Record measurements for compliance evidence
Fire door maintenance best practices
Daily/weekly checks (by building users)
Quick visual check:
- Door closes and latches from any position
- Not propped or wedged open
- No obvious damage visible
- Self-closer appears to be working
Takes seconds:
- Part of general premises walk-around
- Anyone can do this
- Helps catch problems early
Monthly inspections (by responsible person)
More detailed check:
- Door closes properly and latches shut
- Self-closer functioning correctly
- No visible damage to door leaf or frame
- Seals appear intact (not damaged or painted over)
- Hinges secure, no screws missing
- Certification label present and legible
- Signage present and legible
- Door not obstructed
Record:
- Date of inspection
- Any issues identified
- Action taken or required
Professional surveys (six-monthly to annually)
Comprehensive inspection by competent person:
Documentation review:
- Certification labels checked
- Previous inspection reports reviewed
Physical inspection:
- Gap measurements at multiple points
- Intumescent strip condition checked
- Smoke seal condition verified
- Self-closer operation tested and adjusted
- Hinge security and condition checked
- Latch engagement measured
- Glazing checked (if present)
- Frame condition assessed
- Overall door condition evaluated
Report:
- Detailed findings for each door
- Prioritized recommendations
- Photographs of defects
- Cost estimates for remediation
Remediation priorities
Immediate action (serious defects):
- Door doesn't close or latch
- Self-closer missing or completely non-functional
- Gaps significantly exceed 3mm (6mm+)
- Significant damage to door leaf
- Seals completely missing or severely damaged
- Door wedged or propped open
Urgent action (within weeks):
- Gaps slightly exceed 3mm (4-5mm)
- Self-closer working but needs adjustment
- Minor damage to seals
- One or more screws missing from hinges
- Latch engagement less than 12mm
Plan and budget (within months):
- Certification label missing (if door otherwise sound)
- Minor cosmetic damage not affecting performance
- Signage faded or damaged
- Doors nearing end of life but currently functional
Common fire door problems and solutions
Problem: Door won't stay closed
Causes:
- Self-closer not working or missing
- Door has dropped and binds on frame
- Latch doesn't engage properly
- Door warped
Solutions:
- Adjust or replace self-closer
- Adjust hinges, add third hinge if only 2 present
- Adjust strike plate or latch position
- Replace door if warped beyond adjustment
Urgency: Immediate — door provides no protection if not closed
Problem: Excessive gaps
Causes:
- Door dropped due to worn hinges
- Frame movement
- Door warped
- Incorrect installation
Solutions:
- Replace or add hinges (minimum 3)
- Repack and refix frame
- Replace door if warped
- Rehang door correctly
Urgency: Urgent if gaps exceed 4-5mm; immediate if over 6mm
Problem: Damaged or missing seals
Causes:
- Paint buildup over seals
- Physical damage
- Age deterioration
- Never fitted (non-compliant door)
Solutions:
- Replace intumescent and smoke seals
- Ensure correct specification for door rating
- Don't paint new seals
Urgency: Urgent — seals are critical to fire door function
Problem: No certification or wrong rating
Causes:
- Non-fire door fitted in fire door location
- Certification label removed or painted over
- Door replaced by tenant/previous owner
Solutions:
- Replace with certified fire door of correct rating
- If door otherwise compliant, may be able to certify retrospectively (specialist assessment needed)
Urgency: Immediate if clearly not fire door; urgent if certification cannot be verified
Problem: Unauthorized modifications
Causes:
- Holes drilled for cables
- Cat flaps fitted
- Non-compliant letter plates
- Door trimmed excessively
Solutions:
- Replace door if significantly compromised
- Repair with fire-stopping materials if minor (specialist advice needed)
- Remove non-compliant fittings
Urgency: Depends on extent — significant modifications require immediate replacement
Grenfell Tower: Fire door failures contributed to tragedy
The Grenfell Tower fire in June 2017 tragically killed 72 people. While rapid external cladding fire spread was the primary issue, fire door deficiencies contributed to the catastrophe.
- ✗Many flat entrance doors were not properly certified fire doors
- ✗Doors had been replaced over years without maintaining fire resistance
- ✗Gaps around doors significantly exceeded 3mm tolerances
- ✗Self-closers missing or not functioning on numerous doors
- ✗No systematic inspection regime for flat entrance doors
- ✗Some doors had unauthorized modifications (letter plates too large)
- ✗Smoke spread through defective doors despite 'stay put' policy
The Fire Safety Act 2021 was introduced specifically to address flat entrance door issues, making building owners explicitly responsible for ensuring flat entrance doors meet required fire resistance standards. The tragedy led to wholesale changes in fire safety regulation and enforcement.
Fire doors in residential buildings are literally life-critical. They must be certified, correctly installed, and properly maintained. The responsible person must know the specification of every flat entrance door and have a systematic inspection regime. The cost of proper fire doors and maintenance is utterly trivial compared to the consequences of failure. Post-Grenfell, enforcement has intensified, but the real lesson is that people's lives depend on these doors working properly.
Frequently asked questions
Look for: certification label or plug (usually on top edge), intumescent strips in door edge or frame grooves, self-closing device, heavier weight than standard doors, and usually at least 3 hinges. If you're unsure, assume it may be a fire door and have it professionally assessed.
Intumescent strips contain chemicals that expand dramatically when heated (typically around 200°C). The strip swells to 10-20 times its original thickness, filling the gap between door and frame. This expanded material creates a pressure seal that prevents flames and hot gases passing through gaps. Without them, even a solid fire door would allow fire spread through gaps.
A fire door only provides protection when it's closed. Self-closers ensure the door closes and latches after every use, maintaining continuous protection. Most fire deaths involve smoke spreading through open fire doors. Self-closers are not optional — they're an essential component of the fire door assembly.
FD30 is a 30-minute fire door with intumescent strips. FD30s is a 30-minute fire door with intumescent strips PLUS smoke seals. The 's' indicates smoke control. FD30s doors prevent cold smoke spread as well as fire spread. FD30s is preferred for flat entrance doors and residential premises because smoke kills faster than fire.
Yes, you can paint fire doors with standard paint. However: don't paint over intumescent strips or smoke seals (prevents proper function), don't paint over certification labels (need to remain legible), and avoid thick paint buildup that might prevent door closing properly or seals expanding. Use thin coats.
Generally no. Cat flaps create holes that compromise fire resistance and are not part of the tested door assembly. Any modification invalidates the door's certification. If you must have a cat flap, you'd need to find a fire-rated cat flap specifically certified for fire doors (rare) or accept that the door is no longer a compliant fire door.
Gaps must not exceed 3mm because larger gaps allow smoke and hot gases to pass through before intumescent strips can seal them. Even after strips activate, large gaps may not seal completely. Gaps over 4-5mm can reduce fire resistance by 50% or more. The 3mm rule (approximately the thickness of a £1 coin) is critical for fire door effectiveness.
Visual checks weekly or monthly, professional fire door surveys every 6-12 months depending on premises type and risk level. High-traffic locations need more frequent checks. After any building work affecting fire doors, or following any fire incident, immediate inspection is required. Your fire risk assessment should specify inspection frequency.
Next steps
Understand your fire door legal requirements:
Fire door requirements and regulations →
Check if you need fire doors:
Learn about fire safety in flats:
Fire safety in blocks of flats →
Concerned about fire door compliance or condition? A qualified fire door inspector can conduct a comprehensive survey of your premises, assess every fire door against British Standards, and provide a prioritized action plan with cost estimates.
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