Fire Safety Regulations for RSJ Steel Beams – UK Building Regulations Guide 2026

Steel RSJ beams, while structurally strong, lose their load-bearing capacity rapidly when exposed to fire. Understanding fire safety regulations and proper fire protection methods for RSJ beams is crucial for Building Regulations compliance and, more importantly, the safety of building occupants.

This comprehensive guide covers fire resistance requirements, protection methods, costs, and compliance strategies for RSJ beam installations in the UK as of 2026.

Why Fire Protection Matters

The Steel Fire Problem:

Unlike concrete or timber, unprotected steel beams have poor fire resistance:

• At 400°C: Steel loses approximately 50% of its strength
• At 550°C: Steel loses approximately 75% of its strength
• At 600°C: Structural failure becomes likely
• Standard building fire: Reaches 800-1000°C within 30 minutes

Consequences of unprotected steel:

• Rapid structural collapse during fire
• Reduced evacuation time for occupants
• Potential for progressive collapse affecting multiple floors
• Insurance claim denial for inadequate fire protection
• Building Regulations non-compliance

Building Regulations Requirements

Part B (Fire Safety)

Building Regulations Approved Document B sets out fire safety requirements including structural fire resistance.

Key principle: Buildings must provide adequate means of escape and structural stability during fire for sufficient time to allow safe evacuation and firefighting operations.

Fire Resistance Periods:

Fire resistance is measured in minutes (e.g., 30, 60, 90) representing how long an element maintains:

1. Load-bearing capacity (R)
2. Integrity – prevents fire spread (E)
3. Insulation – limits temperature rise (I)

For RSJ beams, load-bearing capacity (R) is the primary concern.

Required Fire Resistance by Building Type

Residential Buildings:

Building Height/Type	Required Fire Resistance
Single-storey houses	30 minutes (often exempt if not supporting habitable room)
2-storey houses	30 minutes
3-storey houses	30 minutes (60 minutes if basement conversion)
4+ storey flats	60 minutes
High-rise (>18m)	90-120 minutes

Commercial Buildings:

Building Type	Required Fire Resistance
Small shops/offices (<280m² floor area)	30 minutes
Medium shops/offices	60 minutes
Assembly buildings (>500 people)	60-90 minutes
Industrial/storage	30-90 minutes (depends on use)

Special Cases:

• Basement conversions: Often require 60 minutes minimum
• Listed buildings: May need special fire protection solutions
• Open-plan living: May require higher standards if escape routes affected

When Fire Protection is Required

You MUST provide fire protection if:

• Beam supports floor or roof separating different dwellings
• Beam forms part of protected escape route
• Beam supports habitable room above
• Building Regulations specify fire resistance period >0 minutes
• Building Control inspector requires it based on specific circumstances

You MAY NOT need fire protection if:

• Single-storey structure with no upper floor
• Garage or outbuilding with no habitable space
• Beam does not support habitable rooms
• Alternative protection measures adequate (e.g., separation distance)

Always confirm with Building Control – do not assume exemption.

Fire Protection Methods

1. Intumescent Paint

How it works: Special paint that expands when heated (up to 50× original thickness), creating insulating char layer that protects steel.

Application:

• Applied by brush, roller, or spray
• Typically 2-6mm dry film thickness
• Multiple coats usually required
• Can be overpainted with standard paint after curing

Fire Resistance Achievable:

• Thin film (2-3mm): 30 minutes
• Thick film (4-6mm): 60 minutes
• Ultra-thick film (8-10mm): 90 minutes

Advantages:

• ✅ Minimal impact on beam profile (thin coating)
• ✅ Can achieve aesthetically pleasing finish
• ✅ Suitable for exposed beams in modern interiors
• ✅ Relatively quick to apply
• ✅ Can be DIY with careful application

Disadvantages:

• ❌ More expensive than boarding (£40-80 per m² coverage)
• ❌ Requires smooth, clean surface preparation
• ❌ Must be applied by qualified applicator for certification
• ❌ Regular inspection/maintenance recommended
• ❌ Can be damaged during subsequent work

Cost (2026):

• Materials: £150-300 for typical 4m beam (30-60 min protection)
• Professional application: £200-400
• Total: £350-700

Best for:

• Exposed beams in contemporary designs
• Where boxing would be visually undesirable
• Renovation projects maintaining industrial aesthetic

2. Plasterboard Boxing

How it works: Encasing beam in fire-rated plasterboard creates physical barrier insulating steel from fire.

Specification:

• Use fire-rated plasterboard (pink/red core)
• Minimum 12.5mm thickness (one layer)
• Often 2× 12.5mm layers for 60 minutes
• Must be continuously sealed joints

Fire Resistance Achievable:

• Single 12.5mm layer: 30 minutes
• Double 12.5mm (25mm total): 60 minutes
• Double 15mm (30mm total): 90 minutes

Advantages:

• ✅ Cost-effective (£200-400 including labor)
• ✅ Widely understood by builders
• ✅ Can be plastered and decorated to match ceiling/walls
• ✅ Readily available materials
• ✅ Provides additional acoustic insulation
• ✅ Protects beam from accidental damage

Disadvantages:

• ❌ Reduces ceiling height by 150-300mm typically
• ❌ Hides beam – removes industrial aesthetic option
• ❌ Additional weight on structure
• ❌ Must be carefully detailed at junctions

Cost (2026):

• Materials: £80-150 (plasterboard, battens, screws, plaster)
• Labor: £150-300
• Total: £230-450

Best for:

• Traditional interiors where beam will be hidden
• Cost-conscious projects
• Where ceiling height not critical concern

3. Concrete Encasement

How it works: Surrounding beam in reinforced concrete provides mass insulation and structural protection.

Specification:

• Minimum 25mm concrete cover to all steel surfaces
• Reinforced with steel mesh
• Typical total section: 100-150mm wider than beam

Fire Resistance Achievable:

• 50mm cover: 60 minutes
• 75mm cover: 90 minutes
• 100mm cover: 120+ minutes

Advantages:

• ✅ Excellent fire protection (all durations achievable)
• ✅ Adds to structural capacity
• ✅ Very durable, low maintenance
• ✅ Good for external or exposed locations

Disadvantages:

• ❌ Very heavy – structural implications
• ❌ Expensive (£500-1,200 typical installation)
• ❌ Slow (formwork, curing time)
• ❌ Difficult to apply to existing beams
• ❌ Significant reduction in headroom

Cost (2026):

• Materials: £200-400
• Formwork and labor: £400-900
• Total: £600-1,300

Best for:

• New construction where weight can be accommodated
• External beams requiring weather protection
• High fire resistance requirements (90-120 min)
• Commercial/industrial applications

4. Spray-Applied Fire Protection

How it works: Cementitious or fiber-based material sprayed onto beam surface, creating thick insulating layer.

Types:

• Cementitious: Cement-based mix, sprayed 15-50mm thick
• Fiber: Mineral fiber mix, lighter, 10-40mm thick

Fire Resistance Achievable:

• 20mm thickness: 60 minutes
• 30mm thickness: 90 minutes
• 40mm+ thickness: 120 minutes

Advantages:

• ✅ Fast application for large areas
• ✅ Can achieve very high fire ratings
• ✅ Lighter than concrete
• ✅ Good for complex shapes and connections

Disadvantages:

• ❌ Requires specialist contractor and equipment
• ❌ Poor aesthetic finish (requires overboarding)
• ❌ Can be friable (easily damaged)
• ❌ May contain fibers requiring careful handling
• ❌ Expensive for single beam (£600-1,000)

Cost (2026):

• Only economical for multiple beams or large projects
• Single beam: £600-1,000
• Commercial building typical: £40-60 per m² coverage

Best for:

• Commercial construction with many beams
• Very high fire resistance requirements
• New build where appearance secondary concern

Design Considerations

Section Factor (Hp/A)

Fire protection thickness required depends on the beam’s “section factor”:

Section Factor = Heated Perimeter (Hp) / Cross-sectional Area (A)

Higher section factor = more fire protection needed

Examples:

Beam Size	Section Factor	Protection Required (60 min)
152×127×37	85 m⁻¹	4mm intumescent or 2× plasterboard
203×133×25	110 m⁻¹	5mm intumescent or 2× plasterboard
254×146×31	95 m⁻¹	4.5mm intumescent or 2× plasterboard

Your structural engineer should specify required protection in calculations.

Four-Sided vs. Three-Sided Protection

Four-sided (beam fully exposed):

• All four faces must be protected
• Standard situation for suspended beams
• Higher protection requirements

Three-sided (beam on wall):

• Top face in contact with structure (insulated by concrete, masonry)
• Only three exposed faces need protection
• Slightly lower protection requirements
• Common for lintels in masonry walls

NOTE: Building Control must approve three-sided protection. Don’t assume without checking drawings.

Compliance and Certification

Third-Party Certification

Why it matters:

• Building Control may require certified products/systems
• Insurance companies often specify CERTIFIRE, LPCB, or IFC certification
• Provides evidence of tested fire performance
• Essential for warranty and liability

What to look for:

• Product certification (intumescent paint, boards)
• System certification (complete assembly tested)
• Installation certification (applicator qualified)

Major certification bodies:

• CERTIFIRE (Warrington Fire)
• LPCB (BRE Global)
• IFC (International Fire Consultants)

Installation Requirements

For certification to be valid:

1. Products must be applied as per manufacturer’s instructions
2. Surface preparation must meet specified standards
3. Film thickness must be verified (for intumescent)
4. Installation must be by trained/qualified operatives (commercial work)
5. Inspection records must be maintained

Typical installation steps:

Intumescent Paint:

1. Surface preparation (grit blasting or wire brushing)
2. Prime coat application
3. Build to required dry film thickness (multiple coats)
4. Thickness verification (use wet film thickness gauge)
5. Top coat (if required)
6. Certification/labeling

Plasterboard Boxing:

1. Install timber or metal framework around beam
2. Fix fire-rated plasterboard with specified screw spacing
3. Seal all joints with fire-rated filler
4. Plaster skim and decorate
5. Label installation

Common Mistakes to Avoid

Mistake 1: Assuming No Fire Protection Needed

Problem: Not realizing fire protection is required until Building Control inspection fails.

Solution: Discuss fire requirements with structural engineer and Building Control at design stage.

Mistake 2: Using Standard Paint Instead of Intumescent

Problem: Standard paint looks similar but provides zero fire protection.

Solution: Only use certified intumescent products. Check certification documentation.

Mistake 3: Inadequate Surface Preparation

Problem: Intumescent paint fails to bond or achieve specified thickness.

Solution: Follow manufacturer’s surface preparation requirements exactly. Usually requires clean, rust-free surface.

Mistake 4: Single Layer Plasterboard for 60 Minutes

Problem: One 12.5mm layer only provides 30 minutes protection.

Solution: Check specification. 60 minutes typically requires 2× 12.5mm layers.

Mistake 5: Damaged Protection During Subsequent Work

Problem: Tradespeople damage intumescent coating when running cables or pipes.

Solution: Install services before fire protection, or repair any damage immediately with certified products.

Costs Summary (2026)

Typical 4-meter RSJ beam (203×133×25):

Protection Method	Materials	Labor	Total	Fire Rating
Intumescent (30 min)	£150-250	£150-250	£300-500	30 minutes
Intumescent (60 min)	£250-400	£200-350	£450-750	60 minutes
Plasterboard 1-layer	£80-120	£150-250	£230-370	30 minutes
Plasterboard 2-layer	£120-180	£200-300	£320-480	60 minutes
Concrete encasement	£200-400	£400-900	£600-1,300	90-120 min
Spray-applied	£300-500	£300-500	£600-1,000	60-90 min

Cost per meter of protection: £75-200 typical

Frequently Asked Questions

Q: Do all RSJ beams need fire protection? A: No, but most do in habitable buildings. Building Control will confirm based on your specific situation.

Q: Can I paint over intumescent paint? A: Yes, once fully cured (typically 7-14 days), you can apply standard decorative paint.

Q: How do I know intumescent paint thickness is correct? A: Use a wet film thickness gauge during application, or hire qualified applicator who will certify thickness.

Q: Can I use fire-rated plasterboard from my loft for boxing? A: Only if it’s genuinely fire-rated (pink or red core) and in good condition. Standard white plasterboard is not fire-rated.

Q: What if I discover I need fire protection after beam installation? A: Can be added retrospectively. Intumescent paint is easiest option, or plasterboard boxing.

Conclusion

Fire safety regulations for RSJ beams exist to save lives. While adding fire protection adds cost (£300-800 typical) and complexity, it’s a legal requirement and crucial safety measure.

Key takeaways:

1. ✅ Always discuss fire protection requirements with Building Control at design stage
2. ✅ Budget for fire protection in project costs
3. ✅ Use certified products and follow manufacturer’s instructions exactly
4. ✅ Engage qualified installers for commercial work or certification requirements
5. ✅ Maintain records of products used and installation certificates

Don’t compromise on fire safety – the modest cost of proper fire protection is trivial compared to the potential consequences of fire in an unprotected building.

This guide provides general information on fire safety regulations in England and Wales as of 2026. Always consult Building Control and your structural engineer for specific requirements for your project.