RSJ vs Concrete Lintel – Which Is Better for Your Opening?

Choosing between an RSJ steel beam and a concrete lintel is a common decision in construction and renovation projects. Both support loads over openings, but each has distinct advantages, limitations, and ideal applications. This comprehensive guide compares RSJ beams and concrete lintels across all critical factors to help you make the right choice for your 2026 project.

What Are the Key Differences?

RSJ (Rolled Steel Joist)

Material: Structural steel (S275 or S355 grade) Profile: I-shaped cross-section Manufacturing: Hot-rolled in steel mills Typical sizes: 152mm to 457mm depth, various widths Weight: 13-120+ kg/m depending on size

Concrete Lintel

Material: Pre-cast reinforced concrete Profile: Rectangular solid or hollow box section Manufacturing: Cast in molds with internal steel reinforcement Typical sizes: 100-225mm depth, 100-380mm width Weight: 15-80 kg/m depending on size

Load Capacity Comparison

Concrete Lintel Capacity

Standard pre-cast lintel (100×65mm):

• Maximum span: ~1.2m for typical loads
• Suitable for: Single doors, small windows
• Load capacity: 15-25 kN/m

Heavy-duty lintel (150×100mm):

• Maximum span: ~2.5m
• Suitable for: Double doors, medium windows
• Load capacity: 30-50 kN/m

Extra-heavy (215×150mm):

• Maximum span: ~3.5m
• Suitable for: Wide openings, cavity walls
• Load capacity: 50-80 kN/m

Limitation: Pre-cast lintels designed for masonry loads only, NOT floor/roof loads in most cases

RSJ Load Capacity

Small RSJ (203×133×25):

• Maximum span: 4.5m+ for typical loads
• Load capacity: 60-100+ kN/m depending on span
• Supports: Masonry + floor + roof loads

Medium RSJ (254×146×31):

• Maximum span: 6m+
• Load capacity: 80-120+ kN/m
• Heavy-duty residential/commercial

Advantage: RSJ designed to support significant floor/roof loads, not just masonry

When to Use Concrete Lintel

Ideal Applications

1. Standard door/window openings (masonry loads only):

• Single doors (900-1050mm wide)
• Standard windows (up to 2m wide)
• No floor loads bearing on opening
• Cavity wall construction

Example: New window in single-story extension wall with roof truss above (no load transfer to lintel)

2. Light construction:

• Partition walls
• Internal non-load bearing walls
• Garden walls
• Garages without rooms above

3. Matching existing construction:

• Extending existing building with concrete lintels
• Aesthetic/architectural consistency

4. Limited budget projects:

• Concrete lintels significantly cheaper
• Suitable when adequate for loads

Advantages of Concrete Lintels

1. Lower cost:

• Standard 100×65×1200mm lintel: £15-25
• Heavy 150×140×2400mm lintel: £45-80
• Much cheaper than equivalent RSJ

2. Fire resistance:

• Concrete inherently fire-resistant
• No additional fire protection needed
• Excellent for party walls

3. Thermal performance:

• Better insulation than steel
• Reduces thermal bridging
• Important for energy efficiency (Part L compliance)

4. Easier handling for small sizes:

• Lighter than equivalent RSJ for short spans
• No special lifting equipment needed
• One or two people can install small lintels

5. No corrosion concerns:

• Concrete doesn’t rust
• No painting/protection required
• Minimal maintenance

6. Simpler installation:

• Place on mortar bed
• Build masonry over immediately
• No need for padstones typically

Disadvantages of Concrete Lintels

1. Limited load capacity:

• Maximum ~3.5m span practically
• Cannot support significant floor/roof loads
• Not suitable for load-bearing structural openings

2. Heavy for longer spans:

• 3m heavy lintel can weigh 120kg+
• Difficult to maneuver
• May require mechanical lifting

3. Brittle material:

• Can crack if dropped
• Damage during transport common
• Cracks compromise strength

4. Longer spans unavailable:

• Largest stock lintels ~4m
• Custom lintels expensive and slow to procure

5. Deflection issues:

• Concrete creeps under load
• Long-term deflection greater than steel
• Can cause cracking in finishes

When to Use RSJ Steel Beam

Ideal Applications

1. Floor/roof loads over opening:

• Knockthroughs with bedroom above
• Garage headers with room above
• Any opening supporting structural loads

Example: 4m opening in load-bearing wall with floor joists bearing on it

2. Long spans (>3m):

• Wide bi-fold doors (3-6m)
• Large bay windows
• Open-plan living spaces

3. Heavy loads:

• Multiple floors above
• Concrete floors
• Commercial buildings

4. Loft conversions:

• Ridge beams
• Floor support beams
• Structural alterations

5. Minimal depth requirements:

• Limited headroom
• RSJ more efficient (thinner for same capacity)

Advantages of RSJ

1. Superior load capacity:

• 203×133×25 carries 60-100 kN/m easily
• Supports floors, roofs, multiple stories
• Spans 4-8m+ depending on size

2. Longer spans possible:

• Standard beams up to 12m+
• Custom lengths available
• Ideal for wide openings

3. Lighter weight for equivalent capacity:

• 203×133×25 at 4m = 100kg vs. concrete lintel ~150kg
• Easier handling for large spans

4. Predictable performance:

• No creep
• Minimal deflection
• Consistent properties

5. Thinner profile:

• Saves headroom
• Easier to conceal
• Better for tight spaces

6. Versatile:

• Can be welded, drilled, modified
• Custom fabrication possible
• Adapts to complex situations

Disadvantages of RSJ

1. Higher cost:

• 203×133×25 at 4m: £270-350
• vs. concrete lintel: £45-80
• 4-7× more expensive

2. Requires structural engineering:

• Professional design essential
• Building Control approval
• Adds £350-600 to project

3. Corrosion risk:

• Needs protection (paint/galvanize)
• Ongoing maintenance
• Rust in damp environments

4. Thermal bridging:

• Steel conducts heat
• Energy efficiency concern
• Requires careful detailing

5. Fire protection may be required:

• Building Regulations may mandate protection
• Intumescent paint or boxing
• Adds cost and complexity

6. More complex installation:

• Padstones usually required
• Precise positioning critical
• Professional installation recommended

Cost Comparison

Small Opening (1.5m single door)

Concrete lintel:

• 100×65×1800mm lintel: £18-28
• Installation (DIY): £0
• Total: £18-28

RSJ equivalent:

• 152×127×37 at 1.8m: £85-115
• Structural engineer: £250-400 (if required)
• Padstones: £30-50
• Installation: £150-250 (professional)
• Total: £515-815

Winner: Concrete lintel (no floor loads = lintel adequate)

Medium Opening (3m wide, floor above)

Concrete lintel:

• NOT SUITABLE (cannot support floor loads)
• Heavy lintel might span but understrength
• Risk of failure

RSJ:

• 203×133×30 at 3.3m: £220-280
• Structural engineer: £350-550
• Padstones: £60-90
• Installation: £300-500
• Total: £930-1,420

Winner: RSJ (only safe option)

Large Opening (5m, two floors above)

Concrete lintel:

• NOT AVAILABLE in this span
• Definitely cannot support loads

RSJ:

• 305×165×40 at 5.3m: £500-690
• Engineer: £400-650
• Padstones: £80-120
• Installation (crane required): £600-900
• Total: £1,580-2,360

Winner: RSJ (only option)

Structural Requirements

Building Regulations

Concrete lintel:

• Usually acceptable under Building Regs for masonry-only loads
• May still require Building Control approval
• Simpler approval process

RSJ:

• ALWAYS requires structural engineer calculations
• Building Control approval mandatory
• Multiple inspections during installation

Load Calculations

Concrete lintel manufacturers provide:

• Load/span tables
• Safe working load data
• Simple to specify from tables

RSJ requires:

• Full structural calculation
• Load analysis
• Deflection checks
• Professional engineering

Installation Comparison

Concrete Lintel Installation

Steps:

1. Prepare bearing (minimum 150mm each end)
2. Mortar bed (10mm)
3. Lift lintel into position
4. Level and check
5. Build masonry over immediately
6. No waiting time required

Time: 1-2 hours typical

Skill level: Competent DIY possible

RSJ Installation

Steps:

1. Install temporary support (acrow props)
2. Create opening with adequate bearing
3. Install padstones (24-48hr cure)
4. Lift RSJ (may need crane)
5. Position on padstones
6. Make good above
7. Allow curing (7+ days)
8. Remove temporary support

Time: 3-7 days depending on complexity

Skill level: Professional recommended

Thermal Performance

Concrete Lintel

Thermal conductivity: 1.0-1.4 W/mK

Insulated lintels available:

• Cavity trays incorporated
• Insulation blocks integrated
• Good thermal performance

U-value impact: Minimal with proper detailing

RSJ Steel Beam

Thermal conductivity: 50 W/mK (steel)

Major thermal bridge concern

Mitigation required:

• Insulated boxing
• Thermal break materials
• Careful detailing

U-value impact: Significant without mitigation

2026 Building Regulations: More stringent thermal performance requirements make steel detailing more critical

Durability and Maintenance

Concrete Lintel

Lifespan: 60-100+ years Maintenance: None required Failure modes: Rare, usually involves cracks from settlement

RSJ

Lifespan: 100+ years (if protected) Maintenance:

• Repaint every 10-20 years (exposed)
• Inspect periodically for corrosion Failure modes: Corrosion if unprotected

Environmental Considerations

Concrete

Embodied carbon: High (cement production CO2-intensive) Recyclability: Difficult (aggregate recovery only) Sustainability: Improving with low-carbon cements

Steel

Embodied carbon: High initially BUT Recyclability: 100% recyclable indefinitely Sustainability: Better lifecycle assessment if recycled

2026 trend: Both improving with green manufacturing

Decision Matrix

Factor	Concrete Lintel Better	RSJ Better
Cost	✓ (much cheaper)
Load capacity		✓ (far superior)
Span length		✓ (longer possible)
Ease of installation	✓
DIY feasibility	✓
Fire resistance	✓
Thermal performance	✓
Maintenance	✓ (none required)
Durability	Both excellent
Versatility		✓

Conclusion

Use concrete lintel when:

• Opening supports masonry loads only (no floors/roofs bearing)
• Span <3m
• Budget constrained
• DIY installation
• Fire resistance important
• Thermal performance priority

Use RSJ when:

• Any floor/roof loads over opening
• Spans >3m
• Heavy loads
• Limited depth available
• Structural alterations (wall removal, loft conversion)
• Professional installation acceptable

Simple rule: If in doubt whether loads are significant, use RSJ. The cost difference is trivial compared to structural failure risk.

Most common scenario: Small openings (doors/windows) in non-load-bearing situations use concrete lintels. Any load-bearing wall alteration or floor support requires RSJ.

Disclaimer: Load-bearing determinations must be made by a chartered structural engineer. This guide provides general comparison only and does not replace professional engineering assessment or Building Regulations compliance.