Timber vs Steel RSJ Beams – Cost, Strength, and Best Use Cases 2026

Choosing between timber and steel beams is a fundamental design decision affecting cost, performance, and long-term durability. Both materials have distinct advantages for different applications. This comprehensive guide compares timber and steel beams across all critical factors to help you make the optimal choice for your 2026 construction project.

Material Properties Comparison

Steel (S275 grade typical)

Strength:

• Yield stress: 275 N/mm²
• Very high strength-to-weight ratio
• Predictable, consistent properties
• No grain direction effects

Density: 7,850 kg/m³ (heavy)

Modulus of Elasticity: 210,000 N/mm² (very stiff)

Durability: 100+ years if protected from corrosion

Fire resistance: Poor (softens at 550°C, fails ~750°C)

Timber (C24 grade typical for structural)

Strength:

• Bending stress: 24 N/mm² (C24 grade)
• ~10× weaker than steel per unit area
• Properties vary by species and grade
• Stronger along grain than across

Density: 420-650 kg/m³ (lightweight)

Modulus of Elasticity: 8,000-12,000 N/mm² (flexible)

Durability: 50-100+ years if protected from moisture/insects

Fire resistance: Better than steel (chars slowly, maintains core strength longer)

Load Capacity Comparison

Example: 4m span, 1m load distribution

Steel (203×133×25 RSJ)

Section properties:

• Depth: 203mm
• Width: 133mm
• Weight: 25 kg/m
• Section modulus: 205 cm³

Capacity: ~80-100 kN at 4m span Deflection: Minimal (~5mm under full load)

Timber (200×75mm C24)

Section properties:

• Depth: 200mm
• Width: 75mm
• Weight: ~7.5 kg/m
• Section modulus: 500 cm³

Capacity: ~15-20 kN at 4m span Deflection: Significant (~15-20mm under full load)

Steel carries 4-5× more load in similar depth!

Equivalent Capacity Comparison

To match 203×133×25 steel RSJ (80 kN capacity at 4m):

Timber requirement:

• 300×100mm C24 beam OR
• DOUBLE 200×75mm beams side by side OR
• Engineered LVL/glulam 240×90mm

Size impact: Timber 2-3× larger cross-section for equivalent capacity

Span Capability

Typical Residential Floor Load (2.5 kN/m, 1.5 kN/m² live load)

Beam Size	Material	Maximum Span
203×133×25	Steel RSJ	5.2m
200×75mm	Timber C24	2.8m
254×146×31	Steel UB	6.5m
300×100mm	Timber C24	4.2m
305×165×40	Steel UB	8.0m+
Glulam 400×140mm	Engineered timber	6.5m

Steel spans 40-60% further than equivalent-size timber

Cost Comparison

Material Costs (2026 UK)

Steel RSJ:

• 203×133×25: £58-75/m
• 254×146×31: £72-95/m
• Weight: 25-31 kg/m

Structural timber:

• 200×75mm C24: £25-38/m
• 300×100mm C24: £55-75/m
• Weight: 7.5-15 kg/m

Per meter: Timber appears cheaper

Equivalent Capacity Cost

For 4m span supporting 80 kN:

Option 1: Steel 203×133×25

• 4.3m @ £66/m = £284
• Total: £284

Option 2: Timber (double 200×75mm)

• 8.6m total (2 × 4.3m) @ £31/m = £267
• Total: £267

Timber marginally cheaper for equivalent capacity

BUT consider:

• Steel single piece (no lateral bracing needed)
• Timber double beam requires connections, spacing, bracing
• Installation complexity higher for timber

Realistic installed costs favor steel for most applications

Installation Costs

Steel beam installation (typical 4m knockthrough):

• Crane/lifting (if >100kg): £100-250
• Padstones: £60-90
• Installation labor: £300-500
• Total install: £460-840

Timber beam installation:

• Manual handling (lighter): £0
• Wall plates: £40-80
• Installation labor: £250-400
• Total install: £290-480

Timber installation ~30% cheaper

Total Project Cost (4m knockthrough)

Steel solution:

• 203×133×25 RSJ: £284
• Installation: £460
• Structural engineer: £400
• Building Control: £280
• Total: £1,424

Timber solution:

• Double 200×75 C24: £267
• Installation: £290
• Structural engineer: £350
• Building Control: £280
• Total: £1,187

Timber ~£240 cheaper (17% saving)

BUT steel provides:

• Slimmer profile (easier to conceal)
• Better future flexibility
• Higher load capacity for changes of use

Durability and Maintenance

Steel

Lifespan: 100+ years if protected

Maintenance required:

• Corrosion protection (paint/galvanize)
• Repainting every 10-20 years (exposed locations)
• Or galvanizing (zero maintenance 50+ years)

Failure modes:

• Corrosion (if unprotected in damp)
• Generally very durable

Timber

Lifespan: 50-100+ years if kept dry

Maintenance required:

• Protection from moisture
• Protection from insects (woodworm, beetles)
• Treatment every 5-10 years (exposed)

Failure modes:

• Rot (if moisture content >20% sustained)
• Insect attack
• Splitting/warping over time

Timber more maintenance-intensive long-term

Environmental Considerations

Steel

Pros:

• 100% recyclable indefinitely
• High recycled content (UK average ~75%)
• Long lifespan amortizes embodied carbon

Cons:

• High embodied carbon (~1.8 tonnes CO2/tonne steel)
• Energy-intensive production
• Mining impacts

Carbon footprint (203×133×25, 4.3m):

• Weight: 107kg
• Embodied carbon: ~193 kg CO2e

Timber

Pros:

• Renewable resource
• Carbon sequestration (stores CO2 absorbed during growth)
• Low energy processing
• Biodegradable end-of-life

Cons:

• Deforestation risk (if not FSC/PEFC certified)
• Lower durability = possible earlier replacement
• Treatment chemicals environmental impact

Carbon footprint (200×75, 4.3m double):

• Weight: ~64kg
• Embodied carbon: ~-50 to +20 kg CO2e (net sequestration if FSC certified)

Verdict: Timber significantly better carbon footprint (~210 kg CO2 difference!)

2026 trend: Sustainability increasingly favors timber for appropriate applications

Fire Performance

Surprising fact: Timber performs better than unprotected steel in fire!

Steel

Behavior:

• Rapidly conducts heat throughout section
• Softens at 550°C
• Loses 50% strength at ~600°C
• Catastrophic failure ~750°C
• Time to failure: 15-30 minutes (unprotected)

Fire protection required:

• Intumescent paint (swells when heated)
• Plasterboard boxing
• Concrete encasement
• Cost: £40-120/m additional

Timber

Behavior:

• Surface chars at predictable rate (~0.7mm/min)
• Char layer insulates inner core
• Core maintains strength
• Fails gradually (warning signs)
• Time to failure: 30-60+ minutes depending on size

Over-sizing provides inherent fire resistance:

• Structural calculation accounts for char depth
• No additional protection often needed
• Cost: £0 (if sized appropriately from start)

Timber advantage in fire (counterintuitive but true!)

Best Use Cases

Use Steel When:

1. Long spans (>5m)

• Steel dramatically more efficient
• Timber becomes impractically large

2. Heavy loads

• Multiple floors above
• Concentrated loads
• Impossible with standard timber

3. Slim/compact profile critical

• Limited headroom
• Desire to minimize boxing depth
• Aesthetic preference for concealed beam

4. Load-bearing alterations

• Knockthroughs with bedroom above
• Garage headers
• Floor support beams

5. Future flexibility desired

• May increase loads later
• Potential change of use
• Steel “over-design” common for adaptability

6. Wet/damp environment (if treated)

• Galvanized steel excellent for damp applications
• Timber risks rot

Example scenarios:

• 5m knockthrough, two floors above: Steel essential
• Balcony support beam (exposed weather): Galvanized steel
• Slim profile concealed lintel: Steel

Use Timber When:

1. Short spans (<4m)

• Timber competitive on cost and performance
• Easier handling

2. Light loads

• Single-story structures
• Roof-only structures (no floor above)
• Timber floor joists (standard application)

3. Environmental priority

• Sustainability critical
• Carbon footprint reduction goal
• Renewable preference

4. Fire resistance important WITHOUT boxing

• Open aesthetic
• Don’t want fire board boxing
• Engineered timber (glulam) excellent

5. DIY installation

• Lighter weight
• Easier manual handling
• Standard carpentry tools/skills

6. Specific architectural aesthetic

• Exposed timber beams (period properties)
• Rustic/farmhouse style
• Feature beams

7. Budget very tight

• Timber can be 15-25% cheaper installed
• If structural requirements permit

Example scenarios:

• 3m opening, single-story garage: Timber viable
• Pergola/garden structure: Timber appropriate
• Oak frame building: Timber for aesthetic
• Loft roof structure (no floor loads): Timber

Hybrid Solutions

Flitch Beams

Concept: Timber-steel-timber sandwich

Advantages:

• Combines timber aesthetics with steel strength
• Better than timber alone, cheaper than steel alone
• Fire performance better than pure steel

Applications:

• Period properties (concealed steel, exposed timber faces)
• Where timber aesthetic desired but pure timber inadequate

Cost: 20-40% more than pure steel (fabrication complexity)

Composite Timber Products

Engineered alternatives to solid timber:

LVL (Laminated Veneer Lumber):

• Stronger than solid timber (~50-80% of steel capacity)
• Consistent properties (no grain defects)
• Cost: ~1.5× solid timber

Glulam (Glued Laminated Timber):

• Large sections possible
• Beautiful appearance if exposed
• Good fire performance
• Cost: ~2× solid timber

CLT (Cross-Laminated Timber):

• Panel product (floors/walls rather than beams)
• Excellent performance
• Cost: Premium but competitive for whole systems

These bridge gap between solid timber and steel

Decision Matrix

Factor	Favor Steel	Favor Timber
Span >5m	✓
Heavy loads	✓
Slim profile needed	✓
Light loads <3m span		✓
Environmental priority		✓
Fire resistance (unboxed)		✓
Budget constrained		✓ (marginal)
DIY installation		✓
Wet environment	✓ (if galvanized)
Durability priority	✓
Aesthetic (exposed beam)		✓ (usually)

Regulatory Considerations

Building Regulations treat both equally for structural purposes

Both require:

• Structural engineer calculations
• Building Control approval
• Proper installation
• Appropriate support

No preference in regulations - engineer specifies most appropriate

Conclusion

Steel and timber beams each excel in different scenarios:

Steel superior for:

• Long spans (>5m)
• Heavy loads
• Slim profiles
• Load-bearing structural alterations
• Ultimate longevity

Timber superior for:

• Short spans (<4m) with light loads
• Environmental sustainability
• Fire resistance without boxing
• DIY projects
• Budget constraints
• Exposed beam aesthetics

Typical costs (4m knockthrough, 80 kN load):

• Steel solution: £1,420 total
• Timber solution: £1,190 total (17% cheaper)
• But steel provides 30-50% more capacity and slimmer profile

For most residential load-bearing alterations (knockthroughs, extensions, loft conversions), steel RSJ beams remain the preferred choice due to superior strength, slimmer profile, and long-term durability, despite slightly higher cost.

Timber competitive for light-duty, short-span applications where sustainability and cost are priorities.

Your structural engineer will recommend optimal material based on your specific loads, spans, and requirements.

Disclaimer: Material selection must be determined by chartered structural engineer based on specific project requirements. This guide provides general comparison only.