Carbon Footprint of European Timber vs Steel vs Concrete

The construction industry accounts for approximately 40% of global carbon emissions, with structural materials — steel, concrete, and timber — representing a significant proportion of a building's embodied carbon. As regulations tighten and clients demand net-zero pathways, understanding the carbon footprint of structural material choices has become essential for architects and engineers.

This guide provides a rigorous, data-driven comparison of the carbon footprint of European structural timber against steel and concrete, with practical guidance for specifiers seeking to minimise whole-life carbon in their projects.

Understanding Embodied Carbon

Embodied carbon refers to the greenhouse gas emissions associated with the extraction, manufacture, transport, construction, maintenance, and end-of-life processing of building materials. It is measured in kilograms of carbon dioxide equivalent (kgCO₂e) and assessed across defined lifecycle stages.

Lifecycle Stages (EN 15978)

For structural materials, modules A1–A3 (cradle-to-gate) typically dominate the embodied carbon profile. However, for timber, the biogenic carbon accounting across all modules — particularly sequestration in A1 and release/avoidance in C and D — fundamentally changes the picture.

Biogenic Carbon: Timber's Unique Advantage

Unlike steel and concrete, timber is a biogenic material — it is grown from atmospheric CO₂ through photosynthesis. Every cubic metre of structural softwood contains approximately 250 kg of carbon, equivalent to approximately 920 kg of CO₂ removed from the atmosphere during tree growth.

The Carbon Cycle in Timber Construction

1. Growth phase: Trees absorb CO₂ from the atmosphere, converting it to cellulose, hemicellulose, and lignin
2. Harvesting: Sustainably managed forests are replanted, beginning a new sequestration cycle
3. Manufacturing: Sawmilling requires relatively low energy input (primarily from biomass residues)
4. Service life: Carbon remains locked in the timber structure for 60–100+ years
5. End of life: Carbon fate depends on disposal route (reuse, recycling, energy recovery, or landfill)

Sequestration Factors by Species

Carbon content calculated as 50% of oven-dry mass. CO₂ equivalent = carbon mass × 3.67 (molecular weight ratio).

Embodied Carbon Comparison: Cradle-to-Gate (A1–A3)

Per Kilogram of Material

Important note: Per-kilogram comparison is misleading for structural design because materials have vastly different densities and structural efficiencies. A functional unit comparison is essential.

Per Functional Unit: Floor Beam Comparison

Comparing materials on a functional unit basis — e.g. a beam spanning 5 metres carrying a domestic floor load of 2.5 kN/m² — provides a meaningful comparison:

Net carbon includes biogenic sequestration for timber options. Steel and concrete have no sequestration benefit.

This functional unit comparison demonstrates that timber solutions are not merely "less bad" than steel or concrete — they are genuinely carbon-negative when biogenic sequestration is included.

Whole-Building Carbon Comparison

Typical 3-Storey Office Building (2,000 m² GIA)

Data based on published case studies and LETI Embodied Carbon Primer benchmarks.

Residential Construction (per dwelling, ~85 m²)

Environmental Product Declarations (EPDs)

EPDs provide the most reliable, product-specific embodied carbon data. They are third-party verified documents following EN 15804+A2 that declare the environmental impact of specific products from specific manufacturers.

Why EPDs Matter for Specifiers

1. Accuracy: Product-specific data rather than generic database averages
2. Comparability: Standardised methodology allows like-for-like comparison
3. Verification: Third-party verified by accredited bodies
4. Regulation: Increasingly required for planning applications and building regulations
5. Rating systems: BREEAM and LEED award credits for EPD-backed assessments

Key EPD Data Points for Timber

When reviewing timber EPDs, focus on:

• GWP-fossil (A1–A3): Emissions from fossil fuel use in production
• GWP-biogenic (A1–A3): Biogenic carbon flows (negative for timber = sequestration)
• GWP-total (A1–A3): Sum of fossil and biogenic (often net negative for timber)
• Module D: Credits for end-of-life reuse, recycling, or energy recovery

Requesting EPDs from Suppliers

When specifying C24 structural timber or BSH glulam, request:

• Product-specific EPD (preferred) or industry-average EPD
• EPD programme operator and registration number
• Declared unit and system boundary (which modules are included)
• Confirmation that the EPD follows EN 15804+A2 (current standard)

Carbon Calculation Methodology

Step-by-Step Whole-Life Carbon Assessment

1. Define functional equivalent: Building type, size, design life, performance requirements
2. Quantify materials: Bill of quantities for structural elements
3. Assign carbon factors: Use EPDs where available, ICE Database for gaps
4. Calculate A1–A3: Material quantities × carbon factors
5. Estimate A4–A5: Transport distances and construction process emissions
6. Assess B1–B7: Maintenance, repair, and replacement over design life
7. Model C1–C4: End-of-life scenarios (demolition, transport, processing)
8. Calculate Module D: Benefits from reuse, recycling, energy recovery
9. Report biogenic carbon: Separately declare sequestered carbon
10. Benchmark: Compare against LETI, RIBA, or WLCA targets

Carbon Factors for Quick Estimates

For early-stage design when detailed EPDs are not yet available:

Regulatory Context

UK Part Z (Proposed)

The proposed Part Z amendment to UK Building Regulations would require whole-life carbon assessments for buildings over 1,000 m². While not yet enacted, it signals the direction of travel and many clients are already voluntarily adopting its principles.

EU Level(s) Framework

The European Commission's Level(s) framework includes indicator 1.2 (Life Cycle Global Warming Potential) as a core sustainability metric. It is increasingly referenced in public procurement and national building regulations across EU member states.

LETI Embodied Carbon Targets

The London Energy Transformation Initiative (LETI) has published embodied carbon targets:

Timber structural systems are one of the most effective strategies for meeting these targets, particularly the 2030 benchmarks which are extremely challenging with steel or concrete frames.

Design Strategies for Minimising Embodied Carbon

Material Selection Hierarchy

1. Reuse existing structure where possible (zero additional embodied carbon)
2. Specify timber as the primary structural material
3. Use recycled steel (EAF production) where timber is not feasible
4. Optimise concrete mixes (GGBS/PFA replacement, lower cement content)
5. Design for disassembly to enable future reuse (Module D benefit)

Timber-Specific Carbon Reduction Strategies

• Specify C24 structural timber from certified sustainable sources (ensures replanting and continued sequestration)
• Use glulam and CLT for longer spans rather than defaulting to steel
• Design connections for disassembly (bolted rather than welded/glued)
• Minimise waste through accurate cutting lists and offcut reuse
• Specify kiln-drying using biomass energy (reduces fossil carbon in A3)
• Consider timber cladding and internal finishes to maximise sequestration volume

Hybrid Structures

Where pure timber is not feasible (e.g. basement structures, high-rise cores), hybrid approaches optimise carbon:

• Timber superstructure on concrete substructure
• Glulam/CLT floors with steel or concrete cores
• Timber roof on masonry/concrete walls
• Timber infill panels in steel or concrete frames

Case Study: Carbon Comparison in Practice

4-Storey Residential Block (24 apartments, 2,100 m² GIA)

Option A: Reinforced Concrete Frame

• Superstructure embodied carbon: 580 tCO₂e
• Carbon intensity: 276 kgCO₂e/m²
• Sequestered carbon: 0 tCO₂e
• Net carbon: +580 tCO₂e

Option B: Glulam Frame + CLT Floors

• Superstructure embodied carbon: 165 tCO₂e
• Carbon intensity: 79 kgCO₂e/m²
• Sequestered carbon: –340 tCO₂e
• Net carbon: –175 tCO₂e

Carbon saving: 755 tCO₂e (equivalent to 190 return flights London–New York)

This demonstrates that material choice alone — without any change to building form, size, or function — can swing a project from significant carbon emitter to net carbon store.

Biogenic Carbon Accounting: The Technical Detail

EN 15804+A2 Approach

The current European standard for EPDs (EN 15804+A2, effective from 2022) requires separate reporting of:

• GWP-fossil: Emissions from fossil fuel combustion and industrial processes
• GWP-biogenic: Carbon flows related to biogenic materials (uptake and release)
• GWP-luluc: Land use and land use change impacts
• GWP-total: Sum of all three components

For timber products, the biogenic carbon accounting works as follows:

The "–1/+1" rule means that biogenic carbon stored in timber is declared as negative in module A1 and positive in module C (when released at end of life). Over the full lifecycle, biogenic carbon is theoretically neutral — but the timing matters. Storing carbon for 60–100 years in a building delays its release, providing a genuine climate benefit during the critical decades of decarbonisation.

Temporary Carbon Storage

The concept of temporary carbon storage recognises that delaying CO₂ emissions has value. A tonne of CO₂ stored in timber for 100 years provides more climate benefit than the same tonne released today, because:

1. It reduces peak atmospheric CO₂ concentration during the critical period
2. It buys time for decarbonisation of other sectors
3. At end of life, the timber can be reused, recycled, or used for energy (displacing fossil fuels)

Some carbon accounting frameworks (e.g. PAS 2080, GHG Protocol) are developing methods to credit temporary storage, though this remains an evolving area.

Communicating Carbon Benefits to Clients

When presenting timber's carbon advantages to clients:

1. Use functional units: Compare beams, floors, or whole buildings — not raw materials
2. Separate fossil and biogenic: Report both GWP-fossil and sequestration clearly
3. Reference standards: Cite EN 15978, EN 15804, and recognised benchmarks (LETI, RIBA)
4. Provide context: Express savings in relatable terms (equivalent car journeys, flights, household emissions)
5. Acknowledge uncertainty: Carbon data has inherent variability — present ranges, not false precision
6. Link to certification: Connect carbon benefits to FSC/PEFC certification ensuring sustainable forest management

Related Resources

• Structural Timber Specification Guide — comprehensive pillar guide
• FSC vs PEFC Certification — ensuring timber comes from sustainably managed forests
• C24 Structural Construction Timber — low-carbon structural softwood
• BSH Glued Laminated Beams — engineered timber for long-span, low-carbon structures
• Passive House Timber Construction — combining low embodied carbon with operational energy efficiency
• Timber Frame vs CLT vs Glulam — structural system comparison