Wood, Steel or Concrete. A Comparison from a Green Perspective

The comparison of wood, steel or Concrete. A comparison from a green perspective requires a methodological framework that simultaneously evaluates environmental and functional indicators for an equivalent functional unit: 1 m2 of usable structural floor area (columns, beams, and floor slab) with equivalent load-bearing capacity (imposed load: 2-5 kN/m2 depending on building typology), a reference study period of 50 years, and the same safety level (ultimate limit state per Eurocodes). Environmental indicators are derived from EPDs prepared per EN 15804+A2: GWP (global warming potential, kgCO2eq), AP (acidification potential, kgSO2eq), EP (eutrophication potential, kgPO4eq), ODP (ozone depletion potential, kgCFC-11eq), PENRT (total non-renewable primary energy, MJ), and ADP (abiotic depletion potential, kgSbeq).

Functional indicators include: fire resistance (R30/R60/R90/R120 per national building regulations), thermal mass (heat storage capacity: kJ/m2K), durability (service life without significant maintenance), recyclability (recycling and reuse rate at end of life), construction speed (m2/day of erected structure), and cost (EUR/m2 of structure). The choice of structural material determines 30-50% of a building's total embodied carbon (modules A1-A5), making it the single design decision with the greatest environmental impact — and the one that must be made earliest in the design process to maximise the potential for optimisation. A rigorous comparison demands that all three materials be assessed for the same structural function, avoiding the common error of comparing unlike quantities.

Structural timber — in its engineered forms: CLT (Cross-Laminated Timber), glulam (glue-laminated timber), and LVL (Laminated Veneer Lumber) — possesses the lowest embodied carbon of the three materials because wood sequesters CO2 during growth (biogenic carbon capture: -1.6 kgCO2/kg of timber). The GWP of CLT for modules A1-A3 ranges from -500 to -700 kgCO2/m3 when biogenic carbon is credited, or from -50 to +50 kgCO2eq/m2 of floor area depending on the accounting methodology (EN 15804+A2 requires separate declaration of biogenic carbon). CLT exhibits a flexural strength of 24-28 MPa, modulus of elasticity of 11,000-13,000 MPa, density of 470-520 kg/m3, and thermal conductivity of 0.12 W/mK.

The fire resistance of CLT is counterintuitive for many practitioners: timber chars at a predictable rate of 0.65 mm/min, forming an insulating carbonised layer that protects the structural core. A CLT panel of 180 mm thickness achieves R90 (90 minutes of fire resistance) without any additional protection — meeting the requirements of most national codes for residential and office buildings. The Mjostaarnet tower (Brumunddal, Norway, 2019: 85.4 m tall, 18 storeys, glulam structure) and the HoHo Wien (Vienna, 2020: 84 m, 24 storeys, hybrid CLT with concrete core) demonstrate the viability of mass timber at height. Limitations: timber requires permanent moisture protection (moisture content below 20% to prevent decay), has lower thermal mass than concrete (85 kJ/m2K for a 200 mm CLT floor versus 250 kJ/m2K for a 200 mm concrete slab), and its availability depends on a sustainable forestry supply chain (certification through FSC or PEFC: 440 million hectares certified globally in 2024). Despite these constraints, the embodied carbon advantage of structural timber — particularly CLT — remains unmatched by any other primary structural material available at commercial scale.

Structural steel has a carbon footprint that varies dramatically with the production route: steel from the blast furnace (BOF — Basic Oxygen Furnace) route emits 2.0-2.5 kgCO2/kg (starting from iron ore plus coke), while steel from the electric arc furnace (EAF) route using above 80% recycled scrap emits 0.3-0.8 kgCO2/kg. For a typical steel structure (weight: 40-80 kg/m2 of usable floor area), the GWP for modules A1-A3 is 70-200 kgCO2eq/m2 (EAF) or 150-300 kgCO2eq/m2 (BOF). Green steel produced using renewable hydrogen (HYBRIT project by SSAB, Sweden) reduces emissions to below 0.1 kgCO2/kg — a 95% reduction — though commercial availability at scale is not anticipated before 2030.

The advantages of steel are: exceptional recyclability (95-98% of structural steel is recycled at end of life — the highest rate of any construction material), direct reuse of bolted sections (90-95% reusable without re-melting, as documented by SCI, 2019), construction speed (a steel structure is erected 2-3 times faster than an in-situ concrete structure), dimensional precision (workshop tolerances of plus or minus 1-2 mm), and fire resistance that can be enhanced through passive protection (intumescent paint: R60-R120 with 1-3 mm thickness) or active systems (sprinklers). Limitations: steel has negligible thermal mass (high thermal conductivity: 50 W/mK), requires corrosion protection (hot-dip galvanising: service life of 50-100 years; paint systems: 15-25 years), and its cost is subject to raw material market volatility (600-1,200 EUR/tonne over 2020-2024, with fluctuations of plus or minus 40%). When evaluating wood, steel or Concrete. from a green perspective, the steel option is distinguished by its end-of-life credentials: near-total recyclability and the potential for direct structural reuse position it favourably in circular economy assessments.

Conventional reinforced concrete has a GWP for modules A1-A3 of 200-350 kgCO2eq/m2 of floor slab (including steel reinforcement). Portland cement is responsible for 85-90% of the concrete's emissions through two mechanisms: clinker production at 1,450 degrees C (thermal emissions) and the chemical decomposition of limestone — calcination releases 0.6-0.9 kgCO2/kg of cement. Decarbonisation strategies for concrete include: substitution of clinker with GGBS (ground granulated blast-furnace slag) at 50-70% replacement (GWP reduction of 40-60%: from 300 to 120-180 kgCO2/m3), use of fly ash at 25-35% replacement (reduction of 20-30%), limestone filler at 15-25% (reduction of 10-15%), LC3 cements (Limestone Calcined Clay Cement: reduction of 30-40%, at industrial scale since 2022 — Scrivener et al., 2018), and geopolymer concrete (alkali activation of slag and fly ash without clinker: reduction of 60-80%, still undergoing standardisation).

The advantages of concrete are: exceptional thermal mass (250 kJ/m2K for a 200 mm slab: thermal lag of 8-12 hours), formal versatility (any geometry achievable with formwork), inherent fire resistance (R120 with 30 mm reinforcement cover, without additional protection), durability in aggressive environments (service life of 50-100 years with appropriate mix design), and generally lower cost (80-150 EUR/m3 of placed concrete). Limitations: concrete has the lowest recyclability of the three materials — 70-80% is recycled as aggregate for fills and sub-bases (downcycling), but below 5% is recycled into new concrete; the demolition process generates dust and noise; and its self-weight (density: 2,400 kg/m3) increases foundation loads and transport emissions. The overall comparison reveals that no single material is universally superior. The optimal choice depends on the climate (concrete's thermal mass is valuable where diurnal temperature amplitude is significant), the building height (CLT timber is optimal up to 8-10 storeys; steel and concrete dominate above that), local availability (proximity of sawmill, steelworks, or concrete batching plant), and the project's carbon targets. A genuinely green perspective demands that each project evaluates these three structural materials against its specific context rather than defaulting to convention.