Is timber as strong as steel?

Per kg yes (modern engineered timber LVL ~0.5× steel strength but ~0.05× density = better strength-to-weight). Per cross-section, no — steel beam 1/4 size of timber I-joist for same load. Use timber where section depth available; steel where space-constrained.

Does Building Control accept timber for loadbearing?

Yes — Eurocode 5 + BS 5268 cover structural timber. C24 graded softwood + LVL + glulam fully specified for residential. Engineer's calcs + grading certificates required, same process as steel.

What about fire resistance?

Timber chars at 40mm/hour — designed for 30-minute fire resistance with 30mm sacrificial char layer. Intumescent paint + fire-resistant gypsum boards provide additional protection. Steel has 0-minute inherent fire resistance; requires intumescent paint (15–60 min) or board encasement. Practical impact: similar fire engineering required either way.

Timber vs Steel Embodied Carbon London 2026 | Builderr

Timber vs Steel for London Extension Structure — Which Has Lower Embodied Carbon?

Timber I-joist 80 kgCO2e/m³ vs structural steel UB 1,540 kgCO2e/m³ — timber ~19× lower kg-for-kg. Use timber I-joists, glulam, or LVL where spans <5m. Use steel where spans >5m, concentrated loads (chimney removal, knock-through), or punching loads. Hybrid optimal: steel transfer beams + timber I-joists between. Substituting steel-where-possible cuts embodied carbon 20–40% on typical extension.

Carbon factors per material

Structural softwood (C24 graded pine/spruce/larch): 0.42 kgCO2e/kg (90 kgCO2e/m³). Engineered timber: glulam 0.51 kgCO2e/kg; LVL (laminated veneer lumber) 0.56; CLT (cross-laminated timber) 0.44; I-joist 0.40. Includes sequestered CO2 storage during tree growth — net negative if forest sustainably managed (FSC certified). Structural steel UB section: 1.55 kgCO2e/kg + ~7.5kg/m for typical 203×102 UB = ~12 kgCO2e/m length. Same span timber I-joist 240mm depth ~2.5 kgCO2e/m. Steel is 5× higher carbon per linear metre at typical residential span. Reinforced concrete column 0.16 kgCO2e/kg.

Where to use what

Timber suitable: floor joists + ceiling joists (I-joist or solid C24); short-span lintels (<3m clear with point load <40kN); roof rafters; non-loadbearing partitions; loft floor build-ups; ground-floor suspended timber floors. Glulam beams suitable: spans 3–8m with distributed loads; visible feature beams (kitchen island spine, exposed roof structure). LVL/Parallam beams suitable: spans 4–7m, concentrated loads up to ~80kN/m, where timber preferred over steel for connection detail. Steel suitable: ridge beams to roof + supporting bedroom walls above (>5m spans + concentrated loads); knock-through beams across kitchen extensions (typical 5–8m clear span + point loads from chimney/floor above); foundations with concrete piles; cantilever supports. Concrete suitable: foundations, substructure, retaining walls, basement structure.

Hybrid approach + practical considerations

Typical London side-return kitchen extension uses: steel ridge beam (5–6m clear span) supporting joists from first-floor above (~150–200 kgCO2e total steel); timber I-joists below for new flat-roof structure (~60 kgCO2e); steel goalpost frame around bifold opening (~80 kgCO2e); rest timber. Vs all-steel approach (steel ridge + joists + frame): ~600 kgCO2e. Saving ~300 kgCO2e by going timber-where-possible. Structural engineer should be briefed at design stage to design timber-first + only use steel where calc requires. Cost: timber typically similar or slightly cheaper than steel per m run for short spans; steel cheaper for long spans + heavy loads. Build time: timber joists 2× faster install than steel; bolted steel goalposts faster than welded.