Steel vs Timber Beams — UK Structural Guide

Steel beams (RSJs/UBs) are needed when spans exceed 4-5m, loads are heavy (loadbearing walls above), or headroom is limited (steel is shallower for the same span). Timber beams (glulam, LVL or engineered) work well for spans up to 4-5m and lighter loads. A structural engineer must specify the beam size regardless of material.

A typical steel beam for a domestic opening (152x89 UB, 3-4m long) costs £200-400 for the steel, plus £800-1,500 for installation including padstones, propping, and making good. Total: £1,000-1,900. Timber glulam for a similar span costs £150-300 for the beam plus £600-1,200 installation. Steel is typically 20-30% more expensive overall.

Yes. Steel conducts heat 400 times faster than timber, creating a significant cold bridge in external walls. This causes condensation, mould and heat loss. Steel beams in external walls must be wrapped in insulation (typically 50mm rigid foam). Timber beams have negligible thermal bridging and are preferred in energy-efficient builds.

Steel loses strength rapidly above 300°C and can fail in a fire. In domestic properties, steel beams are usually encased in plasterboard (2 layers of 12.5mm for 30 minutes fire resistance, or 2 layers of 15mm for 60 minutes). Timber beams char slowly at a predictable rate and often perform better in fire than unprotected steel.

Yes. Any structural alteration requires Building Regulations approval (usually a building notice or full plans application). A structural engineer must calculate the beam size. This applies to both steel and timber beams. The Building Control officer will inspect padstones, bearings and fire protection.