What Size Steel Beam Do I Need?
Quick Answer
Domestic openings 2–4m: typically 152×89 UB to 203×133 UB
Must be calculated and specified by a structural engineer for your specific loading conditions
Important: Steel beam sizing must be carried out by a qualified structural engineer. The sizes below are general guidance only and should not be used for design or construction purposes. Every situation has unique loading conditions that require individual calculation.
Common Domestic Beam Sizes (General Guidance)
- 2m span (single storey above): 152 × 89 UB 16 to 178 × 102 UB 19
- 3m span (single storey above): 178 × 102 UB 19 to 203 × 102 UB 23
- 4m span (single storey above): 203 × 133 UB 25 to 254 × 102 UB 28
- 5m+ span: 254 × 146 UB or larger — engineer essential
Factors That Affect Beam Size
- Span: Longer spans need deeper beams
- Load above: Single storey vs two storeys vs roof only
- Load type: Point loads (from columns above) vs distributed loads
- Deflection limits: Beams must not deflect excessively (typically span/360)
- Lateral restraint: Whether the beam is restrained by the floor structure
What You Need for the Engineer
- Clear span of the opening
- What is above: number of floors, roof type
- Floor construction type (timber joists, concrete)
- Joist span and direction
- Any point loads from above (columns, beams)
- Existing wall construction (solid, cavity)
Cost Guide
- Structural engineer calculation: £200–500
- Steel beam supply: £200–800 depending on size
- Installation (builder): £500–2,000 including temporary support
- Padstones: £30–80 each (concrete bearing pads)
- Fire protection: £100–300 (intumescent paint or boarding)
Building Control
All structural alterations require Building Regulations approval. You must submit the structural engineer’s calculations and drawings to your local authority or approved inspector before starting work.
Last updated: April 2026