When an internal wall comes down or a large opening is formed, a structural steel beam carries the load above. Structural steel is the standard solution on UK domestic and light-commercial projects: it is strong, stiff, compact, readily available from UK stockholders and straightforward to design to Eurocode 3. What trips up many clients — and some contractors — is understanding the section designations, grades and the secondary costs that turn a £200 beam into a £1,500 installed item.
What ‘RSJ’ Actually Means
RSJ stands for Rolled Steel Joist, a term that dates to the early 20th century and is now technically outdated. The current section types are:
- UB (Universal Beam) — wide-flange I-section optimised for bending. The flanges are nearly parallel and the section is deeper than wide. This is the beam to use for horizontal spans carrying vertical loads.
- UC (Universal Column) — wide-flange H-section, nearly square in profile, optimised for axial compression. Used as columns and posts; also used as short transfer beams where headroom limits depth.
- PFC (Parallel Flange Channel) — C-section, used in pairs (back-to-back or toe-to-toe) for lintels, purlins and trimmer beams where only one flange is accessible.
All are hot-rolled in the UK and Europe to BS EN 10025-2 (non-alloy structural steels) and the relevant product standard BS EN 10034 (UB/UC) or BS EN 10279 (PFC).
Common Section Designations
A UB designation such as 203×133 UB25 means: 203 mm nominal depth × 133 mm flange width, approximately 25 kg per linear metre. The actual depth and flange width vary slightly from the nominal — always use the section tables from the Steel Construction Institute (SCI) P363 publication for exact dimensions.
| Section | Depth (mm) | Width (mm) | Mass (kg/m) | Ix (cm⁴) | Typical domestic use |
|---|---|---|---|---|---|
| 152×89 UB16 | 152.4 | 88.7 | 16.0 | 834 | Small domestic openings, spans ≤2.5 m, light load |
| 178×102 UB19 | 177.8 | 101.2 | 19.0 | 1360 | Openings 2.5–3.5 m, single storey above |
| 203×133 UB25 | 203.2 | 133.2 | 25.1 | 2340 | Standard domestic knocked-through opening |
| 254×146 UB37 | 256.0 | 146.4 | 37.0 | 5540 | Larger spans or two-storey load |
| 305×165 UB40 | 303.4 | 165.0 | 40.3 | 8500 | Long spans (≥5 m) or heavy floor loads |
| 305×165 UB54 | 310.4 | 166.9 | 54.0 | 11700 | Heavy-duty domestic or light-commercial |
For UC sections used as posts: a 152×152 UC23 (23 kg/m) is a common slim column for domestic steelwork; a 203×203 UC46 suits heavier loads.
Steel Grades: S275 vs S355
The grade number denotes the characteristic yield strength in N/mm²:
- S275 — yield strength 275 N/mm² (for thicknesses ≤16 mm). The most widely stocked grade; adequate for the majority of domestic applications.
- S355 — yield strength 355 N/mm². Specifying S355 allows a smaller (lighter) section for the same load, reducing depth and self-weight. Premium of roughly 5–10% on material cost, but can save headroom where depth is tight.
In practice, for domestic work the structural engineer will specify S275 as default unless headroom or aesthetics require a shallower section. S355 is more commonly specified on commercial and industrial frames.
Cost Per Metre
Steel beam pricing fluctuates with global scrap and energy prices. As of mid-2026, typical UK stockholder ex-works prices for cut-to-length sections in S275:
| Section | Mass (kg/m) | Supply cost/m (approx.) |
|---|---|---|
| 152×89 UB16 | 16.0 | £30–£45 |
| 178×102 UB19 | 19.0 | £36–£55 |
| 203×133 UB25 | 25.1 | £48–£72 |
| 254×146 UB37 | 37.0 | £70–£105 |
| 305×165 UB40 | 40.3 | £76–£115 |
| 305×165 UB54 | 54.0 | £100–£155 |
These are material-only prices. Additional costs to budget:
- Shot-blasting and priming: £1.50–£3.00/kg (required before painting or fire protection)
- Fabrication (holes, end plates, stiffeners): £500–£1,500 per beam depending on complexity
- Hot-dip galvanising (for exposed or wet locations): £300–£600 fixed + £0.80–£1.50/kg
- Intumescent paint (fire protection to 30 or 60 minutes): £20–£60/m depending on section and protection period
- Structural engineer’s design fee: £400–£900 for a single domestic steel
- Installation labour (crane hire, propping, making good): £600–£2,000+ per beam
Padstones
Where a steel beam bears on masonry, the concentrated end reaction must be spread over enough bearing area to avoid overstressing the masonry below. A padstone — typically a dense-concrete block or engineering brick — is bedded on the masonry before the beam is lowered on.
Common padstone sizes for domestic steels:
- 215 × 140 × 100 mm (one engineering brick wide) — for light-to-medium beams
- 215 × 215 × 100 mm — for heavier beams or weaker masonry (e.g. lightweight aggregate blocks)
- 440 × 215 × 100 mm — where the beam span or load is large and the wall is single-leaf
The structural engineer’s calculation will specify the minimum padstone bearing length. A minimum bearing of 100 mm is commonly specified but this must be confirmed by calculation against the actual reaction force and masonry compressive strength.
Temporary Works and Installation
Before inserting any steel into a load-bearing wall, the wall above must be propped with adjustable steel acrow props on scaffold boards to distribute the load. Props are set at no more than 1.5 m from the opening and at least 1.5 m from each other. The propping scheme should be confirmed with the structural engineer; if the floor above is also loaded, propping must continue through multiple storeys.
Most domestic steel beams require a beam lifter or two-person lift; anything above approximately 300 kg (a 305 UB54 at 5 m is around 270 kg) will need a Hiab or electric chain block. Planning for crane access or temporary lifting points is essential before the beam is ordered.
Relevant Standards and Guidance
- BS EN 10025-2: Structural steel — non-alloy steels
- BS EN 1993-1-1 (Eurocode 3): Design of steel structures
- SCI P363: Steel Section Properties and Member Resistances (the designer’s reference)
- Approved Document A (Structure): Building Regulations structural requirements
- BS 5950-1 (superseded by EC3 for new design but still referenced for existing buildings)
For residential alterations, a structural engineer must calculate and sign off the beam. The Building Control surveyor will check that design calculations are on file before issuing the completion certificate.
