When an internal wall is removed or a new opening is formed in a load-bearing structure, the loads that the wall was carrying — floor, roof, the wall above — must be transferred to new supports at the sides. The most common solution is a steel beam: strong, predictable, readily available, and accepted by every building control authority in the UK. This guide explains what RSJ beams actually are, how engineers select the right size, what installation involves, and what you should budget in 2026.
What Is an RSJ?
RSJ stands for Rolled Steel Joist — a name that dates from the era when structural sections were manufactured by rolling hot steel billets between shaped rollers. The term has stuck in the trade and among homeowners, even though modern structural sections used in UK building work are now more precisely called Universal Beams (UB) and are manufactured to BS EN 10034.
The cross-section is I-shaped: two horizontal flanges (top and bottom) connected by a vertical web. This shape is mechanically efficient for resisting bending loads — the material is concentrated where bending stresses are highest.
What most people call an RSJ in a domestic context is a UB section used to span an opening left after a wall is removed, or to carry loads over a new doorway, bifold opening, or garage door aperture.
How Engineers Size an RSJ
A structural engineer calculates the required beam section through a process of load analysis. The key inputs are:
- Span — the clear distance between supports (padstones)
- Tributary load — what loads the beam must carry (dead loads: floor, roof, walls above; imposed loads: occupancy, snow on roof)
- Deflection limit — beams must not deflect excessively in service; typically span/360 for floor beams
- Bearing length — how much masonry either side the beam can sit on
From these inputs the engineer calculates the required section modulus and selects the lightest standard section that meets the requirement from the standard range of Universal Beams. In a domestic setting:
| Application | Typical section specified |
|---|---|
| Small doorway / 1.8 m opening, light load | 127 × 76 UB or 152 × 89 UB |
| Internal wall removal, 3–4 m span, 1 floor above | 203 × 102 UB or 203 × 133 UB |
| Through-lounge, 4–5 m span, 2 floors above | 254 × 102 UB or 254 × 146 UB |
| Large rear-wall opening, 5–6 m span | 305 × 102 UB or 305 × 127 UB |
| Loft ridge beam, 4–7 m, roof load | 254 × 146 UB or 305 × 127 UB |
The notation describes the beam’s nominal dimensions: 203 × 102 UB means approximately 203 mm deep and 102 mm wide. Note that these are nominal dimensions; the actual depth and flange width vary by weight per metre within a series.
Padstones: The Critical Detail
At each end of the span, the beam sits on a padstone — a block of dense concrete, engineering brick, or natural stone that distributes the concentrated end reaction load into the supporting masonry without crushing it. This detail is frequently underspecified or cut short on site, with damaging consequences.
The structural engineer will specify:
- Padstone dimensions (plan area and depth)
- Minimum bearing length of beam on padstone (typically 150 mm minimum)
- Padstone material (dense concrete block minimum, e.g. Lignacite or equivalent, minimum 7 N/mm²)
A typical padstone for a domestic RSJ is 215 mm × 215 mm × 100 mm dense concrete block (matching the brick-course module). For heavier loads or weaker masonry, a larger or higher-density padstone is specified. Cost: approximately £25–£60 per padstone.
Steel Beam Supply Cost
RSJ supply prices in 2026 depend primarily on the section weight (kg per metre) and the length required. All steel beam supply costs below are for standard Universal Beam sections, cut to length, ex-merchant:
| Beam section | Weight (kg/m) | Typical length (3.5 m cut) | Supply cost (2026) |
|---|---|---|---|
| 152 × 89 × 16 UB | 16 | 3.5 m | £120–£200 |
| 203 × 102 × 23 UB | 23 | 3.5 m | £160–£280 |
| 203 × 133 × 25 UB | 25 | 3.5 m | £175–£300 |
| 254 × 102 × 25 UB | 25 | 4.5 m | £240–£400 |
| 254 × 146 × 31 UB | 31 | 4.5 m | £300–£480 |
| 305 × 102 × 28 UB | 28 | 5.5 m | £380–£620 |
| 305 × 127 × 42 UB | 42 | 5.5 m | £520–£800 |
Supply costs exclude delivery (typically £60–£150) and any steel fabrication (holes, plates, or welded connections) which are additional. Prices are for standard stock sections; non-standard lengths or sections may attract a premium.
Installation Labour Cost
Installing a domestic RSJ requires temporary propping, the beam lift itself, padstone bedding, and making good. Labour cost varies significantly with beam size and site access:
| Job | Labour cost (2026) |
|---|---|
| Small beam (under 100 kg), 2-person lift, simple propping | £500–£900 |
| Medium beam (100–200 kg), standard domestic wall removal | £900–£1,400 |
| Large beam (200–400 kg), requires chain block or mini-crane | £1,200–£2,000 |
| Complex beam with multiple props and narrow access | £1,500–£2,800 |
These figures are for installation only and do not include making good (plaster, floor, decoration).
Total Cost Summary
| Item | Low | Typical | High |
|---|---|---|---|
| Structural engineer (calcs + drawings) | £400 | £600 | £800 |
| Building control fee | £300 | £350 | £600 |
| Steel beam supply | £160 | £380 | £900 |
| Padstones and mortar | £50 | £120 | £250 |
| Installation labour | £500 | £1,100 | £2,000 |
| Making good (plaster/floor/decor) | £500 | £900 | £1,500 |
| Total | £1,860 | £3,450 | £6,050 |
Building Regulations and Structural Calculations
Any beam installation that involves a load-bearing structure requires Building Regulations approval in England. This is not optional and cannot be retrospectively skipped. Building control requires:
- Structural calculations (from a structural engineer)
- Structural drawings showing beam specification, padstone details, and temporary propping strategy
- A completion inspection to verify the installation matches the approved drawings
Unapproved structural alterations are a known problem in the secondary property market. Solicitors conducting searches routinely identify missing building control sign-off, which can delay or collapse sales. If you are buying a property where a previous owner removed walls without approval, you may need a structural engineer’s retrospective assessment and indemnity insurance.
RSJ for Loft Conversions
Steel beams are also used in loft conversions — primarily as ridge beams (carrying the purlin and rafter loads when the ridge is restructured) and as floor beams (providing the loft floor structural support). In a loft conversion context the beams may be larger and the structural analysis more complex, as loads from the roof are being redistributed. Expect structural engineering fees for a loft conversion to run £800–£2,000 for the full package.
Key Points
- What homeowners call an RSJ is technically a Universal Beam (UB), sized by a structural engineer based on span and loads.
- Padstones at the beam ends are not optional — undersized or absent padstones are a common defect that causes masonry to crack and spall over time.
- Supply cost for a typical domestic beam is £160–£600; labour to install is £500–£2,000; engineer’s fee is £400–£800.
- All beam installations in load-bearing walls require Building Regulations approval — no exceptions.
- Get structural calculations before pricing the beam: different engineers may specify different sections, and the builder needs to know the section before purchasing steel.
