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What Is an RSJ? — Rolled Steel Joist Explained
A plain-English guide to rolled steel joists: what they are, when you need one, common sizes, and the installation process in UK construction.
Definition
An RSJ (Rolled Steel Joist) is a structural steel beam with an I-shaped or H-shaped cross-section, used to support loads where walls or other structural elements have been removed. In modern UK construction, the term RSJ is commonly used to refer to any steel beam, though technically most beams used today are Universal Beams (UBs) manufactured to BS EN 10025.
When Is It Used?
RSJs are most commonly needed when removing a load-bearing wall to create an open-plan layout, such as knocking through between a kitchen and dining room. They are also used for supporting floor joists over wide spans, creating wide window or door openings, loft conversions where roof structure is altered, and supporting masonry above openings in external walls.
Key Facts
- A structural engineer's calculation is always required to specify the correct beam size
- Common domestic sizes include 152 x 89 UB 16, 203 x 133 UB 25, and 254 x 146 UB 31
- Beams must bear on adequate padstones (typically 225 x 225 x 100 mm concrete) or steel spreader plates
- Minimum bearing length is usually 150 mm at each end
- Fire protection is required in most situations: 30-minute intumescent paint or plasterboard encasement
- Acrow props and needles are used for temporary support during installation
- Building Regulations approval and Building Control inspection are mandatory
- Steel beams are typically primed with red oxide or galvanised for corrosion protection
Related Calculators
Use the Steel Beam Calculator to estimate beam sizes, or the Lintel Sizing Calculator for opening support. The RSJ Installation Calculator helps plan bearing lengths, padstones and temporary support.
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Frequently Asked Questions
Yes. Any structural alteration involving a load-bearing wall requires Building Regulations approval in England and Wales. You will need structural engineer calculations specifying the beam size, padstone details, and bearing requirements. Building Control will inspect the installation before it is covered up.
Technically, RSJ refers to an older I-shaped profile with tapered flanges, while UB (Universal Beam) has parallel flanges and is the modern standard. In everyday UK trade language, "RSJ" is used loosely to describe any steel beam used in domestic construction, even though most beams installed today are actually UBs.
A typical domestic RSJ installation (e.g., removing a load-bearing wall between a kitchen and dining room) costs between £1,200 and £3,500 including the beam, structural engineer fees (£200-£500), Building Control fees, and labour. The steel beam itself typically costs £30-£60 per metre depending on size.
The beam size depends on the load it must carry, not just the span. A structural engineer must calculate this based on the floor loads, roof loads, and wall above. For a typical domestic single-storey load over 4 metres, a 203 x 133 UB 25 or 254 x 146 UB 31 is common, but this must always be verified by calculation.
While there is no legal requirement for a professional installer, RSJ installation involves temporary support (Acrow props), heavy lifting, and precise bearing details. Most councils require a competent builder, and the work must be inspected by Building Control. Incorrect installation can cause structural failure, so professional installation is strongly recommended.
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Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.