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 Load-Bearing Wall Calculator helps assess whether a wall is structural.
How We Calculate This
Frequently Asked Questions
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Last updated: March 2026
All calculations are estimates. Verify with your supplier.