RSJ vs Universal Beam: What's the Difference?
"RSJ" and "universal beam" are often used to mean the same thing, but strictly they're different sections. Here's what the terms really mean.
By J&N Steelcraft UK Ltd · 14 August 2026
If you're planning an extension or wall removal you'll hear both "RSJ" and "universal beam", often for the same thing. They're related but not identical — here's the difference in plain terms.
What "RSJ" really means
RSJ stands for Rolled Steel Joist. Strictly, it's a specific I-shaped section with slightly tapered inner flange faces. In everyday building talk, though, "RSJ" has become a catch-all term for pretty much any steel beam used to support a wall — even when the section fitted is actually a universal beam.
What a universal beam is
A universal beam (UB) is the modern I-section with parallel flange faces, which makes connections simpler and gives more strength for its weight. Most structural beams specified today are universal beams (or universal columns for uprights). When your engineer's drawing says something like "203 x 133 UB 25", that's a universal beam.
Which do you need?
You don't choose — your structural engineer does, based on the span and the load. They'll specify the exact section, and that's what gets fabricated. So while it's useful to know the difference, the practical answer is: go by the engineer's drawing.
| Term | Section shape | In practice |
|---|---|---|
| RSJ | Tapered inner flanges | Often used loosely for "any beam" |
| Universal beam (UB) | Parallel flanges | The section usually specified today |
| Universal column (UC) | Squarer, parallel flanges | Used for posts and columns |
The fabrication is the same to us
Whatever the engineer specifies, we fabricate it — cut to length, drilled for the connections, and primed or galvanised. See steel beams & RSJs or, for uprights, steel columns & posts.
Frequently asked questions
Is an RSJ stronger than a universal beam? Not inherently — strength depends on the section size and grade, not the label. For a given weight, modern universal beams are efficient and widely used.
My builder said "RSJ" but the drawing says "UB" — is that a problem? No. "RSJ" is just being used loosely. Fabricate to what the engineer's drawing specifies.
Who decides the size? Your structural engineer, from the span and loads. Send us the drawing and we'll make it.
Need this for a real project?
Tell us what you're planning and our estimating team will advise and quote.
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