If you’re planning a home renovation or extension in the UK, chances are you’ll hear terms like RSJ, UB, and UC. These all refer to types of steel beams, but the jargon can be confusing. Choosing the right one isn’t about guesswork—it depends on what the beam needs to support and how it fits into the structure. Here’s a straightforward guide to help you understand the differences.
What Is an RSJ?
RSJ stands for Rolled Steel Joist. It’s a traditional term still widely used by builders and homeowners to mean a steel beam in general. Strictly speaking, RSJs are older designs that are less common today, but in everyday language, “RSJ” has become shorthand for any load-bearing steel beam.
So if someone says you need an “RSJ,” they usually mean a steel beam such as a UB or UC.
What Is a UB (Universal Beam)?
A Universal Beam (UB) is the modern version of the joist. It has a characteristic “I” shape when viewed from the end, with wide flanges at the top and bottom and a thinner central web.
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Best for: Carrying loads mainly in one direction (e.g. spanning across an opening).
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Common uses: Supporting walls, floors, and roofs in extensions or when removing a load-bearing wall.
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Advantages: Strong for their weight, efficient at spanning longer distances, widely available in the UK.
In most domestic building projects, the beam specified by the structural engineer will be a UB.
What Is a UC (Universal Column)?
A Universal Column (UC) looks similar to a UB but is deeper and has thicker flanges, making its proportions closer to a square.
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Best for: Vertical loads, where the beam is acting like a column or supporting weight from above.
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Common uses: Pillars, posts, or very short spans that need to resist heavy loads.
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Advantages: Excellent for compression loads, often used alongside UBs in a steel frame.
In some cases, a UC may also be used horizontally if the design calls for greater strength in a more compact section.
RSJ vs UB vs UC: Which One Do You Need?
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RSJ: Everyday term—if you hear it, expect your engineer to actually specify a UB or UC.
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UB (Universal Beam): The go-to choice for most horizontal spans in UK home renovations. Ideal for supporting walls or floors when creating open-plan spaces.
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UC (Universal Column): Used when you need a vertical support or a compact but very strong section for shorter spans.
Why a Structural Engineer Decides
While it’s useful to know the differences, the final choice should never be left to guesswork. A structural engineer will calculate:
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The load the beam must carry (walls, floors, roof, live loads, etc.)
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The span length
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The bearing points and padstones required
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Whether a UB or UC is more efficient for the job
These calculations are required for Building Control approval in the UK, ensuring the installation is safe and compliant.
Final Thoughts
When people talk about “RSJs,” they usually mean UBs or UCs. Universal Beams are the most common choice for spans in extensions and wall removals, while Universal Columns are used where loads are heavy or vertical.
For homeowners, the key is to understand the basics but rely on a structural engineer to specify the right beam. That way, you know the steel chosen is safe, approved, and perfectly suited to your project.
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