Do I Need an RSJ or a UB Beam? Understanding Which Steel Beam Is Right for Your Project

If you’re planning to remove a wall, extend your home, or create an open-plan space, you’ll almost certainly come across the terms RSJ and UB beam. They’re often used interchangeably, but what do they actually mean? And which one do you really need for your project?

This guide explains the difference between an RSJ and a UB, when each is used, and how to choose the right one for your structural work.

What Is an RSJ?

RSJ stands for Rolled Steel Joist — a traditional term that’s been used in UK construction for decades. In practice, it refers to the familiar I-shaped steel beam used to support loads across an opening, such as when removing a load-bearing wall.

Today, “RSJ” is a general term, not a precise specification. It’s what most builders and homeowners call any type of steel support beam, even though modern standards now use different section names like UB (Universal Beam) and UC (Universal Column).

When someone says “install an RSJ,” they usually mean a Universal Beam — but technically, RSJ is just an old-fashioned way of saying the same thing.

What Is a UB Beam?

A UB, or Universal Beam, is the modern name for the same type of steel section once called an RSJ. It’s still I-shaped, with a deep web and narrow flanges, designed to resist bending and carry loads over a horizontal span.

Every UB is designated by three numbers, such as 203 × 133 × 30 UB, meaning:

• 203 mm deep

• 133 mm wide

• 30 kg per metre

Universal Beams are now the standard structural sections specified in engineering drawings and steel supply catalogues. They’re used in everything from small domestic openings to large commercial frames.

RSJ vs UB: What’s the Difference?

While people often think RSJ and UB are different types of beams, in most modern contexts they refer to the same thing. The key differences are mainly in terminology and standardisation: