RSJ vs Steel Beam: What's the Difference?

If you're planning a structural opening or wall removal, you've probably heard terms like "RSJ" and "steel beam" thrown around. But what's the actual difference, and which one do you need?

What is an RSJ?

RSJ stands for Rolled Steel Joist. It's a type of steel beam with an I-shaped cross-section – imagine the letter "I" when you look at it end-on. RSJs have been used in construction for over a century and are still one of the most common solutions for supporting loads above openings.

The "rolled" part refers to how they're made – hot steel is passed through rollers to form the distinctive I-shape. This makes them strong, relatively lightweight, and cost-effective.

RSJ vs Other Steel Beams

Here's where it gets a bit confusing. "Steel beam" is a general term that includes several types:

• RSJ (I-beam) – The classic I-shaped beam, great for most domestic structural openings
• Universal Beam (UB) – Similar to RSJ but with slightly different proportions, now the UK standard
• Universal Column (UC) – Wider flanges, used when you need to support loads from above
• Parallel Flange Channel (PFC) – C-shaped, useful for specific applications

In everyday conversation, most people use "RSJ" and "steel beam" interchangeably. Your structural engineer will specify exactly which type and size you need.

Which One Do You Need?

The honest answer: your structural engineer decides. They'll calculate the loads involved and specify the exact beam type, size, and grade of steel required.

For most domestic wall removals and structural openings, a Universal Beam (the modern equivalent of an RSJ) is typically used. The size depends on:

• The span of the opening (how wide)
• What's above (single storey, two storeys, roof load)
• The type of construction (brick, block, timber frame)
• Point loads from above (other beams, columns)

The Installation Process

Regardless of beam type, the installation process is similar:

1. Temporary support (Acrow props) installed to carry the load
2. Opening formed in the wall
3. Padstones installed at each end to spread the load
4. Steel beam lifted into position
5. Beam packed and secured
6. Props removed once everything is set

Key Takeaways

• RSJ is a type of steel beam – the terms are often used interchangeably
• Your structural engineer specifies exactly what you need
• The beam size matters more than the type for most domestic projects
• Always get proper calculations – never guess beam sizes