RSJ Beams Explained: Types, Sizes & Common Use Cases for Builders

If you’re working on a structural renovation, extension, or new build, chances are you’ll need an RSJ beam. But what exactly is an RSJ? How do you choose the right type and size? And what are the most common use cases?

In this guide, we’ll break it all down in builder-friendly terms, so you can order with confidence and avoid costly mistakes.

🏗️ What is an RSJ Beam?

RSJ stands for Rolled Steel Joist. These steel beams are widely used in construction to support structural loads — especially when removing load-bearing walls or creating open-plan spaces.

They’re strong, durable, and designed to carry weight horizontally over long spans, making them ideal for both residential and commercial builds.

🔧 Also known as:

• Universal Beams (UBs)
• I-Beams
• H-Beams (depending on the shape and load capacity)

🧱 Common Types of RSJ Beams

There are a few variations of RSJs, each suited to different applications:

1. Universal Beams (UBs)

• Also known as “I-beams” due to their shape.
• Commonly used for horizontal load-bearing.
• Standard for most building projects.

2. Universal Columns (UCs)

• Squarer in shape.
• Used vertically to support loads from above.
• Ideal for columns or upright support frames.

3. Parallel Flange Channels (PFCs)

• C-shaped beams, often used in smaller applications.
• Great for lintels and wall openings.

4. Tapered Flange Beams

• Stronger flange for heavier loads.
• Used in industrial and commercial construction.

📐 RSJ Beam Sizes (And How to Choose)

RSJs come in a wide variety of lengths, depths, and weights. The size you need will depend on:

• The span (how far the beam must stretch)
• The load it must carry
• Whether the load is from above, below, or lateral
• Building control or structural engineer specifications

🔍 Common UK RSJ Sizes Include:

Beam Size (mm)	Weight (kg/m)	Application
152 x 89 x 16	16.1	Small openings, single-storey
203 x 102 x 23	23.0	Medium openings, house extensions
254 x 146 x 31	31.1	Large structural support
305 x 165 x 40	40.0	Commercial and heavy-duty spans

✅ Tip: Always check with a structural engineer before ordering — and don’t forget to factor in any padstones or supports.

🧰 Common Use Cases for RSJ Beams

🏡 Removing Load-Bearing Walls

RSJs are essential for supporting the structure when opening up living spaces or knocking through between rooms.

🧱 Supporting Upper Floors

Whether you’re adding a loft conversion or building a multi-storey extension, RSJs help distribute the load safely down to the foundations.

🚪 Creating Large Openings

For doors, bifolds, and open-plan kitchen-diners, RSJs enable wide, unsupported openings without compromising structure.

🏢 Commercial Fit-Outs

RSJs are used in everything from office refurbishments to warehouse builds — offering reliable, tested structural strength.

🚚 RSJ Delivery & Cutting Services in the South West

At South West Steel Supplies, we stock a wide range of RSJ beams ready for quick delivery across:

• Cheltenham
• Exeter
• Yeovil
• Swindon
• Worcester and more

We offer:

• Cut-to-size service
• On-site collection
• Fast regional delivery
• Expert advice from steel specialists

💬 Need Help Choosing the Right RSJ?

If you’re unsure about the correct size or specification, we’re happy to assist. Our team works closely with builders, architects, and structural engineers to ensure you get exactly what you need — on time and within budget.

📞 Contact South West Steel Supplies today or get a fast quote online.