What is an RSJ Beam and Do I Need One for My Extension?

Title: What is an RSJ Beam and Do I Need One for My Extension or Loft Conversion? Target keyword: RSJ beam extension loft conversion

If you've started researching extensions or loft conversions in London, you've probably come across the term "RSJ beam" — often written into contractor quotes with a price that surprises homeowners. This guide explains what an RSJ is, why it's needed, and what it costs.

What Does RSJ Stand For?

RSJ stands for Rolled Steel Joist. It refers to a structural steel beam with an I-shaped or H-shaped cross-section — the most common type of structural beam used in residential construction. You'll also see the terms "universal beam" (UB) or "universal column" (UC) — these are technically more accurate modern terms for the same kind of section, but RSJ has stuck as the colloquial name in the industry.

Why is an RSJ Needed?

When you create an opening in a wall — for bifold doors in an extension, a new kitchen doorway, or a structural alteration removing part of a load-bearing wall — the wall above the opening can no longer carry the load from the floor, roof, or structure above in the same way as before.

An RSJ is inserted above the opening to span the gap and transfer those loads to the structure on either side of the opening. Without a properly specified RSJ, the structure above would have nothing to bear on, and in the worst case could fail.

In residential construction, RSJs are most commonly needed for: - New large openings in rear walls for bifold or sliding doors (almost always requires an RSJ) - Removing chimney breasts (the floor and roof above need to be supported in a new way) - Creating open-plan spaces by removing internal load-bearing walls - Loft conversions where a new ridge beam or valley beam carries roof loads - Structural lintels above new windows, particularly large picture windows or sliding doors

How is an RSJ Specified?

The size of an RSJ depends on the loads it needs to carry and the span of the opening. A 3-metre bifold door opening in a single-storey rear extension needs a smaller beam than a 6-metre span in a double-storey extension or a loft ridge beam.

This is why a structural engineer is required. They calculate the loads, specify the beam size (expressed as, for example, "203×102×23 UB" — meaning 203mm deep, 102mm wide, 23kg per metre), and prepare structural drawings and calculations that are submitted to Building Control.

Never let a contractor install an RSJ without structural engineer specification and Building Control inspection. An undersized beam can deflect, crack the structure above, or in extreme cases fail.

What Does an RSJ Cost in London?

For a standard residential project:

Scenario	Typical RSJ Cost (supply + install, not including builder's making-good)
Single rear extension bifold opening (3–4m)	£800–£1,800
Large bifold/sliding door opening (4–6m)	£1,500–£3,500
Chimney breast removal (ground or first floor)	£1,000–£2,500 per floor
Loft ridge beam	£1,500–£4,000
Structural wall removal (internal)	£800–£2,000

These figures are for the steel and installation only. Making-good around the beam (plaster, decoration, ceiling) is a separate cost.

What Happens at Building Control?

Your structural engineer's calculations and drawings are submitted to Building Control as part of your full plans application (or reviewed during a site inspection on a building notice). The BCO inspector will check the steel installation at a site inspection — typically at the point when the steel is installed but before it's boxed in. Don't cover it up until the inspector has been.

At RCB, structural steel coordination is part of our standard service. We work with trusted structural engineers, supply and install the steel, and coordinate the Building Control inspection stages.