What Is an RSJ? UK Steel Beam Meaning Explained
An RSJ is the steel beam builders fit to carry a load when a wall is removed or an opening is formed — the horizontal "I"-shaped section you see spanning a knocked-through kitchen or a new bifold opening. RSJ stands for Rolled Steel Joist, and although the term is now used loosely for almost any structural steel beam, it has a specific meaning worth understanding before you buy. This guide explains what an RSJ is, what the letters stand for, how it differs from a universal beam, and how to work out the size you need.
What Does RSJ Stand For?
RSJ stands for Rolled Steel Joist. The name describes how it is made: the steel is hot-rolled into a joist with an "I" or "H"-shaped cross-section. In everyday building talk, "RSJ" has become a catch-all term for the steel beam used to support a load over an opening — but strictly speaking the original RSJ is a specific section type, now largely superseded by the modern universal beam.
What Is an RSJ Beam?
An RSJ beam is a structural steel section shaped like a capital "I". It has two horizontal flanges top and bottom, joined by a vertical web in the middle. This shape is extremely efficient: the flanges resist the bending forces while the web carries the shear, so the beam supports far more load than a solid bar of the same weight. That efficiency is why steel beams replaced timber for spanning large openings in UK construction.
The original Rolled Steel Joist had tapered flanges — thicker where the flange meets the web and thinner at the edges. Modern sections have parallel flanges of even thickness, which are easier to bolt and connect. Today, the beams sold and specified as "RSJs" are almost always parallel-flange universal beams, defined in BS EN 10365.
RSJ vs Universal Beam (UB) vs Universal Column (UC)
These terms cause endless confusion on site, so here is the plain version:
- RSJ — the traditional Rolled Steel Joist with tapered flanges. Now a colloquial term for any load-bearing steel beam.
- UB (Universal Beam) — the modern parallel-flange beam, deeper than it is wide, optimised to span horizontally. This is what most "RSJs" actually are today.
- UC (Universal Column) — a parallel-flange section roughly as wide as it is deep, optimised to carry vertical load as a post or column.
For a fuller breakdown, see our guide to UB vs UC steel beams and the overview of types of steel beams in UK construction.
Common RSJ Sizes
An RSJ is described by its depth, width and weight per metre — for example a 152×89 UB16 is roughly 152mm deep, 89mm wide and weighs 16kg per metre. Domestic beams commonly run from the compact 127×76 up to heavier 203×133 sections for wider spans and bigger loads:
| Size | Approx. depth × width | Weight | Typical use |
|---|---|---|---|
| 127×76 UB13 | 127 × 76mm | 13 kg/m | Short spans, light loads |
| 152×89 UB16 | 152 × 89mm | 16 kg/m | Typical ~3m domestic opening |
| 178×102 UB19 | 178 × 102mm | 19 kg/m | Wider openings, heavier loads |
| 203×133 UB30 | 203 × 133mm | 30 kg/m | Wide spans, two-storey loads |
These are indicative only — the correct size depends on your span and the load above, and must be confirmed by a structural engineer. Our RSJ size guide lists sizes and weights in more detail.
RSJ vs Lintel: What's the Difference?
A lintel and an RSJ both bridge an opening, but they are not the same thing. A lintel is a smaller, often pre-formed beam — concrete or a light pressed-steel profile — fitted over a standard window or door to carry the masonry directly above it. An RSJ is a larger structural steel beam used where the load is heavier or the span is wider, such as removing a load-bearing wall to knock two rooms together. As a rule of thumb: a standard window or door opening takes a lintel; a knock-through takes an RSJ sized by calculation.
What Is an RSJ Used For in Building?
An RSJ carries the load from the structure above when the wall or support beneath it is removed. Common UK uses include spanning a knocked-through opening between two rooms, supporting the structure above a new patio-door or bifold opening, carrying floor and roof loads in loft conversions and extensions, and replacing a failed timber lintel over a wide span. Any time a load-bearing wall comes out, a correctly sized steel beam goes in to take its place.
What Size RSJ Do I Need?
Beam size depends on the span, the load above it (single wall, wall plus floor, or wall plus roof), and the bearing at each end. A 3m domestic opening might use a 152×89 UB16, while a wider span carrying two storeys needs a deeper, heavier section — but these are illustrations, not specifications. Under Part A of the Building Regulations, any load-bearing beam in the UK must be sized by structural calculation, and most local authorities require those calculations from a structural engineer for building control sign-off.
To get a feel for sizes and weights before you commission calculations, use our RSJ steel beam size guide. If you are taking out a wall, our guide on how to remove a load-bearing wall in the UK walks through the full process, costs and approvals.
Buy RSJ Steel Beams at Rackerman
Rackerman supplies RSJ steel beams cut to size in S355J0 grade — including the 127×76 UB13 and 152×89 UB16 sizes common in domestic projects — in mill finish or red oxide primer. Beams are cut to your exact length and delivered across the UK, typically in 5–10 working days for RSJs, with offload assistance for heavy sections.
Browse the full RSJ steel beams range at Rackerman →
Frequently Asked Questions
What does RSJ stand for?
RSJ stands for Rolled Steel Joist — a steel beam hot-rolled into an "I" or "H" shape. The name is now used loosely for any load-bearing steel beam used over an opening, though the original RSJ was a specific tapered-flange section.
What is an RSJ in building?
In building, an RSJ is the steel beam fitted to carry the load when a load-bearing wall or support is removed — for example over a knocked-through opening or a new doorway. It transfers the weight of the structure above to the supports at each end.
Is an RSJ the same as a universal beam?
Not exactly. A true RSJ has tapered flanges, while a universal beam (UB) has parallel flanges and is the modern standard. In practice, the beams sold today as "RSJs" are usually parallel-flange universal beams, so the terms are used interchangeably on site.
What is the difference between an RSJ and a UC?
A UB or RSJ is deeper than it is wide and is designed to span horizontally as a beam. A UC (universal column) is roughly square in proportion and designed to carry vertical load as a post or column. Use a beam to span an opening and a column to support a point load.
What size RSJ do I need for a 3m span?
A 3m domestic opening is often spanned with a section around 152×89 UB16, but the correct size depends entirely on the load above. Under Part A Building Regulations, the beam must be sized by a structural engineer's calculation — never fit a beam based on a rule of thumb.
Do I need a structural engineer for an RSJ?
Yes, in almost all cases. Removing a load-bearing wall and fitting a steel beam requires structural calculations for building control approval under Part A of the Building Regulations. A structural engineer specifies the beam size, end bearings and any padstones.
What is the difference between an RSJ and a lintel?
A lintel is a smaller beam — often concrete or light pressed steel — used over a standard window or door to carry the masonry directly above. An RSJ is a larger structural steel beam for heavier loads or wider spans, such as removing a load-bearing wall. A window opening takes a lintel; a knock-through takes an RSJ sized by calculation.
What grade of steel are RSJ beams?
Modern RSJ and universal beams are commonly supplied in S355J0 grade — a structural steel with a minimum yield strength of 355 N/mm². Older beams may be S275. Your engineer's calculation will state the grade required.
How much does an RSJ cost?
RSJ cost depends on the section size, length and finish, plus delivery and installation. Steel-only prices for a domestic beam typically run from a low base upward by size; installation is usually the larger cost. See our dedicated RSJ cost guide for current price bands and a cost estimator.
What finishes are available on an RSJ?
RSJ beams are usually supplied in mill finish (untreated steel), red oxide primer (a protective base coat ready for painting), or galvanised for exposed and damp locations. Mill finish suits dry internal use; red oxide is the common domestic choice.