The short answer
An RSJ (rolled steel joist) is the steel beam fitted over an opening to carry the load that a removed wall used to support. When you take out a load-bearing wall, the floors, walls or roof above need a new path to the ground — the beam spans the new opening and transfers that load down onto padstones at each end. Modern beams are usually universal beams (UB) or universal columns (UC), standard I-shaped steel sections described by depth × width × weight per metre. The size is calculated by a structural engineer from the span, the load above and a deflection limit (it mustn't visibly sag). The deeper the section, the stronger it is in bending — which is why depth matters most.
If you're removing a wall, the beam is the part doing the real work — and understanding what it is and how it's sized helps you read a quote and ask the right questions. Here's the plain-English version.
Steel beam basics
- What it doesCarries load over an opening
- Common typesUB, UC (the old 'RSJ')
- Described byDepth × width × kg/m
- Sits onPadstones at each end
- Sized byStructural engineer
What a steel beam is and why it's needed
A load-bearing wall carries weight from above — a floor, the wall on the storey above, or the roof — straight down to the foundations. Remove the wall and that weight has nowhere to go, so it has to be caught and redirected. The steel beam does that: it spans the new opening and gathers the load that used to pass through the wall, then channels it down through its end bearings onto the masonry at each side.
The term RSJ (rolled steel joist) is the traditional name and is still used loosely for any such beam, though true RSJ sections have largely been superseded by universal beams and universal columns. The beam is the single most important component of a wall removal — get it right and the change is invisible and permanent; get it wrong and you risk sagging, cracking or failure. That's why it's never sized by eye.
How a beam is sized
Sizing is a calculation, not a lookup. The engineer works through three things:
- The load: they total everything the beam will carry — the weight of the wall, any floor bearing on it, a wall on the storey above, and (on top-floor or single-storey cases) the roof, plus the people and furniture load (imposed load).
- The span: the clear width the beam has to bridge. Bending forces rise sharply with span, so wider openings need much stronger, deeper beams.
- Deflection: even a beam strong enough not to break must not sag enough to crack the plaster or open joints above. UK design limits deflection to a small fraction of the span, and this often decides the final section.
From those, the engineer selects a standard steel section deep and strong enough for both strength and stiffness. They also size the padstones — concrete or steel spreader pads — so the concentrated load at each end doesn't crush the masonry beneath, and check the supporting walls and foundations can take it.
Reading a beam specification
When you see a beam called out on the engineer's drawing, it follows a standard format. Knowing how to read it helps you check the right steel turns up:
A label like '203×133×25 UB' means a universal beam roughly 203mm deep, 133mm wide, weighing 25kg per metre. A heavier number for the same depth means a chunkier, stronger section. The engineer's schedule will also specify the grade of steel, the length, the bearing length at each end, the padstones, and any connections where beams meet. Your builder orders to that exact schedule — steel suppliers cut and supply to spec — so it's worth a quick check that what's delivered matches the drawing.
| Part of spec | Example | Means |
|---|---|---|
| Depth × width | 203 × 133 | Section size in mm |
| Weight per metre | 25 | kg/m (heavier = stronger) |
| Type | UB / UC | Universal beam / column |
| Bearing & padstone | Per schedule | End support details |
Indicative steel beam notation for guidance. Source: steel sections to BS 4-1 / EN 10365; IStructE practice.
Installation, padstones and getting it right
A beam is only as good as its bearings. All the load it carries is dumped onto small areas at each end, so it sits on padstones — solid pads that spread that load into the wall below. If the padstones are undersized, missing, or the masonry beneath isn't sound, the load concentrates wrongly and the supporting wall can crack or crush, even if the beam itself is perfectly sized. During installation, the structure above is held on temporary props until the beam is in and the bearings are made good, then the props come out. Building Control inspects the installed beam to check it matches the calculations and is properly seated. The takeaways for a homeowner: the beam size, the padstones and the end bearings are all part of one design, they come from the engineer's calculations, and none of them should be improvised on site. A correctly sized and seated beam makes a removed wall a permanent, invisible improvement.
One question homeowners often have is whether the beam will be visible once the job is finished, and the answer shapes how the engineer designs it. A downstand beam sits below the line of the ceiling, leaving a visible bulkhead across the opening — simpler and often cheaper to install, but a step in the ceiling. A flush beam is raised up into the floor zone above so the ceiling stays level and the steel is hidden, which looks cleaner but is more involved, because the engineer has to work the beam in among the floor joists and the joists have to be hung off it. Which is possible depends on the depth of steel the loads demand and the room available in the floor above: a deep beam carrying a heavy load may be hard to conceal flush without significant work. This is worth raising with the engineer early, because it affects both the look of the finished room and the cost — and it's another reason the beam isn't just a part to be ordered off a shelf but a designed element that ties the structure, the loads and the finished appearance together.
Frequently asked questions
Is an RSJ the same as a steel beam?
Effectively yes in everyday use. 'RSJ' (rolled steel joist) is the traditional name for the steel beam over an opening, though modern beams are usually universal beams (UB) or universal columns (UC). People still call any such beam an RSJ regardless of the exact section.
What do the numbers on a steel beam mean?
A beam is described by depth × width × weight per metre, for example 203 × 133 × 25 UB — roughly 203mm deep, 133mm wide, 25kg per metre. For the same depth, a higher weight means a stronger section. The engineer specifies which the job needs.
What are padstones and why do they matter?
Padstones are solid concrete or steel pads the beam's ends sit on. They spread the concentrated load from the beam into the wall below so it doesn't crush or crack the masonry. They're sized as part of the engineer's calculation and are as important as the beam itself.
Sources & further reading
- Institution of Structural Engineers — find an engineer
- Checkatrade — steel beam (RSJ) costs and guide
- Planning Portal — structural alterations and Building Regulations
Figures on this page are typical UK ranges drawn from published sources and depend on your specific project. They are guidance, not a quotation.