What size RSJ do I need to remove a load-bearing wall?

People want a quick number — '6 inch by 3 inch will do' — but steel sizing isn't like that, and getting it wrong is dangerous. Here's what the size actually depends on and why it has to be calculated rather than guessed.

RSJ, UB, UC — what the terms mean

'RSJ' (rolled steel joist) is the traditional name people still use for the steel beam over an opening, but in practice modern beams are usually universal beams (UB) or universal columns (UC) — standard hot-rolled I-shaped sections. A beam is described by its depth × width × weight per metre, for example a 203×133×25 UB. The depth is the most important dimension for a beam: a deeper section is far stronger in bending and resists sagging better than a shallow one of the same weight.

Which section suits your opening isn't a matter of taste — it's the output of a calculation. The engineer balances strength (it mustn't fail under load) against deflection (it mustn't visibly sag or crack the finishes above), and against practical limits like the depth you can fit. That's why two openings of the same width can need different beams if one carries more above it.

The three things that drive the size

Beam sizing comes down to load, span and how far it's allowed to bend:

• Span: the clear width of the opening. Bending forces rise sharply as the span grows, so a wide knock-through needs a much deeper, heavier beam than a narrow doorway-sized opening.
• Load above (the tributary load): the engineer adds up everything the beam will carry — the wall's own weight, any floor bearing on it, a wall on the storey above, and on top-floor or single-storey cases the roof. A wall carrying only a ceiling needs far less steel than one carrying a floor plus a wall plus the roof.
• Deflection: even a beam strong enough not to break must not sag enough to crack the plaster or open joints above it. UK practice limits deflection to a small fraction of the span, which often governs the choice of a deeper section.

The engineer also checks the end bearings — the beam dumps all that load onto small areas at each end, so it sits on padstones (concrete or steel spreader pads), and the masonry beneath must be able to take the concentrated load without crushing.

Why guessing or copying a neighbour fails

A beam that worked for next door, or for a job you saw online, tells you almost nothing about yours. The variables that change the answer:

Two houses on the same street can need different beams for the same-width opening because one is a single storey and the other carries a bedroom and roof above, or because one has a point load (another beam or a chimney) landing near the opening. Under-sizing a beam risks sagging, cracking and in the worst case failure; over-sizing wastes money and may not even fit the available depth. The calculation exists to hit the right section for your loads — strong enough, stiff enough, and properly supported at the ends.

Indicative drivers of beam size for guidance only. Source: IStructE guidance; Eurocode-based design practice.

How to get the right answer

The correct route is straightforward: appoint a chartered structural engineer to assess the opening. They measure the span, work out the loads above, calculate the required steel, and specify the exact section, the padstones, and the bearing details — all in a calculation pack your builder follows and Building Control checks. Engineer fees for a single beam are commonly in the region of £300–£700, a small part of the overall job. Once you have the specification, your builder orders that exact beam (steel suppliers cut and supply to the engineer's schedule) and installs it on the specified padstones. Don't let a builder fit a beam 'sized by eye' or copied from another job — if there are no calculations, there's nothing for Building Control to sign off, and no assurance the steel is right for your loads. The number you want is specific to your house, and only the calculation produces it.

It also helps to understand why the engineer often specifies a beam that looks bigger than people expect. Homeowners frequently picture a slim steel and are surprised when the calculation calls for a deep, heavy section — but that depth is usually being driven by deflection rather than strength. A beam can be more than strong enough not to break and still need to be deeper, because the limit that governs is how little it is allowed to bend: even a few millimetres of sag over a wide opening can crack the plaster, open joints in the floor above, or make a door upstairs stick. UK design practice deliberately keeps deflection to a small fraction of the span, which on a wide knock-through often forces a deeper beam than strength alone would require. So if your engineer specifies something chunkier than the beam you saw at a friend's house, it is not over-engineering — it is the section that keeps the finishes above intact as well as carrying the load. Trying to substitute a shallower, cheaper beam to save money or gain headroom is exactly the kind of change that should never be made without the engineer re-checking the calculation, because the consequence shows up not as a dramatic failure but as a slowly cracking ceiling.

Frequently asked questions

Sources & further reading

• Institution of Structural Engineers — find an engineer
• Planning Portal — structural alterations and Building Regulations
• Checkatrade — remove a load-bearing wall (steel beam)

Figures on this page are typical UK ranges drawn from published sources and depend on your specific project. They are guidance, not a quotation.