RSJ, steel beam and lintel are three words people reach for when they want to put a piece of steel over an opening, and they are often used as if they mean the same thing. They do not. They describe different members, used in different situations, and getting the terms straight makes it much easier to talk to a builder, an engineer or a steel fabricator. Here is what each one actually is, how they differ, and when you need each, with the important caveat that the right choice and size for your project always come from a structural engineer’s calculation rather than a rule of thumb.
What an RSJ actually is
RSJ stands for rolled steel joist. It is an older name for a hot-rolled steel section: a length of steel rolled at the mill into an I-shape or H-shape with a web in the middle and flanges top and bottom. The name has stuck in everyday building language, so most people still call any structural steel beam an RSJ, even though the term is decades old.
In modern UK steel tables the sections that builders call RSJs are now usually a Universal Beam (UB) or a Universal Column (UC). A UB is deeper than it is wide and is used mainly for spanning, such as a beam over an opening. A UC is closer to square and is used mainly to carry vertical load as a post. So when someone asks for an RSJ, what they almost always need is a UB sized for their opening. For the full naming system, see our guide to universal beam sizes.
What “steel beam” means
Steel beam is simply the general term for any horizontal steel member that spans an opening and carries the load above it. An RSJ is a type of steel beam. A UB is a type of steel beam. So is a UC used on its side, and so are fabricated and built-up sections for larger jobs.
This is the key point: an RSJ and a steel beam are not two different things you choose between. An RSJ is one kind of steel beam, and in normal conversation the two words are interchangeable. What matters is not the label but the section: the depth, width and weight of steel the job requires, which is worked out from the load and the span.
What a lintel is, and how it differs
A lintel is the member that bridges a smaller opening such as a window or a door and carries the masonry or wall above it. The job is the same idea as a beam, transferring the load above the opening down into the walls either side, but the scale is usually much smaller.
For openings like a standard window or door, a pre-formed steel lintel often suffices. These are made-to-pattern profiles, frequently galvanised, designed and load-rated by the manufacturer for common opening widths and wall types. They arrive ready to build in, which is why they are the normal choice for everyday openings in a new build or a straightforward replacement.
A lintel can also be made from materials other than steel, such as concrete, depending on the opening and the wall. The defining feature is the role it plays, bridging a smaller opening, rather than the material it is made from.
RSJ vs beam vs lintel: the short version
Put plainly:
- Steel beam is the umbrella term for any steel member spanning an opening.
- RSJ is an older name for a rolled steel section, now usually a UB or UC. It is a type of steel beam.
- Lintel is the member over a smaller opening like a window or door, where a pre-formed steel lintel often does the job.
The practical line is one of scale and load. Smaller, standard openings tend to call for a pre-formed lintel. Larger openings and heavier loads call for a structural beam or RSJ sized specifically for the job.
When do you need an RSJ rather than a lintel?
You typically move from a pre-formed lintel to an RSJ or structural beam when the opening gets wider, when the load above is heavier, or when you are altering the structure of the building. The most common example is removing a load-bearing wall, where the wall, and often a floor or roof above it, has to be carried on a new beam across a span far greater than a window or door. Wider openings, knock-throughs between rooms, and openings carrying a storey above them all tend to need a designed steel beam rather than an off-the-shelf lintel.
The honest answer to “which do I need?” is that you cannot tell from a web page. It depends on the exact span and the exact load being carried, and only a structural engineer can confirm it. If you are taking out a wall or forming a large opening as part of an extension or wall removal, assume a designed beam until an engineer tells you otherwise.
Who decides the size, and why it matters
Whichever member your job needs, the size is not a guess and it is not picked from a chart. A structural engineer calculates the load above the opening and specifies the exact section, its length and how it bears onto the walls or padstones at each end. For a pre-formed lintel the engineer or the manufacturer’s load tables confirm the correct profile; for an RSJ or beam the engineer specifies the steel section in full.
For load-bearing work, including removing a load-bearing wall, that calculation and Building Control sign-off are required. This is not red tape for its own sake: it is what confirms the structure above the opening is safely carried. Skipping it risks both the safety of the building and problems at sale or survey later.
As a CE approved fabricator working to BS EN 1090 execution standards, we supply and fit the steel to the engineer’s specification, from cutting and drilling in our own Banbury workshop to fitting it on site. We do not size the beam ourselves; we make and install the section your engineer has specified, correctly and to standard.
Frequently asked questions
What is the difference between an RSJ and a steel beam?
There is no real difference in everyday use. RSJ stands for rolled steel joist, an older name for a hot-rolled steel section. Today those sections are usually a Universal Beam (UB) or Universal Column (UC) in the steel tables. Steel beam is just the general term for any horizontal steel member that spans an opening, so an RSJ is a type of steel beam. The exact section you need comes from a structural engineer’s calculation, not the name.
What is the difference between a lintel and an RSJ?
A lintel is the member that bridges a smaller opening such as a window or door and carries the load of the wall above it. For openings like these a pre-formed steel lintel, often a galvanised profile, is frequently enough. An RSJ or larger structural beam is used for bigger openings and heavier loads, such as removing a load-bearing wall or supporting a wider span. The size and type for any opening must come from a structural engineer’s calculation.
When do you need an RSJ instead of a lintel?
You typically need an RSJ or structural beam rather than a pre-formed lintel when the opening is wider, when you are removing a load-bearing wall, or when the load above is heavy, such as a floor, roof or another wall. A pre-formed lintel often suffices for a standard window or door opening. Only a structural engineer can confirm which is correct, because it depends on the span and the load being carried.
Do I need Building Control approval for an RSJ or beam?
Yes. Installing a steel beam or RSJ to support a load, including removing a load-bearing wall, is structural work that requires a structural engineer’s calculation and Building Control sign-off. The engineer specifies the section, length and bearings, and Building Control checks the work. We supply and fit the steel to that specification across Banbury and Oxfordshire. Contact us for a free quote.