If you are opening up a wall, building an extension or looking at structural drawings, there is a good chance you will come across the term RSJ. It stands for Rolled Steel Joist and is one of the most familiar names used in the UK for a structural steel beam.
There is an important terminology point, though. Builders and homeowners often use "RSJ" as a general name for a steel beam, while modern structural drawings and steel suppliers commonly use more precise section designations such as Universal Beam (UB) and Universal Column (UC).
This guide explains what an RSJ is, what these beams are used for and how the terminology fits together.
What does RSJ stand for?
RSJ stands for Rolled Steel Joist. The name refers to a structural steel section produced by rolling steel into a profile designed to carry structural loads.
In everyday UK building terminology, however, RSJ has become a much broader term. Someone talking about "putting an RSJ in" will often simply mean installing a structural steel beam, regardless of the exact section designation shown on the structural engineer's specification.
This is why the beam described as an RSJ during a conversation with a builder might be identified as something like 203 × 102 × 23 UB on the drawings or steel order.
The distinction matters when ordering. RSJ may be useful everyday terminology, but the full structural section specified for the project is what identifies the steel that is actually required.
Is an RSJ the same as a Universal Beam?
Not precisely. The terms are closely associated in modern building work, but they should not simply be treated as interchangeable product names.
Universal Beams (UBs) and Universal Columns (UCs) are recognised families of hot-rolled structural steel sections. Their nominal dimensions are covered by BS EN 10365. "RSJ", meanwhile, remains a long-established term that is widely used in the building trade and by homeowners when referring more generally to structural steel beams.
| Term | Meaning | How you are likely to encounter it |
|---|---|---|
| RSJ | Rolled Steel Joist | A long-established term still commonly used for structural steel beams in UK building work. |
| UB | Universal Beam | A standard modern structural section designation commonly found on drawings, calculations and steel listings. |
| UC | Universal Column | Another standard structural section family, generally with different proportions from a UB. |
Universal Beams
A Universal Beam is a standard hot-rolled structural section with a relatively deep web between its two flanges. UBs are widely used where a structural member needs to resist bending, although whether a particular UB is suitable depends on the structural design rather than simply its dimensions.
Universal Columns
Universal Columns have different proportions, typically appearing broader in relation to their depth. Despite the name, the designation describes the section family rather than imposing a rule that a UC can only ever be used vertically as a column.
The practical point is simple: if an engineer specifies a particular UB, UC or other section, use the complete specification rather than substituting another steel section because both might casually be described as an "RSJ".
What does an RSJ look like and how does it work?
A typical Universal Beam has the familiar I-shaped cross-section associated with structural steel. It consists of three easily recognised parts:
- the top flange;
- the vertical central web; and
- the bottom flange.
The shape is an efficient way of arranging steel within a structural member. In simple terms, the flanges contribute strongly to resistance against bending, while the web connects them and has an important role in carrying shear.
That does not mean two beams that look broadly similar will perform in the same way. Changes to the overall dimensions, flange and web thicknesses, steel grade, restraint and loading can materially affect structural performance.
What are RSJs used for?
Structural steel beams appear in both new construction and alterations to existing buildings. In domestic work, they are particularly familiar where an existing loadbearing arrangement is being changed.
Supporting structure above a new opening
If part of a loadbearing wall is removed to create a wider doorway or an open-plan space, the loads previously carried through that section of wall still need an adequate route to the supporting structure below. A structural steel beam may form part of the engineered solution.
Supporting floors
Steel beams can form part of a floor structure, carrying loads across spans and transferring them to supporting walls, columns or other structural elements.
Supporting roof structures
Structural steel is also used in roof construction, including situations where the design needs support across an open area or where roof loads need to be transferred into other parts of the building.
Extensions and structural alterations
Large openings are common where an extension is joined to an existing house. Depending on the structural arrangement, the design may incorporate one or more steel members to transfer loads around the new opening.
Supporting masonry after chimney breast removal
Removing a chimney breast can leave masonry above that still requires structural support. A steel beam is one possible engineered solution, but it should not be assumed that every chimney can be treated in the same way. Our guide to RSJ chimney support and chimney breast removal looks at this situation in more detail.
Why is structural steel used for beams?
Rolled structural steel sections provide engineers with well-defined section properties and are available in a wide range of profiles. This makes them useful where loads need to be transferred across an opening or incorporated into a larger structural arrangement.
Steel can also provide substantial structural capacity without requiring an excessively bulky member, which is one reason it is frequently encountered in alterations where available space is limited.
That does not make steel automatically preferable to timber, concrete or another structural system. The appropriate material and member depend on the structure being designed, the loads involved, available space, supports, connections, fire requirements and practical construction constraints.
How are RSJ sizes described?
Modern steel section designations contain more information than they may appear to at first glance.
Take a beam listed as 203 × 102 × 23 UB. This is a section currently found in Wade's RSJ and structural steel beam range.
- 203 × 102 identifies the nominal section size.
- 23 identifies its nominal mass in kilograms per metre.
- UB tells us that it belongs to the Universal Beam section family.
These first two numbers should not be read as guaranteed exact measured dimensions. Published section tables provide the actual dimensions and other properties for each section.
The mass-per-metre figure is also useful for estimating how heavy a beam will be to transport and handle. For example, total beam mass can be estimated from the section's kg/m value and its length. We explain that separately in our RSJ weight guide.
What the designation does not tell a homeowner is whether that beam is suitable for their opening. That requires structural design.
Can you choose an RSJ yourself?
You can use section designations to understand what has been specified, but choosing the structural beam itself is a different matter.
A beam's suitability can depend on factors including the loads it supports, its span, the way it is restrained, its bearing and support conditions, its connections and the surrounding structure. Two openings of the same width can therefore require quite different structural solutions.
For alterations involving loadbearing structure, the beam and its supporting arrangement will normally need structural design, commonly provided by a structural engineer or another appropriately competent structural designer.
Does installing an RSJ need Building Regulations approval?
In England, structural alterations affecting loadbearing parts of a building generally fall within the Building Regulations. Removing part of a loadbearing wall and replacing its support with a structural beam is a typical example.
Approved Document A provides guidance on structural safety under the Building Regulations in England, including the loads on buildings and structural elements such as walls, floors, roofs and chimneys.
Building Control may therefore need structural information and calculations demonstrating that the proposed alteration is adequate. The precise approval route and information required will depend on the project.
Planning permission is a separate issue. Work that does not require planning permission can still require Building Regulations approval because the two systems address different matters.
The construction process matters as well as the finished beam. HSE guidance states that potentially loadbearing parts of a structure should be assessed by a competent person before they are altered, so that the work can be planned without compromising structural stability.
If you are planning this type of work, our guide to how an RSJ is installed explains the typical process from structural design and preparation through to positioning, inspection and making good. It is intended to explain what happens during the work rather than provide DIY structural instructions.
RSJs and structural steel from Wade
Once the required section has been established from the structural design, the specification can be matched to the correct steel product. Wade supplies a range of Universal Beams and Universal Columns through our RSJ and structural steel collection.
The important sequence is design first, order second. The product listing should be used to source the section that has been specified, not as a substitute for determining what the building structurally requires.
Frequently asked questions
What does RSJ mean?
RSJ means Rolled Steel Joist. In UK building terminology, it is commonly used as a general term for a structural steel beam.
Are RSJs still used?
The term certainly is. Builders, suppliers and homeowners still regularly refer to RSJs, although modern structural specifications often identify the actual steel section using more precise designations such as UB for Universal Beam or UC for Universal Column.
Is an RSJ an I-beam?
They are related terms but are best kept distinct. "I-beam" describes the familiar I-shaped form associated with many structural steel sections, while RSJ is the established term Rolled Steel Joist. A modern beam casually called an RSJ may in practice be specified as a particular Universal Beam or another structural section.
What is the difference between an RSJ and a steel beam?
"Steel beam" is the broader description. RSJ refers specifically to Rolled Steel Joist terminology, although in everyday UK construction language the term is frequently used much more generally for structural steel beams.
Do I need a structural engineer for an RSJ?
There is not a blanket rule that every use of structural steel legally requires someone with the specific title "structural engineer". However, where a project alters loadbearing structure, the new support arrangement needs to be adequately designed. For domestic structural alterations, this work is commonly undertaken by a structural engineer, with calculations provided for Building Control where required.
