What Size RSJ Do I Need? A Practical Guide for Builders

One of the most common questions we get at Builders Beams R Us is some version of: "I need a beam for a knocked-through wall, what size do I need?" It is a reasonable question, and the honest answer is that the correct size always comes from a structural engineer's calculation based on the specific loads involved in your project.

That said, there is a lot a builder can understand about RSJ sizing without doing the structural maths themselves. This guide covers how beam sizes are defined, what factors affect the section required, and what the typical ranges look like for the most common domestic project types. It also explains exactly what you need to send us to get a fast, accurate quote.

RSJ stands for Rolled Steel Joist, though in practice builders and fabricators use the term interchangeably with Universal Beam (UB). When a structural engineer specifies a steel beam, they will typically refer to it using a designation that looks something like this: 203x102x23 UB.

Those three numbers tell you everything about the section. The first is the depth of the beam in millimetres, measured from the top flange to the bottom flange. The second is the width of the flanges in millimetres. The third is the weight per linear metre in kilograms. So a 203x102x23 UB is 203mm deep, 102mm wide, and weighs 23kg per metre of length.

Deeper beams are generally stronger because more of the material is positioned further away from the neutral axis, which is where bending resistance comes from. Heavier sections within the same depth series have thicker webs and flanges, which also adds strength. Your structural engineer will specify both the section size and the steel grade, most commonly S275 or S355, with S355 being the higher-strength option.

Several factors determine what size beam is needed for any given situation. A structural engineer takes all of these into account when producing their calculations.

The span

The clear span is the distance between the points of support, usually the inner faces of the walls or columns carrying the beam. The longer the span, the greater the bending moment the beam must resist, and therefore the larger the section required. Even a relatively modest increase in span, say from 2.5 metres to 3.5 metres, can push the specification up by one or two section sizes.

The load above

A beam carrying a single-storey load from a ground-floor wall removal is working much less hard than one that also carries a first floor, a second floor, or a roof. In a typical terraced house, a beam installed at ground floor in a through-lounge conversion is carrying relatively modest loads. The same span in an older three-storey property, or one where a heavy flat roof sits above, will need a significantly heavier section. Party walls and chimney breasts add further load considerations that the engineer needs to assess.

The bearing length

The bearing is the length of the beam that sits on the wall at each end. Insufficient bearing concentrates the load onto a very small area of masonry and can cause crushing or settlement. Your engineer will specify a minimum bearing length, often 150mm for domestic work, though this varies depending on the masonry type and the load involved. End plates or padstones may also be required.

The construction type

Timber-framed buildings, masonry cavity walls, solid stone walls and steel-framed structures all behave differently. Older stone properties in Yorkshire, for example, often have thick but irregular masonry with variable compressive strength, which affects how loads are transferred into the bearing. Telling your engineer what the property is built from and when it was constructed helps them produce an accurate specification.

The ranges below are illustrative only. The correct section for your specific project must always come from a structural engineer's calculations. Do not use these figures to order steel without an engineer's sign-off.

Ground-floor wall removal in a two-storey semi or terraced house

For a standard knock-through between two ground-floor rooms in a typical inter-war or post-war semi-detached property, spans are commonly in the 2.5 to 3.5 metre range. Engineers frequently specify sections in the 150x75 to 203x102 UB range for these jobs, depending on the exact span and the floor and roof loads coming down. A 203x102x23 UB is a very commonly encountered section for this type of work.

Rear extension opening with a flat or pitched roof above

A rear extension creating a wide opening between the existing house and a new structure often involves spans of 3 to 5 metres. Where the extension has its own roof structure bearing onto the beam, sections of 254x102 or 254x146 UB are common. Wider openings of 4.5 metres and above frequently move into the 305 series or require a heavier section within the 254 range.

Loft conversion ridge beam or purlin

Steel beams in loft conversions are used in a variety of configurations: as ridge beams running along the apex of the roof, as purlins supporting rafters partway up the slope, or as structural members supporting new dormer structures. Spans vary considerably depending on the roof geometry, but for a typical ridge beam in a semi-detached property engineers often specify sections in the 178x102 to 254x102 UB range. Dormer structures with steeper loads may push into larger sections.

Garage conversion lintel replacement or structural opening

Garage conversions often require a new beam across the front opening where the up-and-over door has been removed, or across an internal wall being opened up to connect the garage to the main house. Front opening spans of 2.4 to 3 metres with a first floor above are commonly handled with sections in the 152x89 to 203x102 UB range, though again the specific loading governs everything.

Commercial or multi-storey loads

Where steel is carrying loads from multiple storeys, or in commercial premises with higher imposed floor loads, the sections move into much heavier territory. Spans of 4 to 7 metres carrying two or more floors can require sections in the 305, 356 or 406 series. These projects will always be specified by a structural engineer and the sections are too site-specific to generalise meaningfully.

Not every structural steel member is a Universal Beam. Parallel Flange Channels (PFC) are used where the beam needs to sit flush against a wall on one side, for example in a flitch beam arrangement or where access for a full UB flange is not available. Universal Columns (UC) are used where the load is primarily compressive rather than in bending, for example as vertical posts or props.

We fabricate all of these sections at our Wakefield workshop, along with angles, flat bar, box section and plate. If your engineer has specified something other than a standard UB, just send us the designation and we will confirm availability and price.

Getting an accurate quote from us is straightforward. The more detail you can provide, the quicker we can turn it around.

The section designation: For example, 203x102x23 UB. This comes from your structural engineer's drawings or calculations.
The length: The overall fabricated length of the beam in millimetres, including the bearing at each end. If you are unsure of the exact length, give us the clear span and the bearing length and we can work out the overall.
Any drilling or plating details: If the engineer has specified end plates, cleats, bolt holes or stiffeners, let us know or send us the relevant drawing.
The finish required: All steel we supply is shot blasted as standard. If you need primer, powder coat or galvanising, tell us at the time of quoting.
Your delivery postcode and preferred week: We deliver across Yorkshire and the wider UK. Give us the site postcode and your preferred delivery week and we will confirm availability.

If you do not yet have engineer's drawings but want a rough idea of cost, send us your best estimate of the section and length and we can give you a ballpark figure while you wait for the final specification.

Yes. For any structural alteration to a building, including wall removals, extensions and loft conversions, building regulations require that the steel is sized by a qualified structural engineer. We fabricate to approved structural designs and do not produce our own calculations.

If you are working with a builder who does not yet have engineer's drawings, a structural engineer can usually produce calculations for a standard domestic project within a few days. The fee for a typical domestic beam calculation is modest relative to the cost of the project, and the calculations are required for building control sign-off regardless.

If your client has sketches but no formal drawings yet, we can still cut and drill to measurements provided, but the responsibility for the structural design sits with the engineer or the builder in charge. We will always flag if something does not look right.