UK structural steel beams are not picked from a shelf by guesswork; they are specified from a national system of standard sections. Every beam has a designation built from three numbers, and the sections are grouped into serial sizes so an engineer can choose the right one for a given load and span. Here is how that system works, what the UB, UC and RSJ labels mean, and why the correct size for your project always comes from a structural engineer’s calculation rather than a chart on a web page.
How a UK universal beam size is written
A universal beam size is written as three parts: depth x width x mass per metre. For example, a 203 x 133 x 25 UB is approximately 203 mm deep, about 133 mm across the flange, and weighs roughly 25 kilograms per metre of length. The first number (depth) is the most important for spanning, the second (width) describes the flange, and the third (mass per metre) tells you how much steel is in the section, which relates to its strength.
The published dimensions and engineering properties for every section live in the UK steel section tables, commonly known as the Blue Book. That is the authoritative source an engineer works from, and it is where exact figures for any given section should be taken, not from a summary like this one.
Serial sizes: grouped by depth
Sections are organised into serial sizes by their nominal depth. Within each depth there are several beams of different weights and flange thicknesses, so an engineer can step up to a heavier section without changing the overall depth. The standard UK depth range runs roughly:
- 127, 152, 178, 203, 254, 305, 356, 406, 457, 533 and 610 mm, with larger sections above that.
Those are the common serial-size depths, not a menu to choose from. A shallow beam can be perfectly correct for a small opening, and a deep beam may be needed for a wide span carrying floors above. Only the calculation decides.
UB, UC and RSJ: what the labels mean
The three terms cause most of the confusion, so here is the plain version:
- Universal Beam (UB): deeper than it is wide, designed mainly to span horizontally. These carry floor and roof loads across an opening and are what most people mean by a steel beam.
- Universal Column (UC): closer to square in section, designed mainly to carry vertical load as a post or column. Because the flange is wider, a UC resists buckling under compression.
- RSJ (rolled steel joist): the older term for a rolled section. In everyday speech, and on many RSJ supply and fit jobs, “RSJ” is still used loosely for any structural beam. In the modern tables the sections you will actually be supplied are UBs and UCs.
So when a homeowner asks for an RSJ over a knocked-through wall, the engineer will usually specify a UB by its full designation, and that is the steel we cut, drill and fit.
Why the section comes from a calculation, not a chart
It is tempting to look up a beam size online and order it. Do not. The correct section depends on the load above the opening (the floors, walls and roof it carries), the clear span, how the beam is supported, and the deflection limits for the finish above. A structural engineer works all of that out and specifies the exact section, the length and the bearing details. For any load-bearing work that calculation, plus Building Control approval and inspection, is required, not optional.
Publishing a “pick your beam” table would be misleading and unsafe, which is why this guide explains the naming system but stops short of telling you which beam to use. For how the steel and the fitting are priced once the section is known, see our RSJ installation cost guide.
What TC Rowan needs from you
If you already have an engineer’s specification, send it over with the depth x width x mass designation and the length, and we can supply the section cut and drilled to suit, or supply and fit it with our own Banbury workshop and erection team. If you do not yet have a calculation, we can point you in the right direction. For the design and frame side of larger projects, see our structural steelwork service.
Frequently asked questions
How are universal beam sizes named in the UK?
UK universal beams use a three-part designation: depth times width times mass per metre. A 203 x 133 x 25 UB is roughly 203 mm deep, 133 mm wide and weighs 25 kg per metre. Sections are grouped into serial sizes by depth, and the exact dimensions and properties are published in the steel section tables (the Blue Book).
What is the difference between a UB, a UC and an RSJ?
A Universal Beam (UB) is deeper than it is wide and is used mainly for spanning, such as floor and roof beams. A Universal Column (UC) is closer to square and is used mainly to carry vertical load as a post. RSJ is the older term for a rolled steel joist; many people still call any structural beam an RSJ, but in modern tables the sections are UBs and UCs.
What are the standard universal beam depths?
Common UK serial-size depths run roughly 127, 152, 178, 203, 254, 305, 356, 406, 457, 533 and 610 mm, with larger sections above that. Within each depth there are several weights. We do not recommend a depth from this list; the correct section comes from the engineer’s calculation for your specific load and span.
Which beam size do I need for my job?
There is no safe way to pick a beam size from a chart. A structural engineer calculates the load above the opening and specifies the exact section, length and bearings. For load-bearing work that calculation and Building Control sign-off are required. Send us the engineer’s specification and we will supply and fit the steel, or get a free quote to talk it through.