The 203 x 133 UB30 steel beam is one of the most widely specified RSJs in UK domestic construction — the standard choice for through-lounge openings, kitchen extensions and bi-fold spans where a 203 x 133 UB25 no longer has the capacity. Rolled to S355J0 structural steel and supplied cut free to any length from 1m to 12m, available in mill finish or red oxide primer, with free UK mainland delivery included in the price.
Steel grade — S355J0 to BS EN 10025-2
Every 203 x 133 UB30 we supply is rolled in S355J0, a 355 N/mm² minimum yield structural steel with impact toughness certified at 0°C. S355 is the grade UK structural engineers now design to by default, and it carries roughly 29% more yield strength than the older S275 material still sold by some merchants. If your engineer has calculated to S275, an S355 beam of the same section will always satisfy the design — but never substitute the other way round without checking.
For a fuller comparison see our guide to S275 vs S355 steel, or the steel grades & standards reference for the full specification we supply against.
Where the UB30 is typically used
At 206.8mm deep and 30kg per metre, this section sits at the point where most single-storey domestic openings are comfortably covered. It is the section most often specified when removing a load-bearing wall between a kitchen and dining room, or carrying a floor over a wide rear opening.
Against the lighter 203 x 133 UB25 it adds around 22% more major-axis stiffness for 4.9kg per metre more weight. If you are still narrowing down the section, the UK steel beam size chart lists every UB, UC and PFC size with weights.
Available finishes
Mill finish is the beam exactly as it leaves the rolling mill: bare structural steel carrying the usual blue-grey mill scale. It is the right choice where the RSJ will be encased in plasterboard, boxed in, fire-protected or painted on site by your own decorator — see our guide to boxing in a steel beam.
Red oxide primer is a factory-applied anti-corrosive coating that gives the beam short-term protection while it sits on site, in transit or in an unheated shell before the roof and windows go in. It is a primer, not a topcoat — designed to be overpainted, and not a substitute for intumescent fire protection or galvanising in an exposed external location. We cover this in what red oxide primer actually means, and whether RSJs need painting at all.
Cut to size — free, to the millimetre
Tell us the length you need and we cut it. There is no charge for cutting, and no minimum order beyond a single metre. Enter your exact dimensions in the calculator above — you can specify several different cut lengths on one order, and the tool works out what you need. Steel is supplied and billed in whole metres, so a 4.2m requirement is charged as 5m; where you order more than one cut in the same finish, the total is rounded once rather than cut by cut, which keeps the cost down on multi-beam jobs.
Cut tolerance is ±3mm on length. If your opening is tight, allow the usual bearing at each end and confirm the padstone detail with your engineer before ordering. More on what lengths RSJ beams come in.
Section dimensions
| Depth of section (h) | 206.8 mm |
| Width of section (b) | 133.9 mm |
| Web thickness (s) | 6.4 mm |
| Flange thickness (t) | 9.6 mm |
| Root radius (r) | 8.0 mm |
| Depth between fillets (d) | 171.6 mm |
| Mass per metre | 30.0 kg/m |
| Cross-sectional area | 38.2 cm² |
| Surface area per metre | 0.923 m²/m |
Section properties
| Second moment of area, y-y (Iy) | 2,900 cm⁴ |
| Second moment of area, z-z (Iz) | 384.6 cm⁴ |
| Elastic modulus, y-y (Wel,y) | 280.4 cm³ |
| Elastic modulus, z-z (Wel,z) | 57.45 cm³ |
| Plastic modulus, y-y (Wpl,y) | 314.8 cm³ |
| Plastic modulus, z-z (Wpl,z) | 88.25 cm³ |
| Radius of gyration, y-y (iy) | 8.7 cm |
| Radius of gyration, z-z (iz) | 3.1 cm |
Nominal dimensions and properties to BS EN 10365. Figures are published for reference only.
Structural engineer notice
Beam sizing is not something to work out from a table. The correct section for your opening depends on span, the loads carried above, bearing conditions and the padstone detail — all of which must be calculated by a qualified structural engineer, and in most cases will form part of your building control submission. We supply the steel to the specification you give us; we do not provide structural design.
Delivery
Cut-to-size beams are dispatched on a 5–10 working day lead time, with free delivery to UK mainland addresses included in the price — no carriage is added at checkout. A 203 x 133 UB30 weighs 30kg per metre, so a 5m beam is 150kg. Deliveries are kerbside and offloading assistance is required — please have suitable labour or lifting equipment available.
See how RSJ steel beams are delivered for what to expect on the day, or our delivery & lead times page. Ordering several beams for one project? Trade discounts are available.
Related sections
- 203 x 133 UB25 steel beam — the lighter section in the same serial size
- 254 x 146 UB31 steel beam — wider flange for longer spans
- 203 x 102 UB23 steel beam — narrower alternative
- All RSJ steel beams & columns
- Universal beams (UB)