In short
A universal beam is named by three numbers: nominal depth and width in millimetres, then weight in kilograms per metre. A 203 x 133 x 25 UB is about 203 mm deep, 133 mm wide and weighs 25 kg a metre. Depth does most to decide how far it can span.
Reading the name
Every hot-rolled section made to the British standard has a designation in the same form: depth x width x mass per metre, followed by the type.
So 254 x 146 x 31 UB means a Universal Beam in the 254 x 146 family that weighs 31 kg for each metre of length. Multiply by the length and you have the weight to be carried through the house: a 4.5 m beam of that size is about 140 kg.
The first two numbers are nominal. They name a family, and each family comes in several weights. A heavier member of the family has thicker flanges and web, so its real depth is a few millimetres more than a lighter one. The 254 x 146 family, for example, runs from about 251 mm deep at 31 kg/m to about 260 mm at 43 kg/m. If headroom is tight, ask the engineer for the actual depth, not the nominal one.
Beam sizes common in houses
These Universal Beam families cover nearly all domestic work.
| Family (depth x width, mm) | Weights available (kg/m) | Typical domestic use |
|---|---|---|
| 127 x 76 | 13 | Short lintels and trimmers |
| 152 x 89 | 16 | Openings up to about 2 m |
| 178 x 102 | 19 | Internal wall removals |
| 203 x 102 | 23 | Internal wall removals |
| 203 x 133 | 25, 30 | Knock-throughs with a floor above |
| 254 x 102 | 22, 25, 28 | Longer spans, light load |
| 254 x 146 | 31, 37, 43 | Rear openings of 3 to 5 m |
| 305 x 102 | 25, 28, 33 | Long spans, light load |
| 305 x 165 | 40, 46, 54 | Wide rear openings, two storeys above |
The uses in the last column are a rough guide to what turns up on drawings. They are not a way to pick a beam. The right section depends on the span, on everything the beam carries, and on how much it is allowed to deflect.
Universal columns used as beams
Universal Columns are squarer: about as wide as they are deep, with thicker flanges. They are designed to stand upright, but engineers often lay one flat as a beam when depth is limited.
| Family (depth x width, mm) | Weights available (kg/m) |
|---|---|
| 152 x 152 | 23, 30, 37 |
| 203 x 203 | 46, 52, 60, 71, 86 |
| 254 x 254 | 73, 89, 107, 132, 167 |
A 203 x 203 x 46 UC is a frequent choice where a beam must hide within the depth of a floor. It weighs about half as much again per metre as a UB that would do the same job with 50 mm more depth, which tells you what shallowness costs.
Why depth matters most
A beam resists bending mainly through its flanges, and the further apart they are, the harder they work. Stiffness rises steeply with depth. That is why a 254 mm deep beam at 31 kg/m will outperform a 203 mm beam at 30 kg/m while using much the same weight of steel.
For the homeowner this is a design conversation worth having early. If the beam can drop below the ceiling, or sit up inside a deeper floor, the engineer can use a lighter, cheaper section. If it must be flush with a shallow ceiling, expect a heavy UC, bigger padstones and more people to lift it.
Width and bearing
Width matters at the supports. A 102 mm flange sits neatly on a single leaf of blockwork. A 146 or 165 mm flange is wider than a 100 mm inner leaf, so the detail at the bearing needs thought, particularly in a cavity wall.
Where a beam carries the full thickness of a cavity wall above, a wide plate is usually welded under a narrower beam to pick up both leaves, in place of two beams bolted side by side.
Steel grade and marking
Almost all sections sold in the UK today are grade S355. Older drawings and tables may assume the weaker S275, so the grade on the engineer’s calculation and the grade on the delivery ticket should match.
Fabricated structural steel must be UKCA or CE marked under BS EN 1090-1. The fabricator supplies a declaration of performance, and building control are entitled to ask for it.
RSJ or universal beam?
Strictly, an RSJ is a rolled steel joist: an older, smaller section with tapered flanges. A few sizes are still rolled. In everyday speech, though, RSJ just means a steel beam, and what your builder calls the RSJ will be a UB or UC on the drawing.
Can I choose my own size?
No, and building control would not accept it. Span tables for steel assume specific loads that rarely match a real house. A structural engineer’s calculation for a single beam is a small job, and it is the document the inspector asks for. The same engineer will size the columns and connections if the opening leads into a steel frame extension.