What Size Steel Beam Do I Need?

It's the first question almost everyone asks, and the honest answer is that nobody can tell you the size from a photo or a rough description. The section you need is calculated from the span and the load, and getting it wrong is the difference between a floor that's fine and a floor that sags, cracks or worse.

What actually determines the size

Four things drive it:

• The span — how far the beam has to bridge between supports. Span is the biggest factor, and the required section grows quickly as it increases.
• What's above it — a single-storey roof is a very different load to a floor, a load-bearing wall, or a wall plus a floor plus a roof on a two-storey extension.
• Point loads — anything concentrating weight onto one spot, such as another beam landing on it, a purlin, or a stack above.
• Bearing and deflection — the beam has to sit on something that can take the load (padstones or posts), and it has to not flex too much. Deflection limits often decide the final size even when strength alone would allow something smaller.

Why you can't just use a table off the internet

Span tables and beam calculators you find online are generic. They don't know what's above your opening, what the walls are made of, whether there's a point load landing mid-span, or what's happening at the bearings. They're fine for a rough sanity check on feasibility — they are not a design, and building control will not accept one.

How the size actually gets decided

A structural engineer surveys or works from your drawings, calculates the loads, and specifies the exact section — something like a 203 x 133 UB 25 — along with the grade, the bearing details and any padstones or posts. That specification is the document everything else follows from: building control checks it, and we fabricate to it.

If you want to understand the section names themselves, see RSJ vs universal beam. And yes, you do need building regulations approval for this work.

What we need to fabricate it

Once you have the engineer's calculations and drawing, send them over. From that we cut to length, drill for the connections, add cleats or base plates, and supply primed or galvanised, ready for your builder to install. See steel beams & RSJs or, if it's a wider opening needing posts, goalpost frames.

Frequently asked questions

Can you tell me the size if I send a photo? No — and be wary of anyone who does. Size comes from calculated loads and spans, not from looking at the opening.

Does a bigger beam always work? Not necessarily. Oversizing adds weight and cost, can create bearing problems, and still needs the right bearing detail. The engineer specifies what's correct, not just what's big.

Who engages the engineer? Normally you or your builder, directly. It's a small cost relative to the build and it's what makes the rest of the job possible.

Got a drawing already? Send it to us and we'll quote the steel.