LEINSTER STEEL"What size RSJ do I need?" is the question we hear most from homeowners taking out a wall. Here's how it's decided — and why the final size must come from a structural engineer.
What determines the beam size
- Clear span — the width of the opening the beam has to bridge. Wider spans need deeper, heavier beams.
- What it carries — just the wall above, or floors and roof too? A beam under a two-storey wall carries far more than one under a single-storey.
- Point loads — anything concentrated bearing onto the beam, like another beam or a purlin.
- Bearing — how the beam sits on the walls at each end (usually a minimum of 150mm onto padstones).
Rough ballpark sizes
For a typical domestic opening these UB (universal beam) sizes come up a lot, but treat them only as a sense of scale, never as a spec:
- Openings up to ~3m, light load: 152x89 or 178x102 UB
- ~3–4m or carrying more: 203x133 or 254x146 UB
- Wider spans / two-storey loads: 305x165 UB and up
Always confirm with an engineer
Beam sizing is a structural calculation — get it wrong and it's dangerous. A structural engineer will size the beam and specify the padstones and bearing. It's a small cost for peace of mind and for building-control sign-off.
Then we cut it
Once you have the size and length, Leinster Steel supplies the beam cut to length with mill certificates (S275 or S355), black or primed. Send the spec through our enquiry form or get an instant price.
This guide is general information, not structural advice. Always use a qualified structural engineer to size structural steel.
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