What size steel beam do I need? (And why only an engineer can answer it)
How steel beams are described
UK steel sections are named by their nominal depth and width in millimetres, then their weight per metre. A "203 × 133 × 25 UB" is a universal beam about 203 mm deep and 133 mm wide weighing about 25 kg per metre. RSJ (rolled steel joist) is the everyday name for all of them, even though most beams used today are universal beams (UB) or universal columns (UC).
What decides the size
Three things: the span, the load and the bearings. Span is the clear opening plus the length the beam sits on at each end. Load is everything above it: the first floor, any wall above, the roof, and people and furniture. The bearings are the padstones or posts the beam sits on, which have to spread the load into masonry strong enough to take it.
Deflection matters as much as strength. A beam can be strong enough not to fail but still bend enough to crack the plaster above, so engineers limit how far it is allowed to sag.
- Longer span: a deeper or heavier section
- More floors or a roof above: a heavier section, sometimes two beams side by side
- Point loads, such as a wall or post landing on the beam: extra checks
- Weak masonry at the ends: bigger padstones or steel posts
Sections commonly seen in UK houses
As a rough idea of what turns up on domestic jobs: short internal knock-throughs often use sections around 152 × 89 UB, typical kitchen-diner openings around 203 × 133 UB, and wider rear openings deeper sections or a goalpost frame. Treat these as illustrations, not a specification. Two houses with the same opening can need different beams because of what sits above.
Why you can't guess it
Building control will ask for structural calculations before they sign off a beam, and an undersized beam can crack ceilings or worse. Oversizing costs money and can make the steel too heavy to get through the door. The engineer's calculations are usually £350–£1,000 for a domestic beam, and they are the cheapest insurance on the whole job.
We arrange the engineer's calculations as part of every installation, or fit to a builder's existing calculations on a fit-only basis.
Related questions
Can a builder tell me what size beam I need?
A builder can make an educated guess, but building control needs calculations from a structural engineer. Without them you won't get a completion certificate, which causes problems when you sell.
What does UB mean on a steel beam?
Universal beam, the standard I-shaped section used for most domestic openings. UC means universal column, a squarer section used for posts and heavier loads.
Do steel beams need fire protection?
Often, yes. Building Regulations usually require the steel to be protected, typically by boxing it in fire-rated plasterboard or using intumescent paint. The engineer and building control will confirm what's needed.
Ready to take it further? See our steel beam & rsj installation service.