What steel actually costs, how sizing works, and why the cheapest beam isn't always the cheapest job.
A typical domestic steel beam (RSJ), supplied and fitted, costs £600–£2,000 depending on span, depth and access — separate from the engineer's calculations and Building Control fee. The beam itself is rarely the biggest line item; sizing it correctly is what actually controls the total cost.
A beam's size isn't a matter of taste — it comes out of a calculation. Three things drive it: the span (the distance it has to cross unsupported), the load it's picking up (how much floor or roof area is bearing onto it, and how many storeys are above), and the allowable deflection — how much it's permitted to sag under load without cracking the finishes above it. Longer spans and heavier loads need a deeper, heavier section. There's no shortcut to this that doesn't risk the building.
Left: the two dimensions that matter — depth and flange width. Right: longer spans generally need a deeper section (illustrative only — every beam is sized by calculation, not by eye).
Most house-scale beams are "Universal Beam" (UB) sections, described by their depth and width in millimetres — for example 152x89, 203x133, 254x146 or 305x165. As a very rough feel for scale: a 2–3m kitchen doorway-width opening often lands around a 152–203 section; a 3.5–4.5m open-plan span often needs a 203–254 section or two beams side by side. These are illustrative only — your actual load and span always decide the real number.
It's tempting to shave cost by specifying the smallest section that "should" work. In practice, a beam sized too close to the limit deflects more than it should, which shows up months later as cracked plaster or a door that starts sticking — and fixing that after the fact costs far more than the few hundred pounds saved on steel. On the flip side, an oversized beam isn't free either: it's heavier to handle, needs a bigger padstone (sometimes a bigger foundation), and simply costs more than necessary. Correct sizing, checked by calculation rather than judgement, is what actually keeps the job cost-effective.
For shorter spans and lighter loads, an engineered timber beam (LVL or glulam) is sometimes a genuine alternative to steel — lighter to handle, easier to fix other timber to, and often left exposed as a feature. It typically costs a similar amount to a steel section supplied and fitted, but doesn't suit every span or load case. Your engineer can tell you within the same quote whether it's an option worth pricing alongside steel.
A beam number on its own isn't enough for a builder to price or Building Control to approve. A complete specification includes the exact section size and grade, the padstone or bearing detail at each end, connection details to adjoining structure, and a deflection check confirming it won't crack the finishes above. That's what turns a beam into a buildable, sign-off-ready piece of the calculation pack.
No obligation, no jargon — just a clear fixed fee and an honest view of what your project needs.