What is an RSJ? Steel beams explained for homeowners

RSJ, UB, UC, PFC — the jargon translated

'RSJ' stuck as the household word decades ago, but modern beams come in a few standard families. What matters to you is that the engineer chooses; what helps is understanding the words on the quote.

• UB (universal beam): the standard I-shaped beam; the workhorse for wall removals
• UC (universal column): squatter section, often used where headroom is tight or as posts in goalpost frames
• PFC (parallel flange channel): C-shaped; often used in pairs or where the beam sits against something
• Flitch beam: steel plate bolted between timbers; handy for matching existing timber depths
• Box/goalpost frame: two columns plus a beam, used for full-width openings

How the size is chosen

The engineer works out the loads the wall carries — floors, walls above, roof, snow, people — and specifies a section that carries them with the right safety factor and without bending enough to crack plaster (deflection). That's why two identical-looking walls can need very different steels, and why guessing from a neighbour's job is a false economy.

The beam's ends matter as much as the beam. Loads concentrate where the steel bears onto the wall, so the engineer specifies padstones — concrete blocks that spread the load — and a minimum bearing length, typically 100–150mm each end.

What an RSJ costs

The steel itself is the cheap part: most domestic beams cost £150–£500 supplied, cut to length. Installation — propping, cutting out, lifting, packing, fire protection — is where the real cost sits, which is why a typical supplied-and-fitted single beam lands between £1,800 and £4,500 including the wall removal.