What is the difference between a steel and a lintel?

Lintels span standard window and door openings, are precast concrete or galvanised steel in stock lengths, and the bricklayer fits them as the walls go up. Structural steels, or UBs, carry the heavy loads at the main opening, wide door spans, and ridge, individually calculated and fabricated to size.

RSJ is the old name everyone still uses for a structural steel beam. The modern term is Universal Beam, or UB. A designation like 305 x 102 x 33 UB means 305mm deep, 102mm wide across the flanges, and 33kg per metre.

How long does steel fabrication take for an extension?

Standard stock sections typically arrive within a couple of weeks, and custom fabrication with specific lengths, bolt holes, or welded plates takes longer. Complex multi-element packages can run several weeks, so order early because delays here stall the roof and everything downstream.

What are padstones and why are they needed?

Padstones are dense concrete or engineering brick blocks beneath each end of a steel beam. They spread the steel's concentrated load into the wider blockwork below. Without them the steel would crush the blocks, so building control checks for them at every bearing point.

Why does extension steelwork cost more than expected?

A single online figure is for one beam, but an extension needs several: the main opening, lintels, often a ridge beam, and sometimes a cantilever. Design changes, cavity width problems on older houses, and extra fabrication are near-universal, so budget well above the initial quote.

Can a builder swap a steel ridge beam for timber?

Not without approval. If the approved specification says steel, it must be steel unless your structural engineer confirms timber is adequate in writing and building control is notified before the change. Builders substituting timber for steel unannounced is a documented and serious deviation.

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Phase 4 · Structure · Task 03 of 09

Structural Steels and Lintels for Extensions

What structural steels do, who specifies them, how to verify your order, and why extension steelwork routinely exceeds initial quotes. Covers cantilevers, cavity width problems, and the ordering failures that delay builds by months.

17 min readUpdated 2026Premium

Process infographic showing the steelwork journey from structural engineer specification through fabrication, delivery, and installation, with common failure points highlighted at each stage

Prerequisites

Complete these first

Your extension walls are a dead weight without steels. Every opening you create, whether it's knocking through to the existing house, spanning across bifold doors, or supporting the ridge of your new roof, needs a steel beam to carry the load that the wall can no longer carry. Get the steelwork right and nobody ever thinks about it again. Get it wrong and you're looking at structural failure, building control refusal, or months of delays while fabricators redesign what should have been specified correctly the first time.

Steelwork is where the money goes, the arguments start, and the programme stalls. On real projects, the steel saga can stretch across 25 or more separate events spanning nearly a year: ordering failures, design changes, cavity width problems, unauthorised material substitutions, and multiple fabricator visits. That's not normal. But it's not unusual either.

Do this first

Your structural engineer's calculations must be finalised before steels can be ordered. Walls must reach lintel height before lintels go in. Order steels as early as possible because fabrication takes 2-6 weeks, and delays here stall everything downstream: roof structure, windows and doors, and your building control inspection.

What this guide covers

01What Steels Actually Do
02Steels vs Lintels: Know the Difference
03Who Does What
04The Ordering Problem
05What It Costs
06The Cavity Width Problem
07The Cantilever Option
08The Ridge Beam Question
09Installation: What Actually Happens
10Fire Protection
11Design Changes: The Cost Multiplier
12Regional Differences
13The Steel Specification Checklist
14What Comes Next

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