Structural Steel for Extensions: UBs, RSJs, Padstones, Fire Protection, and What It All Costs

Structural steel only looks optional when the person trying to remove it is not the engineer who specified it. On extension work, an unauthorised substitution can bury delay, extra cost, and non-compliance inside the structure before anyone realises. The homeowner does not need to calculate the beam, but does need to understand what was specified and why. That is often the only thing stopping a convenient site shortcut from becoming an expensive rebuild.

This page covers everything a homeowner needs to know about structural steel for extension work in the UK: what the different sections are called, what they cost, how they're installed, why padstones matter, what fire protection is required, and how to spot when something has gone wrong.

What it is and what it's for

Structural steel is hot-rolled mild steel formed into long sections with specific cross-sectional shapes designed to carry load efficiently. In domestic construction, structural steel does one job: it bridges openings or spans that masonry alone can't span. Anywhere you have a wall that needs to stay up but you want to remove the bit underneath (a window, a door, a kitchen-diner opening, a wall taken out for open-plan), there's almost always a steel beam doing the work.

The UK industry uses confusing overlapping names for the same thing.

Universal Beam (UB) is the modern UK term. Cross-section is an I-shape with parallel flanges (the top and bottom horizontal bars of the I). This is what you'll see on every structural engineer's drawing for a current project.

Rolled Steel Joist (RSJ) is the older name. Historic RSJs had tapered flanges (thicker at the web, thinner at the tips). True tapered-flange RSJs are largely obsolete in new work. But "RSJ" has stuck as a generic trade term, and most builders, suppliers, and homeowners still call any I-section beam an RSJ. When your builder says "RSJ," they almost certainly mean a modern parallel-flange UB.

I-beam is the American term, sometimes used in UK informal speech.

All three usually refer to the same thing in a domestic extension context. Don't get hung up on the naming. The structural engineer's drawing will specify the exact section.

The standards apply regardless of what you call it. Structural steel work in UK buildings is governed by Building Regulations Approved Document A (structural safety) and Approved Document B (fire safety), with structural design carried out to BS EN 1993, the European standard known as Eurocode 3. Eurocode 3 replaced the older British standard BS 5950 in April 2010, although BS 5950 can technically still be used. Most UK structural engineers now design to Eurocode 3 by default.

There's one more standard that matters as of 2025: UKCA marking is mandatory for all structural steel sold in Great Britain from 1 July 2025, replacing CE marking. Any fabricator producing structural steel for your project must hold a Factory Production Control (FPC) certificate from a UK Approved Body, a welding certificate per BS EN 1090-1, and must issue a UK Declaration of Performance with the steelwork. If a fabricator can't produce these documents on request, walk away. Building control will not accept uncertified structural steel.

Section types you'll meet

Five section types come up on extension drawings. Knowing the difference helps you read what your engineer has specified.

The two letters and three numbers on a structural drawing follow a strict format. A "203x133x25 UB" is a Universal Beam, nominally 203mm deep, 133mm wide flange, weighing 25kg per metre. The depth is what carries bending load efficiently, the width sets how much bearing the masonry above gets, and the kg/m number is how the supplier prices and weighs it.

Section size and what your engineer will specify

The structural engineer will pick the section size based on three things: the span, the load coming down on top of the beam, and the deflection limit (usually span divided by 360, so a 4m beam can deflect at most about 11mm under load).

This is indicative only. Always use the section your structural engineer specifies on the drawing.

A documented extension over 30m² used 305x127x48 UBs as the main spans across the rear opening. That's at the heavy end of typical residential because the build had a wide bifold opening with a roof load above and a goal-post connection at one end. A simpler rectangular extension with a 4m kitchen opening might only need a 203x133x30 UB. The variation is large and is driven entirely by the load path: what's above the beam (joists, roof, upper floor) and how far that load travels.

S275 vs S355 grades

Steel grade is the second specification on the drawing. Two grades dominate UK construction.

S275 has a yield strength of 275 N/mm². Adequate for most single-storey residential extensions. Better for welding. Cheaper.

S355 has a yield strength of 355 N/mm². About 29% stronger, allowing 15-25% lighter sections for the same capacity. Roughly 10-15% more expensive per kg. S355 is now the dominant grade in UK commercial construction generally, but for residential extensions S275 is still the common default unless the engineer specifies otherwise.

S460 exists for very high-load applications. You won't see it on an extension drawing unless something unusual is going on.

If the engineer's drawing says "S275" or "S355" with no further detail, the fabricator will supply that grade. Don't let anyone substitute a different grade without going back to the engineer for sign-off.

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How structural steel actually gets specified, made, and installed

Steel arrives on site through a three-trade chain: structural engineer specifies it, fabricator makes it, builder installs it. Knowing who does what stops you from blaming the wrong person when something goes wrong.

The structural engineer's role

A chartered structural engineer (member of the Institution of Structural Engineers, IStructE, or the Institution of Civil Engineers, ICE) carries out load calculations, picks the section sizes, designs connections, specifies padstones, and produces a structural drawing. The drawing goes into your building control submission under the engineer's stamp. Their fee for the steelwork element of an extension typically runs £150–250 for initial calcs, or £500–1,500 as a complete package covering site visit, calculations, and drawings.

Skipping the engineer is the single most damaging mistake a homeowner can make. A 2018 CROSS Safety report found that 30-40% of structural applications received from non-professional engineers were potentially under-designed. Beams calculated as fully restrained when they should have been unrestrained, software used without understanding, copy-pasted spans from neighbours' projects. The cost of getting it wrong runs from cracked walls to collapse. The cost of getting it right is a few hundred pounds.

The fabricator's role

The fabricator is who turns mill stock into the exact piece you need. There are two purchasing routes:

Stock supply (online merchant or steel stockist). Cut to length from standard 6m or 12m mill lengths. No drilling, no welding, no end plates. Fast: 1-5 days delivery. Suitable for a single straight beam with simple bearing connections. Suppliers include FH Brundle, Southend Steels, metals4U, Pratley Builders Beams, and Cardiff Steels.

Fabricated to drawing. The structural engineer's drawing goes to a steel fabricator who cuts to exact length, drills bolt holes, welds end plates, and prepares column-to-beam connections. Lead time is 2-8 weeks depending on the fabricator's workload. Required for any multi-beam frame, goal-post arrangement, cantilever connection, or where one beam bolts to another.

The 30m² documented extension used a local fabricator (a small Oxfordshire firm) for the full sequence: main beams in March 2022, ridge beam in May 2022, cantilever and welded plates from November 2022. Total fabrication invoices ran around seven to eight thousand pounds across the project, plus builder's labour for installation. Five separate fabrication phases is unusual; most extensions complete fabrication in one or two phases.

Lead time matters more than price

Steel lead time is the project management trap. Order steel too early and the engineer's design might still change. Order it too late and the build sits idle waiting for the beams. The pattern that works:

1. Engineer issues drawings.
2. Building control accepts the structural calcs (no point fabricating against a design that fails BC review).
3. Order fabrication before the wall opening is created on site, so the steel arrives roughly when the masonry is ready to receive it.
4. Builder confirms padstone size and bearing detail before the steel arrives.

Plan four to eight weeks for fabricated steel from order to delivery. If your build programme depends on the steel being on site in week six, the order needs to be placed in week one or two.

The builder's role

The builder receives the steel, lifts it into position, beds it onto padstones, bolts up connections, fits temporary support during the lift, and applies fire protection. Most extension builders have done dozens of steel installations and know the drill. The risk areas are: under-engineered lifting (especially in confined spaces), wrong padstone size, inadequate bearing, and skipped fire protection.

Padstones and bearing detail

A steel beam ends concentrate load into a small area of masonry beneath. Without something to spread that load, the masonry crushes and the beam settles, which cracks walls, jams doors, and triggers expensive remediation. The standard solution is a padstone: a dense concrete or engineering-brick block placed under the beam end, sized to spread the point load into the wall safely.

Standard UK padstone sizes for domestic work are 440x215x100mm, 440x215x140mm, and 440x215x215mm. The depth (100, 140, or 215mm) depends on how much load needs to be spread. Most extensions use precast concrete padstones at C40/50 strength class. £8 – £20 per padstone from a builders' merchant. A typical extension needs 2 to 4. Engineering bricks (Class B) are an acceptable alternative where a SE confirms the masonry strength is enough, but precast concrete is the default.

Three things must be right at every bearing:

Bearing length. UK guidance (Planning Portal, LABC, most building control officers) calls for a minimum 150mm bearing at each end. NHBC's Chapter 6.5 cites 100mm as a floor minimum, but in practice 150mm is what BCOs enforce. Your engineer's drawing will state the required bearing length; achieve at least that.

Padstone size. The padstone must be wider than the beam flange and deep enough to spread the load through the masonry below. If the engineer specifies 440x215x100, install 440x215x100. Don't substitute 300x215x100 because it's what the merchant had in stock.

Level and level bedding. The padstone must be level (spirit-level confirmed before beam delivery), bedded on a full mortar joint, and the top surface clean and dry when the beam arrives. Slate shims to pack out a low padstone are not acceptable; building control will reject slate because it crushes under load.

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Fire protection: 30 minutes or 60 minutes?

Bare structural steel loses strength rapidly above 550°C and will buckle in a fire. UK Building Regulations require structural elements to maintain their load-carrying capacity for a minimum period in a fire, expressed as REI minutes (Resistance, Integrity, Insulation). The required period for an extension steel comes from Approved Document B Volume 1, Table 4.1.

The decision tree is simpler than the regulations make it sound:

Single-storey extension, no occupied floor above the beam. 30 minutes REI. Achieved by encasing the beam in two layers of 12.5mm standard plasterboard with staggered joints, fixed to the beam soffit and sides per the British Gypsum White Book installation specification.

Beam supports an upper floor or any occupied space above. 60 minutes REI. Achieved with two layers of 15mm fire-rated plasterboard (the pink board, often Gyproc Fireline or equivalent) installed exactly to the manufacturer's spec, including correct fixing centres, joint stagger, and metal corner beads.

Beam left exposed as a feature. Intumescent paint applied to a tested system thickness. The paint expands when heated to form an insulating char that protects the steel. Domestic-grade products like Zeroflame Aquasteel WB exist. Surface preparation matters: clean, dry, oil-free, with primer applied first. The required dry film thickness depends on section size and number of exposed faces, and your engineer or a fire-protection specialist will calculate it. Document every coat thickness for building control. Material cost runs roughly £40 – £200 per linear metre of beam.

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The community confusion on fire protection (visible across BuildHub, MoneySavingExpert, and Screwfix forums) is mostly about which scenario applies. If you have a single-storey rear extension with no bedroom or attic above, you're in the 30-minute camp. If your beam is under a converted loft, an upper bedroom, or a flat roof where someone walks during maintenance, you're at 60 minutes.

Thermal break at the column base

Where a steel column lands on the ground floor inside the warm zone of the building, raw steel acts as a thermal bridge: heat conducts directly out through the steel into the cold ground. This is a Part L compliance issue and a condensation risk. The standard fix is a thermal break block at the column base, the most common product being a Marmox Thermoblock.

Marmox Thermoblock is a high-strength, low-conductivity block (compressive strength 9 N/mm², thermal conductivity 0.05 W/mK) that sits under the column base plate to break the conductive path while still carrying the load. Available in 100mm, 140mm, and 215mm widths to match wall thicknesses, in 600mm length and 65mm or 100mm thickness. Most homeowners have never heard of it; most engineers and building control officers will expect to see it where steel columns penetrate the insulation envelope.

If your steel design includes a goal-post frame with vertical columns landing on the ground floor inside the warm zone, ask your engineer if a thermal break block is specified. If not, raise it before the design is finalised.

Lifting and handling

Steel weight catches homeowners out. A 4m length of 152x89x16 UB weighs 64kg, manageable for two people. A 4m length of 254x146x37 UB weighs 148kg, which is at the limit of what four people can safely lift in a confined space. A 4m of 305x127x48 UB is 192kg and effectively requires lifting equipment.

Manual Handling Operations Regulations 1992 sets no absolute weight limit but requires the principal contractor to risk-assess the lift. The HSE recommended limit per person is 25kg in good conditions, less in awkward postures or confined access. For anything over about 80kg total, expect to see acrow props with strongboy heads (the bracket that supports the masonry above while you remove the wall under it), bottle jacks, or a hired telehandler/small crane.

CDM Regulations 2015 require the structural engineer to provide an installation methodology for heavier or awkward beams, not just structural calcs. If your engineer hasn't documented how the beam goes in, that's a missed deliverable.

The documented build's main steels were 305x127x48 UBs and were lifted using acrow-and-strongboy rigs over two days. No crane access was possible. Plan well ahead for lifting on tight terraced or end-of-terrace access.

How much does a typical extension's steelwork cost

For an extension with a single 4m kitchen opening, expect:

• Structural engineer fee for steel design: £500–1,500
• Steel supply (cut to length, simple beam): a few hundred pounds depending on section size, calculable from the per-metre table below
• Padstones: 2 to 4 at £8 – £20 each
• Builder labour for installation: £800–1,500
• Fire protection plasterboard and labour: £200 – £500

A straightforward single-beam project lands at £900–1,600.

A wider opening with a cantilever for corner-free bifolds adds:

• Cantilever steel fabrication: £1,000–1,500
• On-site welding for connections: £700–1,200
• Possible additional pier or foundation work: £300–600

Long-span or complex multi-beam jobs run £3,000–10,000, including the documented 30m² project at the upper end of that range due to repeated design revisions.

Per-metre supply pricing

If you want to sanity-check what your fabricator is quoting for the steel itself (separate from labour and engineering), per-metre supply prices from UK retailers and stockists are:

These are stock prices for cut-to-length supply. Add 20-50% for fabrication (drilling holes, end plates, welded connections), more for cantilevers or goal-post frames. Prices are ex VAT; add 20% for the consumer total. Delivery is typically £40 – £80 within local area, free above an order threshold of around five hundred pounds at most stockists.

If your builder's steel quote is more than double these per-metre figures multiplied by the beam length, plus a fair margin, ask what's in the difference. Some fabricators bundle install labour into the steel supply line, which is fine if disclosed.

External resource

Southend Steels UB price list

UK steel stockist with current per-metre prices for all common UB sizes. Useful sanity check on a fabricator's quote.

southendsteels.com

Building control inspections for steel

Building control (LABC or an Approved Inspector) inspects extension steelwork at three points typically:

1. Before steel arrives. Padstones in place, bedded, and level. Bearing locations confirmed against structural drawings.
2. Steel installed but not yet covered. Beam in position on padstones, bearing length achieved, connections bolted and welded as specified, no obvious deflection. The BCO will physically check this.
3. Fire protection complete, before plastering. Boarding installed to manufacturer spec, or intumescent paint applied to documented thickness.

The hard rule: never plaster, board, or otherwise hide structural steel before the BCO has inspected it. Builders sometimes try to keep the programme moving by covering ahead of inspection. If they do, the BCO can require the covering to be removed at your expense to verify what's underneath. On the documented 30m² build, three separate steel inspection points happened across 11 months of structural work, including the May 2022 ridge beam crisis described in the next section.

What can go wrong: a real ridge beam substitution

The most instructive structural failure isn't a collapse, it's a substitution. On a 30m² rear extension with a vaulted shallow-pitch roof, the structural engineer specified a steel ridge beam to control the outward thrust from the rafters onto the wall plates. The builder, reasonably enough from his perspective, decided the steel ridge was unnecessary and substituted two bolted-together 8x2 timber beams. He didn't tell the homeowner, didn't tell the architect, and didn't tell the engineer.

The building control officer caught it on a routine inspection a few weeks later. Timber, however well-bolted, is less stiff than steel. The beam would deflect more under roof load, the rafters would push out at the wall plates, the wall plates would pull on the masonry, and over time the walls would crack and the roof would sag. The BCO required the steel ridge to be reinstated. The builder had to lift the timber out, order the steel beam from the fabricator (a few hundred pounds invoiced for that section), reinstall, and call the BCO back to re-inspect. Total delay: about a fortnight, plus the inspection re-attendance fee.

The lesson isn't that builders are dishonest. Most aren't. The lesson is that on any structural project, the contract should explicitly prohibit substitution of any structural element without written approval from the structural engineer. Add a clause. Get it signed before work starts. If your contract doesn't contain that clause already, BCO inspection is your only catch, and BCOs only inspect after the work is done.

Common mistakes

Using an online RSJ calculator instead of paying an engineer. Online calculators give a section size but ignore foundation suitability, wall capacity, building stability, and load combinations. Cheap is expensive when the wall cracks two years later.

Ordering steel before building control approves the structural calcs. A failed BC review can change the section size or connection detail. Fabricated steel ordered against a rejected design either gets re-fabricated (at cost) or sits in a yard while the rest of the build waits.

Padstone undersized or substituted on site. The builder uses what's in the merchant's yard rather than what the engineer specified. Visible at inspection if you photograph everything; expensive to fix if the wall is already plastered.

Mistaking 30 minutes fire resistance for "30 minutes of any plasterboard." The fire rating only applies if you follow the manufacturer's tested system specification. Random plasterboard offcuts fixed with random screws don't qualify.

No thermal break at column bases. The Part L issue surfaces years later as condensation forming on cold spots near the column base, then mould.

Substituting timber for steel without engineer approval. As above; don't let it happen, and write the prohibition into the contract.

Treating "RSJ" and "UB" as different products. They're the same thing in modern domestic work. Don't pay a premium for a "real RSJ" or accept a substitution from "RSJ" to "UB" as if it matters.

Where you'll need this

Structural steel is involved at multiple points in any extension or renovation project where load-bearing walls are altered or wider openings are formed.

• Steels and lintels - primary horizontal beams across the rear opening, plus lintels above all new windows and doors
• Roof structure - structural ridge beams and roof support steels for vaulted or wide-span roofs
• Walls and blockwork - the masonry that receives steel beam ends, with padstones bedded into the blockwork courses
• Building control inspection structure - the BCO visit confirming bearing, padstones, welding, and fire protection

These elements show up across the structure phase of any single-storey or two-storey extension, loft conversion, or wider knock-through project where masonry walls are altered.