Structural Steelwork in London Extensions and Renovations: RSJs, Universal Beams, Sizing, and What to Expect

**The terminology: RSJ vs. UB vs. UC**:

The term 'RSJ' (Rolled Steel Joist) is the common colloquial term used by London homeowners and many tradespeople for any horizontal structural steel beam. However, the RSJ (a specific older section with tapered flanges, no longer commonly produced in the UK) has been largely superseded by more efficient modern sections:

• *Universal Beam (UB)*:
• The most common structural steel section for horizontal spanning elements in London residential construction — floor beams, lintel beams across structural openings, roof support beams. Key features:
• •Parallel flanges (unlike the tapered flanges of an RSJ)
• •Deeper than it is wide — optimised for bending strength under vertical loads
• •Designated by depth × flange width × weight per metre length: e.g., 254×146×37 UB (254mm deep, 146mm wide flanges, 37 kg/m)
• •Produced in standard lengths to 18m from UK steel stockholders (e.g., Metals4U, Brown McFarlane, Corus/Tata Steel)
• •Available in standard lengths or cut to length for delivery

• *Universal Column (UC)*:
• Optimised for vertical (axial) loading — a UC section is approximately as wide as it is deep (square-ish profile). UCs are used as columns (vertical members carrying load from above to foundations). In residential projects, UCs appear as:
• •Steel post in a narrow wall void where the wall has been removed (e.g., chimney breast support column)
• •Column in a steel goal-post frame at the head of a wide garage opening
• •Column supporting a long beam at mid-span (reducing the effective span and beam size)

• *Rectangular and Square Hollow Sections (RHS/SHS)*:
• Hollow sections (tube steel) have excellent torsional (twisting) resistance and are used where:
• •A neat architectural finish is required (hollow sections have no exposed flanges that accumulate dirt)
• •The member must be used in both bending and axial compression
• •Connection geometry does not suit open sections

*Flats and plates*: Steel flats and plates are used for connection details — gusset plates, base plates, and packing plates at beam-to-wall connections and padstone arrangements.

**The most common steelwork configurations in London residential projects**:

*1. Single beam spanning a structural wall opening*: The most common steelwork installation in London residential projects — a single UB spanning the width of a structural opening (kitchen wall removal; chimney breast removal; merging two reception rooms). The beam sits on padstones (dense concrete blocks bedded into the masonry on each side of the opening) and is installed by temporary propping the floor above before the wall below is removed.

• Typical beam sizes for common London residential openings:
• •2.0m opening (e.g., chimney breast removal): typically 178×102×19 UB or 203×102×23 UB
• •2.5m opening: typically 203×133×25 UB or 254×102×22 UB
• •3.0m opening (one room width): typically 254×146×31 UB or 305×102×25 UB
• •3.5–4.5m opening (full rear wall removal or wide rear extension opening): typically 305×165×40 UB to 406×178×54 UB depending on loading
• •5.0–6.0m opening (very wide rear extension or underpinning span): 457×191×67 UB to 533×210×92 UB

Note: these are indicative only — the structural engineer specifies the actual beam size based on the precise loading, span, and deflection criteria. Never install a steel beam without SE calculations.

*2. Pair of beams in a compound arrangement (wall-to-wall)*: For extensions where the new extension floor or roof is to be supported from the side walls rather than from a mid-span column, a pair of beams (one on each side of the wall cavity or partition) may be used in a 'flitch plate' or 'double beam' arrangement. More commonly, a single deeper beam is used.

*3. Padstone and holding-down arrangement*: A padstone is a dense concrete or natural stone block bedded into the masonry wall at each end of the spanning beam. The padstone distributes the concentrated beam end reaction over a larger masonry area, preventing local crushing of the brickwork under the beam. Padstone size is specified by the SE (typically 215mm × 100mm × minimum depth to suit the wall type; dense concrete padstones typically 40N/mm²). Steel beams are fixed to padstones using either epoxy resin anchor bolts or, for lightly loaded spans, simply wedged with mortar.

**The structural engineering approach to beam sizing**:

A structural engineer sizes a beam by checking that it has adequate strength and stiffness for the loads it must carry over the required span. The key calculations are:

• *1. Loading assessment*:
• The engineer determines the total load the beam must support. For a London residential floor beam or a beam spanning a structural opening, the loads include:
• •Dead load (DL): permanent loads — floor structure, floor finishes, partition allowance, ceiling below
• •Imposed load (IL): variable loads — people, furniture, contents (typically 1.5 kN/m² for residential floors; 0.5 kN/m² for non-accessible roofs)
• •Point loads from above: if columns, beams, or structural elements above transfer load to the beam, these are accounted for as point loads
• •Self-weight of the beam

*2. Bending moment and shear force calculation*: For a simply supported beam (supported at both ends, no intermediate support), the maximum bending moment occurs at mid-span and is calculated as: M_max = wL²/8 (for uniformly distributed load w over span L)

The structural engineer selects a beam section with a 'plastic moment of resistance' (M_pl) that exceeds M_max with the appropriate safety factor (typically using Eurocode 3 / BS EN 1993 with the UK National Annex).

• *3. Deflection check*:
• A beam that is structurally strong enough but too flexible will deflect excessively under load — causing cracking of plaster finishes, sticking doors, and visible sagging. The Building Regulations' deemed-to-satisfy limit for deflection is typically:
• •Span/360 for beams carrying plaster or brittle finishes
• •Span/250 for beams without brittle finishes
• •Absolute maximum: typically 20–25mm

For a 4.5m span, Span/360 = 12.5mm maximum deflection. A structurally adequate beam may still need to be upsize if deflection governs rather than strength. This is particularly important for long-span beams in open-plan kitchen extensions.

*4. The main factors that increase beam size in London residential projects*:

• •Longer span: beam size increases approximately with the square of the span — doubling the span quadruples the required beam section size
• •Additional floors above: every floor above the beam adds to the dead and imposed loading — a beam carrying one floor above is much smaller than a beam carrying two floors and a roof
• •Concentrated point loads: a column or pilaster bearing onto the beam at mid-span dramatically increases the bending moment compared to the same load uniformly distributed
• •High imposed load areas: if the space above is a bathroom (heavy floor screed + tiles + bath + water) rather than a bedroom, the imposed load is significantly higher
• •Tight deflection limits: where the beam supports a tiled floor or plasterboard ceiling below (brittle finishes), the stricter deflection limit governs and increases the required beam depth

**Beam specification and procurement — the contractor's role**:

Once the SE has specified the beam (e.g., '254×146×31 UB Grade S275 to BS EN 10025-2, cut to 3600mm; 30mm minimum bearing each end'), the contractor is responsible for:

• •Obtaining a quote from a steel stockholder or fabricator and ordering the beam
• •Confirming that the supplied beam matches the SE's specification (grade, weight, section designation)
• •Arranging delivery and crane/telehandler lift where required (long or heavy beams typically require a HIAB vehicle or crane delivery; 406×178×54 UB at 3.6m = approximately 195kg — requires two people and mechanical assistance to lift into position)
• •Installing the beam on the specified padstones, checking level and alignment
• •Notifying Building Control that the structural steelwork is ready for inspection before any boxing in or plasterboarding

**Fire protection for structural steelwork**:

Bare steel loses structural strength rapidly at elevated temperatures — a typical structural steel section will lose 50% of its yield strength at approximately 500–550°C, which can be reached in a building fire within minutes of flashover. Building Regulations Part B requires that structural steelwork is protected to maintain its structural performance for the required period of fire resistance:

• •Lintels (beam spanning a wall opening) within an external wall: often not required to have fire protection (the masonry wall itself provides fire containment)
• •Beams supporting a floor structure: typically require 30-minute fire resistance (REI 30) in a domestic dwelling — achieved by plasterboard boxing (2 × 12.5mm plasterboard, or 1 × 15mm plasterboard)
• •Beams in an HMO where a longer fire resistance period is required: may need 60-minute fire protection (intumescent paint or thicker plasterboard boxing)

The most common fire protection method in London residential projects is plasterboard boxing — the beam is wrapped in two layers of 12.5mm plasterboard (standard pink plasterboard) to achieve 30-minute fire resistance. The boxing is completed by the dry-lining or plastering subcontractor after the structural frame inspections are done.

Alternative: intumescent paint — a thin coat of special paint that expands dramatically on heating (intumescent = swells when hot), forming an insulating char around the steel. Intumescent paint is used where the architectural finish requires the beam to remain visible — in contemporary open-plan interiors where an exposed steel beam is an aesthetic feature. Intumescent paint systems for residential applications: 60-minute rating with appropriate coat thickness; supplier assessment required; specified by the SE or building inspector. Typical cost: £20–£60/m of beam length for intumescent paint application vs. £10–£30/m for plasterboard boxing.

**Galvanising of steelwork in exposed positions**:

• For steelwork in wet or exposed conditions (external lintels, steel in contact with masonry in a cavity wall, structural elements in a basement or ground floor slab), hot-dip galvanising (HDG) protects the steel from corrosion. The galvanising process:
• •Steel is dipped in molten zinc at approximately 450°C
• •The zinc forms an intermetallic layer with the steel surface
• •Provides corrosion protection typically for 50–80 years in UK atmospheric conditions
• •Cost: typically £200–£600 to galvanise a single residential beam, depending on size and the galvaniser's minimum job charge

For internal structural beams in a dry, warm space (inside a plastered wall or floor), galvanising is not required — painting with a suitable primer and top coat before installation is sufficient.

**Structural steelwork costs for London residential projects (2025)**:

*Supply cost of common beam sections (UK steel stockholder, cut to length, ex-works, grade S275)*:

| Section | Weight (kg/m) | 3.0m length (approx.) | 4.5m length (approx.) | |---|---|---|---| | 203×133×25 UB | 25.1 kg/m | £70–£110 | £105–£165 | | 254×146×31 UB | 31.1 kg/m | £85–£130 | £130–£200 | | 305×102×28 UB | 28.2 kg/m | £80–£120 | £115–£175 | | 305×165×40 UB | 40.3 kg/m | £110–£170 | £165–£255 | | 406×178×54 UB | 54.1 kg/m | £150–£230 | £225–£340 | | 457×191×67 UB | 67.1 kg/m | £185–£285 | £275–£425 |

Note: steel prices fluctuate with the global market — these are 2025 indicative figures. Add delivery cost (typically £50–£150 for London residential sites) and crane/HIAB delivery for heavier beams.

*Installation cost for beam installation (London residential, 2025)*:

• Beam installation labour cost depends on:
• •Size and weight of beam (heavier = more complex lift)
• •Height of installation (ground floor lintel vs. first floor beam requiring scaffolding or podium)
• •Number of beams (two beams together are often only marginally more expensive than one)
• •Complexity of temporary propping (number of floors above; occupied house; restricted access)

• Typical labour-only installation cost for one beam in a standard London residential extension:
• •Small lintel (up to 203×133 UB, span up to 2.5m): £400–£800
• •Medium beam (up to 305×165 UB, span up to 4.0m): £700–£1,500
• •Large beam (457×191 UB and above, span 4.5m+): £1,200–£2,500+

Note: these are labour-only costs. Add supply cost, padstone supply and installation, temporary propping hire, and structural engineer inspection.

*Total cost for a typical London structural opening (all-in: supply, temporary propping, installation, padstones, Building Control inspection)*:

• •Non-load-bearing partition removal (no steel required): £1,200–£2,500
• •Single storey rear wall removal with small UB (2.5–3.0m span): £2,500–£4,500
• •Wide rear wall removal with medium UB (3.5–4.5m span): £4,000–£7,500
• •Double-storey opening (ground and first floor combined): £7,000–£15,000+

**Programme — when steelwork is installed in a London extension project**:

In a standard London single-storey rear extension programme, structural steelwork is installed during the 'structural phase' — typically Week 5–8 of a 16-week programme:

1. Weeks 1–2: excavation and foundations 2. Weeks 2–4: substructure (walls to DPC, floor slab) 3. Weeks 4–6: superstructure (extension walls to wall plate) 4. **Week 6: structural steelwork in existing house (wall removal and beam installation)** 5. Weeks 6–7: roof structure 6. Week 7: roof waterproofing 7. Weeks 7–8: windows and doors installation (weathertight) 8. Weeks 8–10: first-fix (M&E) 9. Weeks 10–12: insulation and plastering 10. Weeks 12–14: second-fix 11. Weeks 14–16: tiling, decorating, and snagging

The structural steelwork in the existing house (week 6) is often on the programme critical path — it enables the extension to connect to the house. Delayed beam delivery (common if beam is non-standard or must be galvanised, which adds 2–3 weeks) can hold up the whole programme.

**Rule: order steelwork as soon as SE specification is confirmed — not after groundwork starts**.

For standard section beams from UK stockholders, lead time is typically 2–5 working days cut-to-length from stock. For non-standard sections, fabricated connections, or galvanised items, allow 2–4 weeks. For steel portal frames or complex welded assemblies, allow 4–8 weeks.