Structural Steel Beams in London Extensions and Refurbishments: RSJ Sizing, Installation, and Building Regulations

**When a structural steel beam is required in a London extension or refurbishment**:

• A structural steel beam is required wherever a new opening or penetration is made in a load-bearing element (a wall, floor, or roof structure that carries vertical or horizontal structural loads from elements above or adjacent). In a London Victorian terrace, load-bearing elements typically include:
• •**External walls**: all external walls of a Victorian terrace are structurally significant. The rear elevation wall (through which the extension connection is made) typically carries the rear first-floor wall/floor and possibly roof loads
• •**Spine walls**: many Victorian terraces have a central load-bearing spine wall running front-to-back through the middle of the house plan — parallel to the party walls. This spine wall typically carries the staircase, first-floor landing, and the first-floor flat between the front and rear rooms. Creating an open-plan ground floor by removing the spine wall requires a significant structural beam to carry the transferred loads
• •**Chimney breast walls**: a chimney breast is part of the party wall or an internal load-bearing wall. Removing a chimney breast (at any level) requires a structural beam to carry the loads that the breast was carrying — including the weight of the breast structure above the removal level
• •**First-floor joists**: where a loft conversion creates a new room in the loft space, the first-floor joists (which also serve as the ceiling of the rooms below) may need structural beams at the loft floor edges

• **Which structural elements in a London Victorian terrace are NOT load-bearing**:
• •Front bay window front wall (bay window cheeks and face are typically non-structural — but check with a structural engineer)
• •Cross-partition walls (plasterboard stud walls running perpendicular to the party walls — typically added in the 20th century; not original load-bearing elements)
• •Timber stud partitions in 1970s–1990s extensions or loft conversions

*The only way to confirm whether a wall is load-bearing is to have a structural engineer inspect the property and the building's structural layout. Assumptions should not be made without professional input.*

**Structural beam selection — how the engineer calculates the required beam size**:

The required beam size (depth, width, and weight per metre) is calculated by a structural engineer based on: 1. **The span** — the clear distance between the bearing points (the walls or columns the beam sits on). The larger the span, the deeper and heavier the beam required 2. **The total load** — the total dead load (self-weight of the structure above; floor finishes; partition walls) plus the live load (occupancy load; furniture; snow load on the roof, if relevant) carried by the beam. The longer the span AND the greater the load above the beam, the larger the beam required 3. **The deflection limit** — the beam must not deflect (bend under load) more than span/360 under the live load, to avoid cracking plasterwork on the ceiling below the beam. This deflection limit often governs the beam selection for longer spans (over 4–5m) 4. **The steel grade** — structural steel in UK residential construction is typically Grade S275 (yield strength 275 N/mm²) or S355 (yield strength 355 N/mm²). S355 steel is stronger per unit weight — it allows a slightly lighter beam to carry the same load, but costs slightly more per tonne

**Typical RSJ (UB) sizes for common London extension opening spans**:

| Span (clear opening) | Typical UB size (S275) | Typical load scenario | |---|---|---| | 2.0m (narrow doorway) | 152 × 89 × 16 UB or 178 × 102 × 19 UB | Single-storey wall opening; no floor above | | 2.5m | 178 × 102 × 19 UB or 203 × 133 × 25 UB | Single-storey; light load above | | 3.0m (standard rear wall opening, single-storey extension) | 203 × 133 × 25 UB or 254 × 102 × 22 UB | Single-storey extension; floor above | | 3.5m | 254 × 102 × 25 UB or 254 × 146 × 31 UB | Ground-floor opening; first floor above; standard terraced house | | 4.0m (wide rear extension opening) | 305 × 102 × 28 UB or 305 × 127 × 37 UB | Ground floor; first floor and roof above; standard load | | 5.0m (very wide rear opening or ground-floor spine wall removal) | 356 × 127 × 39 UB or 356 × 171 × 45 UB | Ground floor; 2 floors above; significant load | | 6.0m (large open plan ground floor) | 406 × 140 × 46 UB or 406 × 178 × 54 UB | Multiple floors and roof above; engineer to confirm |

*UB notation explained*: `254 × 146 × 31 UB` means: 254mm nominal depth × 146mm flange width × 31 kg/m weight. Always confirm the beam specification with a structural engineer — the above are indicative only and must be checked for the specific load scenario.*

**Padstone specification — the critical bearing element**:

A padstone is the reinforced concrete block or engineering brick placed under each end of the structural beam to spread the concentrated point load from the beam end into the wall below over a larger bearing area, preventing local crushing of the masonry.

• *Part A structural safety requirements for padstones (Building Regulations)*:
• •Padstone dimensions must be confirmed by the structural engineer as part of the beam design — the padstone size depends on the bearing stress at the wall and the beam end reaction load
• •Typical padstone specification for London residential extension RSJ beams:
• - Concrete padstone: minimum C25/30 concrete; minimum 215mm × 140mm × 65mm (a standard half-brick depth padstone) for lighter beams; larger padstones for heavier loads
• - Engineering brick padstone: a course or two of Class B or Class A engineering brick (Staffordshire Blue) can be used as a padstone — very high compressive strength
• •Minimum bearing length of the beam on the padstone: typically 150mm minimum — the beam end must sit on at least 150mm of padstone bearing
• •The padstone must be fully bedded in cement mortar (1:3 OPC:sand) on the masonry below — no hollow bedding

**Temporary propping — the critical safety requirement during RSJ installation**:

Before any load-bearing masonry is removed to create the opening for a new structural beam, the loads carried by that masonry must be temporarily transferred to an alternative structural path — this is achieved by temporary propping. Failure to provide adequate temporary propping before masonry removal is a serious structural safety risk and is one of the most common causes of structural collapse during renovation works in London.

*Temporary propping principles for London extension RSJ installation*: 1. Establish the load path above the proposed opening — identify what is supported by the masonry to be removed (floor joists; first-floor walls; roof structure; chimney stack) 2. Design the temporary propping arrangement to carry these loads safely to the ground — typically via adjustable steel acrow props (Acrows) with prop heads; or via temporary steel or timber needles through the wall; or via a combination of both 3. Install the temporary propping before any masonry removal begins 4. Do not remove masonry until the propping is in place and the loads have been confirmed to be transferred — check that each prop is correctly loaded (firm; plumb; not loose) 5. Once the beam is installed, fully bedded, and the masonry above the beam is rebuilt (or the lintel above is correctly supported), the props can be de-propped in a controlled sequence

• *Temporary propping requirements from the Building Regulations (Approved Document A — Structure)*:
• •The propping design must be assessed by a competent person — for significant spans (above approximately 3m) or complex load scenarios (chimney breast over two floors; spine wall), the propping arrangement should be designed by the structural engineer as part of the beam design package
• •Propping must be on solid ground or a solid floor — acrow props must not be placed on suspended timber floors (which can deflect or fail under the concentrated prop load) without a spreader arrangement

**RSJ installation sequence for a London rear extension opening**:

1. Structural engineer completes beam design; issues sketches/spec 2. Building Control plan check submitted and approved (or building notice issued) 3. RSJ beam ordered from a steel stockholder (typically 2–5 working days lead time for standard sections in London; longer for non-standard sections) 4. Temporary props erected under floor/ceiling above the proposed opening 5. Opening in the masonry cut (typically using a disc cutter for dense blockwork; or by picking out bricks for brick masonry) 6. Padstones placed and bedded at each bearing position 7. Beam installed into the opening — typically lifted by: (a) a small crane or UNIC spider lift if access allows; (b) a tirfor manual come-along winch; (c) a Porta-Lift or beam trolley; or (d) directly by two or more workers for shorter, lighter beams. For single-storey rear extension openings (typically 3–4m span, 25–45kg/m), a 3–4m beam weighs 75–180kg — manageable by 2–3 operatives with appropriate lifting equipment and care 8. Beam packed tight up to the underside of the structure above (using slate packs or proprietary steel packs) — ensuring no gap between the beam top flange and the masonry above 9. Masonry above the beam rebuilt where required (infilling gaps; making good) 10. Building Control stage inspection — structural stage (Building Control officer inspects the beam installation before plasterwork is applied) 11. Fire protection applied (see below) 12. Beam boxed in or left exposed (client/architect choice)

**Steel beam fire protection — Part B compliance**:

Structural steel exposed to fire loses strength rapidly as its temperature rises. At approximately 550°C, structural steel retains only 60% of its ambient-temperature yield strength — and will deflect significantly under structural load at this temperature. Part B (Fire safety) of the Building Regulations requires that structural elements in a dwelling maintain their structural integrity for the required fire resistance period:

*Fire resistance requirements for structural beams in London residential buildings (Part B Approved Document B, Volume 1)*:

| Application | Required fire resistance period | |---|---| | Ground-floor beam (single-storey extension, no habitable room above) | 30 minutes | | First-floor beam (beam under a first floor; habitable room above) | 30 minutes | | Beam under a second floor or roof in a multi-storey extension | 30 minutes to 60 minutes (depending on the building height and configuration; confirm with Building Control) |

*Steel beam fire protection methods*:

**Method 1 — Intumescent paint**: Intumescent paint is the most common fire protection method for RSJ beams in London residential extensions. The intumescent coating is a thin layer (typically 0.5–3.0mm DFT — dry film thickness) that expands dramatically when heated, forming a thick insulating char layer around the steel that delays the temperature rise in the steel.

Specification: the intumescent paint system consists of: 1. Primer (surface preparation; blast clean to Sa 2.5 or grind clean to St 3) 2. Intumescent base coat (the reactive layer — applied at the specified DFT to achieve the required fire resistance period) 3. Topcoat (optional protective and decorative finish — may be coloured if the beam is to be left exposed)

For a 30-minute fire resistance on a standard residential UB (up to approximately 305 × 127 UB), intumescent paint typically requires a DFT of approximately 1.0–1.5mm. The required DFT depends on the beam section factor (Hp/A — the ratio of the heated perimeter to the cross-sectional area) — the engineer or intumescent paint manufacturer will specify the required DFT based on the specific beam size.

**Method 2 — Plasterboard board encasement**: Encasing the steel beam in plasterboard (fire-rated Type F or Type X gypsum board, 12.5mm or 15mm thick) fixed to a light steel or timber sub-frame around the beam provides fire protection by insulating the steel from the heat of the fire. 30-minute fire resistance is typically achieved with one layer of 15mm Type F plasterboard on a steel sub-frame. This method also provides a clean architectural finish and is standard practice for concealed beams.

*Exposed beam fire protection*: Where the client wants the RSJ beam to be exposed (visible as an architectural feature), intumescent paint is the standard fire protection method. The intumescent coating can be overcoated with a decorative finish (typically satin or matt black or RAL 9005 for the popular exposed-steel aesthetic in London contemporary extensions).

**Building Control and structural steel — the plan check and stage inspection process**:

• *Plan check / building notice*:
• •Where a structural beam is part of a notifiable extension project (which all extensions to London dwellings are), the structural beam must be included in the Building Regulations application
• •The application must include: structural drawings showing the beam position, span, and bearing details; the structural engineer's calculation pack or specification; the padstone specification; and the fire protection specification
• •Building Control may accept a building notice (for smaller residential projects) — but structural steel typically warrants full plans application for plan check

• *Stage inspections*:
• •Structural stage: the Building Control officer visits to inspect the beam installation before the beam is covered or boxed in — typically called for by the contractor
• •The inspector will check: beam size (compared with approved drawings); padstone bedding and size; bearing length; and that the beam is packed up tight
• •After the inspection, the beam can be boxed in or the intumescent paint applied (if exposed)

*Structural warranty implications*: New structural beams installed under Building Regulations approval are covered by the completion certificate. For new-build or conversion projects with an NHBC Buildmark warranty, all structural beams must be installed and inspected per NHBC Standards. For single-dwelling extensions, the Building Control completion certificate typically satisfies mortgage lenders.

**Flitch beams — when and why they are used**:

A flitch beam (also called a flitched beam) is a composite beam made from a steel plate (the flitch plate — typically a mild steel flat bar or a UB web cut to size) sandwiched between two timber members (typically C24 structural timber joists or LVL — laminated veneer lumber) and bolted together through the assembly at regular intervals. The steel carries the tensile stress (bottom of the beam under bending) and the timber members carry the compressive stress and provide lateral stability and a means of fixing the beam to adjacent joinery and structure.

• *When flitch beams are preferred over plain UB beams*:
• •**Where depth is limited**: the composite flitch beam can achieve a smaller overall depth for a given span-to-load ratio compared with a timber joist — and a larger depth than a steel UB alone. Flitch beams are useful in Victorian first floors where the existing floor joist depth is 200–225mm and a conventional UB would be significantly deeper (causing a step in the ceiling)
• •**Where the beam must match the existing floor joist depth**: the flitch plate is selected to be the same depth as the timber (e.g., a 200mm deep flitch plate between two 200mm × 50mm C24 joists gives a combined beam depth of 200mm — the same as the adjacent floor joists)
• •**Where fixing into the beam is required**: fixing wood to a plain UB requires welding or bolting. A flitch beam with timber members on each face allows direct nailing or screwing from joists into the timber members
• •**For lighter spans (up to approximately 3.5–4.0m) in residential loft conversions**: the flitch beam is very common in London loft conversions for carrying the new loft floor on both sides of the staircase opening

*Flitch beam design*: flitch beams must be designed by a structural engineer — the steel plate size, timber member size, and bolt pattern must all be calculated for the specific span and load scenario. Do not attempt to specify a flitch beam without structural engineering input.

**Chimney breast removal beams — a specific and common London application**:

Chimney breast removal is one of the most common structural alterations in London Victorian and Edwardian terraces. When a chimney breast is removed from a room (typically the front reception room at ground floor, or a bedroom on the first floor), the loads previously carried by the chimney breast stack above must be redistributed to the remaining structure.

*Typical structural scenario for chimney breast removal in a London terrace*:

Removing the chimney breast from the ground-floor front reception room: 1. The chimney breast typically continues through the first floor and into the roof as the chimney stack 2. The weight of the first-floor and above chimney breast is carried on the ground-floor chimney breast — when the ground-floor breast is removed, this weight must be redistributed 3. Solution: install a steel beam across the chimney flue opening at ceiling level (first-floor level above the removed section) to carry the weight of the chimney above the removal point. This beam is typically: a purpose-formed RSJ spanning between the party wall and the room's spine wall (or another suitable bearing point); or a needled arrangement where short steel sections ('needles') pass through the party wall to bear on the wall thickness 4. The precise arrangement depends on the specific house structure and must be designed by a structural engineer 5. The first-floor chimney breast can then be retained, cantilevered on the new beam structure, or also removed (with a further beam at the second-floor level if the chimney continues above)

*Important — chimney breast removal notification*: Chimney breast removal is a notifiable structural alteration under Building Regulations (Part A — Structure). It requires Building Regulations approval (full plans application or building notice). Chimney breast removal without Building Regulations approval is a common hidden defect that emerges on property searches — a Level 2 or Level 3 RICS survey will typically flag undisclosed chimney breast removal.

**Cost of structural steel beam installation in London 2025**:

The cost of a structural steel beam installation in a London house extension or refurbishment depends on the span, load, access, and project complexity:

*Cost components*:

| Cost element | Typical cost range (London 2025) | |---|---| | Structural engineer design (beam calculation + specification) | £500–£1,500 (single beam or simple multi-beam scheme); £1,000–£3,000 (complex multi-beam; chimney breast scheme; loft conversion structural package) | | Steel beam supply (UB, standard sections, from stockholder) | £600–£2,500 (based on £900–£1,500/tonne steel + cutting/delivery; a 3m × 25 kg/m UB = ~75kg = ~£70 steel value — but minimum order charges apply) | | Temporary propping hire and setup | £300–£800 | | Installation labour (including masonry opening; padstones; beam lift and pack; making good) | £800–£2,500 (single beam in standard opening) | | Fire protection (intumescent paint or plasterboard encasement) | £200–£600 (intumescent on a single beam) | | Building Regulations application fee | £150–£400 (London Borough Building Control or Approved Inspector) |

*Typical all-in cost for a standard London rear extension RSJ installation (3–4m span; single beam; single-storey)*: **£2,500–£5,000** (inclusive of SE design; beam supply; propping; labour; fire protection; Building Control)

*Typical all-in cost for ground-floor spine wall removal (London terrace; 4–6m span; significant load)*: **£4,000–£10,000** (inclusive of SE design; larger beam(s); propping; labour; making good; fire protection; Building Control)

*Typical chimney breast removal (one floor; London terrace)*: **£2,500–£6,000** (inclusive of SE design; needle steel; propping; demolition; making good; Building Control)