Structural Steel in London Extensions: RSJs, Beams and What to Know

The two steel section types most commonly used in London residential construction are: RSJ (Rolled Steel Joist), also known as I-beam or UB (Universal Beam): the standard beam section used to span openings in walls. The RSJ has a characteristic I-shape in cross-section — wide flanges top and bottom, a thin web connecting them. The depth and width of the section determines its strength and stiffness. Common section sizes in London residential work: 152x89, 178x102, 203x133, 254x146, 305x165 (all in mm, depth x flange width). The size required depends on the span of the opening and the load above. UC (Universal Column): a stockier section with approximately equal depth and width — used as a column or post rather than a spanning beam. Less common in residential work but used where a slim column is needed to support a beam above, particularly in open-plan ground floors. SHS and RHS (Square and Rectangular Hollow Sections): sometimes used for architectural or aesthetic purposes (visible steel details, feature columns) where the clean box section suits the design intent. Structurally less efficient than UB sections for beams of equivalent depth.

The most common applications of structural steel in London residential projects: Opening a wall between rooms (knockout opening): a steel RSJ spans the new opening in the load-bearing wall, supported on padstones at each end. The most frequently encountered steel installation in London home alterations. Rear wall removal for an extension connection: the existing rear wall of the house is opened to connect the new extension — typically a single or double steel beam spanning the full width of the opening. Loft conversion floor beam: a steel beam installed at first-floor ceiling level to support the new loft floor and the dormer structure, reducing the load on the existing ceiling joists. Loft flat roof beam: a ridge or valley beam in a loft conversion where the roof structure is modified to create dormer or Mansard geometry. Ground floor open-plan column and beam frame: in a wrap-around extension or wide-span open plan, a series of steel columns and beams create a structural frame at ground floor level, removing the need for load-bearing walls. Structural glazing support: in roof lanterns, bi-fold door heads, and frameless glass walls, steel sections are used concealed within the construction to carry the glass loads.

Structural steel costs in London have two components: the steel supply cost (based on the weight of the section and the market price for structural steel) and the installation cost (crane hire if required, propping, cutting and fitting, padstone installation, fire protection, making good). Steel supply cost: approximately £800-£1,500 per tonne for standard UK structural steel sections. A typical 3m span 203x133 RSJ weighs approximately 80kg — supply cost around £100-£150. A larger 5m span 305x165 UB weighs approximately 350-500kg — supply cost £450-£750. Installation cost: includes the labour, plant (scaffold, temporary propping, possibly a small crane for heavy sections), padstone materials, concrete infill, and fire protection painting or casing. For a standard single-span wall opening RSJ in a London terrace (2.5-3.5m span): £1,500-£4,000 supply and install including padstones and making good. For a wider span (5-6m, rear wall removal): £3,500-£8,000. For multiple beams in a complex open-plan alteration: £8,000-£20,000. These costs exclude the structural engineer's fee (£500-£1,500 for standard residential beam design) and building regulations.

A padstone is a bearing element placed under each end of a steel beam, distributing the concentrated beam reaction load into the wall or structure below. Without adequate padstones, the concentrated load at the beam ends can exceed the bearing capacity of the brickwork and cause crushing or cracking of the masonry. Padstone specifications: for smaller beams (up to approximately 254mm depth, light loads): a concrete or dense concrete block padstone, minimum 215x215mm plan and 100-150mm depth. For larger beams (305mm depth or more, heavier loads): engineering brick padstones, larger precast concrete padstones, or in some cases a reinforced concrete padstone beam where the wall bearing width is limited. The structural engineer specifies the padstone size and concrete/material grade as part of the beam design. The contractor installs the padstones in cement mortar before seating the beam. A common mistake is under-specifying padstones (using standard bricks rather than dense concrete or engineering bricks) — this can lead to settlement and cracking at the beam ends.

Bare structural steel loses its strength rapidly when exposed to fire — at 550°C, steel retains only 60% of its ambient-temperature strength, and at 700°C only 20%. In a fire, an unprotected steel beam supporting a floor above may fail within 15-30 minutes. Building Regulations Part B requires structural elements to maintain their stability for a defined period during a fire (typically 30 or 60 minutes for residential buildings). Fire protection for steel beams in London residential projects: Intumescent paint: a specialist paint applied to the steel that swells (intumesces) when exposed to heat, creating a thick insulating char that protects the steel from the fire. Applied in coats to achieve the required thickness (determined by the section factor and fire rating required). Most common method for visible steel in residential interiors. Cost: approximately £300-£800 per beam section, depending on size and fire rating. Plasterboard boxing: the steel beam is boxed in with two layers of fire-rated plasterboard — faster and less expensive than intumescent paint, but the steel becomes invisible. Where the beam is to be concealed within the ceiling, this is the preferred method. Concrete or vermiculite encasement: less common in residential work. Almost always either boxed in plasterboard or coated with intumescent paint in London residential extensions.