Structural Steelwork in Extensions: RSJs, Padstones, and Load Transfer

When a load-bearing wall is removed (for example, to create an open-plan kitchen-dining room by removing the rear ground-floor wall of a Victorian terrace), the load that the wall was carrying must be transferred to a new structural path. A steel beam performs this function: it spans the new opening, carries the distributed load above (floor loads, wall loads, roof loads), and transfers them to the supports at each end.

• **What loads act on a beam**:
• •Self-weight of the wall or floor above the beam
• •Live loads (furniture, occupants) on the floor above
• •Roof loads transferred through the wall above
• •In double-storey extensions or loft conversions, multiple floors' worth of load may be accumulating at the beam level

• **How loads are transferred at the beam ends**:
• A steel beam must bear on adequate supports at each end — it cannot simply rest on the brickwork without distributing its point load over a sufficient area. Options:
• •**Padstones**: Precast concrete or engineering brick bearing pads placed into the wall at each end of the beam to spread the concentrated load over a larger area of masonry. Padstone dimensions are calculated by the structural engineer — typically 215 × 215 × 100mm minimum for a domestic span, but dependent on the load.
• •**Stacked courses of engineering bricks**: An alternative to precast padstone — a specified number of courses of high-compressive-strength engineering bricks, bonded with a strong mortar, to replace the equivalent padstone function.
• •**Column or post support**: Where a beam cannot bear onto an existing wall at one end (for example, at a mid-room column), a steel column or fabricated steel post carries the load down to a new foundation pad below.

**Deflection**: All beams deflect (sag) under load. Part A of the Building Regulations limits allowable deflection to span/360 (for beams visible on the ceiling) — which for a 5-metre span means maximum 13.9mm deflection. Structural engineers check both bending strength (capacity to carry the load) and deflection (limiting visible sag and plaster cracking). A beam that is strong enough to carry the load may still need to be upsized if the deflection is excessive.

**Beam types used in domestic extensions**:

*Universal Beam (UB)* — the standard 'I-beam' section used for most domestic spanning situations. Sections are designated by their nominal depth in mm (the '305' in 305 × 165 × 40 UB is approximately 305mm deep) and weight per metre (40 kg/m). Deeper sections carry more load with less deflection. Common domestic sizes: 152 UB, 178 UB, 203 UB, 254 UB, 305 UB.

*Universal Column (UC)* — a wider-flange section used where the beam must resist loads in both directions or where the 'column-like' section shape is appropriate.

*Compound beam (flitch beam)* — two steel plates bolted either side of a timber member. Less common in modern construction; used where steel beam depth is constrained.

*Steel hollow section (SHS, RHS)* — square or rectangular hollow sections, used for columns, lintels, and exposed architectural beams where the box profile is preferred aesthetically.

• **Why you must use a structural engineer for beam sizing**:
• Beam sizing is a calculation — it requires:
• •Accurate load assessment (how much weight is the beam carrying, and from how large an area)
• •Material properties of the steel section chosen
• •Support conditions (how the beam is restrained at each end)
• •Deflection check against Part A requirements

A structural engineer will produce a specification and, for Building Regulations submission, a calculation pack. Building Control will not approve a structural alteration without a structural engineer's design confirmation for any load-bearing wall removal.

Contractors may suggest a beam size based on experience — this is not a substitute for an engineer's calculation. An undersized beam can fail, catastrophically. An oversized beam is expensive, heavy, and harder to handle on site. Neither is acceptable.

• **Typical structural engineer fees for domestic beam design (London 2025)**:
• •Single beam design (structural calculation + specification for Building Regulations): £500–£1,200
• •Multiple opening design (several steels, padstone schedule, drawing package): £1,200–£3,500
• •Full structural package (extension structure, loft, and steelwork): £2,000–£6,000

Installing a steel beam in an existing load-bearing wall is a temporary works challenge — the existing structure must continue to carry its loads during the period when the beam is being installed. This requires temporary propping.

**Temporary propping sequence for a load-bearing wall removal**: 1. **Prop floors and ceilings above**: Adjustable steel props (Acrow props) are erected on either side of the wall at the floor above, to carry the floor load during the operation. Props must be placed on a suitable bearing — not on suspended timber floors without supporting directly to the floor joists (or they can punch through the floor boards). 2. **Remove plaster to expose wall**: The wall is opened to allow working access to the masonry and to confirm the structural construction. 3. **Form opening in stages**: Never remove all masonry at once — the lintel course or a temporary timber needle is installed first in a section, then the opening is extended. 4. **Install padstones**: Padstones are bedded in mortar in the new bearing positions. 5. **Lift and position the steel beam**: This is the most physically demanding operation — large domestic beams (254 UB × 10m span) can weigh 450 kg or more. Specialist lifting equipment (beam trolleys, chain hoists) is required. Structural connection to the padstones must be made as per the engineer's specification. 6. **Make good masonry above the beam**: Repoint or reinstate any masonry disturbed during installation. 7. **Remove props once beam and bearings are confirmed solid**: Allow the beam to take the load before removing temporary props.

• **Common installation issues**:
• •**Beam too heavy for site access**: Longer beams may need to be fabricated in two sections with a site-welded or bolted splice joint — the structural engineer must design the splice.
• •**Existing masonry in poor condition at the bearing**: Corroded mortar, thin brick courses, or historic repairs at the support point may reduce the effective bearing capacity — engineer must assess.
• •**Existing services in the wall**: Gas pipes, electrical runs, or plumbing routed through the wall must be identified and diverted before the wall is opened.
• •**Building Control inspection**: For notifiable structural works, the structural frame must be inspected by Building Control before being hidden — do not encase the beam in plasterboard before the inspection.

• **Cost of beam installation (London 2025, structural labour only)**:
• •Single beam in existing party or internal wall (up to 4.5m span, beam pre-delivered to site): £2,500–£5,500 including temporary propping, opening, padstone installation, and making good
• •Multiple beams (3+ steels): £5,000–£12,000+
• •Supply of the steel beam: £150–£800 depending on section and length (plus delivery to site)