Structural Steelwork for House Extensions: RSJs, Padstones, and What to Expect

**The five most common reasons structural steel is needed in a London extension**:

**1. Opening up the rear wall of the house (creating the extension opening)**:

The most common use of structural steel in a rear extension is to carry the load of the house above the new opening between the existing house and the new extension. When the rear wall is opened up for bifold doors, sliding glass, or a large kitchen opening, the load that was previously carried by the wall must now be redirected around the opening — the steel beam spans the opening width and transfers the load to the jamb piers (the brickwork columns at each side of the opening).

Beam size depends on: the span (the width of the opening), the load above (single-storey above = load from first floor joists and roof; two-storey above = load from first-floor floor, first-floor walls, second-floor ceiling, and roof), and the specification of the bearing (how much wall the beam sits on at each end). A typical 3.6m rear opening in a two-storey Victorian terraced house is usually spanned by a 203mm × 203mm universal column (UC) section or a 254mm × 102mm universal beam (UB) section — but the structural engineer specifies the exact size.

**2. Carrying the new extension roof onto the existing house walls**:

Where the extension roof (flat or pitched) connects to the existing house, the flat roof joists or pitched roof rafters bear on or connect to the existing structure. If the extension is wide, a structural beam may be needed to span between the existing house and the extension's side walls — particularly where the extension is a large open-span kitchen (no internal walls to provide intermediate support).

**3. Removing a load-bearing internal wall within the house as part of the extension project**:

Open-plan rear extensions often include removing an internal load-bearing wall within the house (e.g., the wall between the original kitchen and the rear reception room) to create a continuous open-plan space that flows into the extension. This requires a steel beam to carry the floor joists and wall above the removed wall, spanning between the side walls.

**4. Supporting the new staircase opening in a loft conversion**:

In a loft conversion, the new staircase requires an opening in the first-floor ceiling — which means trimming the first-floor joists to create the stair opening. Where multiple joists are trimmed, a trimmer beam (often steel) is required to carry the load of the cut joists.

**5. Foundation-level beams (steel capping beams for piled foundations)**:

Where the extension requires piled foundations (near trees, in areas of deep made ground, or over utilities), the piles are connected by capping beams — reinforced concrete or steel sections spanning between pile heads to distribute the load of the extension walls above.

**The structural engineer's role**:

All structural steel in a domestic extension must be designed by a structural engineer (a Chartered Engineer with IStructE or ICE membership is the appropriate qualification). The structural engineer's scope typically includes:

1. *Site visit and load take-down*: Assessing the existing structure, the loads on each element, and the condition of the bearing walls 2. *Beam sizing calculation*: Calculating the bending moment, shear forces, and deflection under load for the proposed beam, using BS EN 1993 (Eurocode 3 for steel structures) or BS 5950 3. *Connection and bearing design*: Specifying the minimum bearing length (the length of beam sitting on the jamb pier — typically 150mm minimum for a domestic steel beam), the padstone specification, and any web stiffening or connection plates required 4. *Temporary support specification*: Specifying the required temporary propping arrangement during construction (before the beam is installed)

**What a structural drawing for a steel beam contains**:

• •The beam section size (e.g., 203 × 203 × 46 kg/m UC in grade S275 steel)
• •The beam span (clear span between supports)
• •The minimum bearing length at each end
• •The padstone specification (concrete grade, dimensions, reinforcement if required)
• •The level at which the beam sits (top-of-beam level, or underside-of-beam level)
• •Any propping instructions
• •Fire protection specification (if the beam is in a fire-critical location)

**Common beam sizes for London domestic extensions (indicative only — always follow engineer's specification)**:

| Situation | Typical section | |---|---| | Non-load-bearing (partition) opening up to 1.8m | Steel lintel (Catnic/IG equivalent) | | Load-bearing opening 1.5–2.5m, single-storey above | 152 × 89 × 16 UB, S275 | | Load-bearing opening 2.5–3.6m, single-storey above | 178 × 102 × 19 UB or 203 × 102 × 23 UB | | Load-bearing opening 3.0–4.5m, two-storey above | 203 × 203 × 46 UC or 254 × 102 × 28 UB | | Load-bearing opening 4.5–6.0m, two-storey above | 254 × 254 × 73 UC or 305 × 127 × 37 UB |

UB = Universal Beam (I-section, efficient for horizontal spanning); UC = Universal Column (H-section, efficient for carrying vertical loads — sometimes used as a beam where depth is restricted)

**Grade S275 vs S355**:

Structural steel is available in two common grades: S275 (275 MPa yield strength) and S355 (355 MPa yield strength). S355 is approximately 30% stronger — the same section in S355 can carry 30% more load than in S275. For large spans where beam depth is constrained (e.g., a wide opening in a Victorian terrace where head height is critical), an S355 beam can achieve the same structural performance in a shallower section. The engineer will specify the grade — if you are given a choice, use S355 if depth is critical.

**Padstones — what they are and why they matter**:

A padstone is a block of dense material (dense concrete block, engineering brick, or sometimes a custom precast concrete pad) placed at the bearing point of a structural beam — the point where the beam sits on the wall. The padstone spreads the concentrated load from the end of the beam into the masonry below, preventing the masonry from being overstressed (crushed) under the point load.

Without a padstone, a structural steel beam bearing directly on standard hollow or lightweight blockwork would crush the block under the concentrated point load. Padstone specifications:

• •*Material*: Dense concrete block (minimum 7.3 N/mm² compressive strength) or engineering brick (Class A or B) is commonly specified for loads up to approximately 50kN per end. For higher loads, a custom concrete pad with reinforcement is used.
• •*Dimensions*: The padstone plan dimensions spread the load across a sufficient area of masonry — the engineer calculates the minimum dimensions based on the bearing load and the masonry compressive strength. Typical padstones: 100mm × 215mm × 140mm deep, or 215mm × 215mm × 140mm deep for heavier loads.
• •*Installation*: The padstone must be fully bedded in mortar before the beam is placed; the beam must bear over its full specified bearing length on the padstone.

**Temporary propping**:

Before any load-bearing wall opening is created (or any load-bearing joists or rafters are cut), the loads above must be temporarily supported while the permanent structural beam is installed. Temporary propping uses adjustable steel acrow props with spreader boards above (distributing the load into the floor above) and below (distributing the load into the floor below without point-loading through it).

• The contractor's temporary propping arrangement must reflect the structural engineer's instructions — incorrect propping can allow structural movement during the work. Common errors:
• •Inadequate spreader boards (the prop point-loads through an unsupported floor)
• •Props placed too far from the opening (not within the load path)
• •Props removed before the permanent beam is installed and the temporary support has been maintained long enough for mortar to cure

**Fire protection for structural steel**:

Unprotected structural steel loses its load-bearing capacity rapidly when exposed to fire — at approximately 550°C, most structural steel has lost sufficient strength to cause failure. Building Regulations Part B requires that structural elements in a fire-compartment boundary are fire-protected to the required period (typically 30 minutes for domestic residential: a half-hour fire rating).

• For a domestic extension, structural steel beams that are:
• •*Within the fire compartment of a habitable room (not a corridor or escape route)*: typically require 30-minute fire protection
• •*Within a protected staircase or escape route*: require 30-minute minimum fire protection
• •*Concealed within the floor or ceiling void*: the encasing construction (floor boards above, plasterboard soffit below) typically provides the fire rating — check with Building Control

• Methods of fire protection:
• •*Intumescent paint*: A thin coating applied to the steel that expands when heated, providing an insulating char layer; typically requires 2–3 coats; used where the beam is exposed (visible architecture); requires specialist applicator certification for certain products
• •*Boxing in with fire-rated plasterboard*: The beam is encased in a timber or metal frame lined with 12.5mm or 15mm Type F fire-rated plasterboard; most common method; provides 30-minute or 60-minute rating depending on board thickness and number of layers

**Programme: steel beam installation**:

The installation of a structural steel beam is a critical programme milestone because all downstream trades (brickwork above, floor/ceiling finishes, first-fix carpentry) are blocked until the beam is in place and the temporary propping can be removed.

Typical programme for a steel beam installation in a London domestic extension:

| Stage | Duration | |---|---| | Temporary propping installation | Half day | | Wall opening cut (brickwork removed above beam level) | 1–2 days | | Padstone installation and cure | 1 day (including overnight mortar cure) | | Beam delivered to site and lifted into position | Half–1 day | | Checking bearing, levelling, and securing | Half day | | Temporary propping removal (after pad mortar has cured, typically 24–48 hours) | Half day | | **Total programme time from propping to reinstated opening** | **4–6 days** |

**Steel lead time**: Structural steel is fabricated from stock sections by a steel stockholder or fabricator. For standard universal beam and column sections, delivery is typically 3–5 working days from order — but this should be ordered as soon as the engineer's design is issued, not left until the wall is being opened. For longer lead-time items (fabricated plate girders, special sections), allow 2–4 weeks.