Structural Steel in Extensions and Loft Conversions — What It Involves and What It Costs

Victorian and Edwardian London houses are built with load-bearing masonry walls — every wall that is not a timber-framed partition carries vertical load from the floors and roof above. When an opening is created in one of these walls — for a doorway, a wide kitchen-diner opening, a bi-fold door, or a structural alteration — the load that was distributed through the solid wall has to be redirected around the new opening.

• A steel beam (RSJ or UB) is the most efficient way to do this:
• •Steel has a very high strength-to-depth ratio — a 200mm deep steel beam can span 4–5 metres and carry substantial loads
• •A steel beam concentrates the load at the ends, which means the masonry needs to be rebuilt at the support points (the 'padstones') but can be completely removed across the span
• •Steel is faster to install than concrete (which requires formwork and curing time) and can span further than timber for the same depth

• **Common situations where structural steel is used**:
• •Removal of a chimney breast (two RSJs — one at each floor level)
• •Rear wall opening for bi-fold or sliding doors in a kitchen extension
• •Combining rooms by removing a load-bearing wall (one or two RSJs depending on the number of floors above)
• •Extending a structural opening that already exists
• •Loft conversion: new structural steel floor beams alongside the existing timber joists to carry the new habitable floor load
• •Hip-to-gable loft conversion: a ridge beam to support the new roof structure after the hipped end is removed
• •Flat roof extension: a structural roof beam to carry the flat roof structure

**Who specifies the steel?** Structural steelwork in residential projects must be specified by a structural engineer (chartered structural engineer, MIStructE or CEng). The engineer calculates the loads acting on the beam, determines the required beam size and grade, and specifies the end bearing (padstone) requirements.

**The specification process**: 1. Client or architect engages structural engineer 2. Engineer carries out a desk study of the existing structure (using drawings where available, or a site inspection where not) 3. Engineer calculates loads and specifies beam size (typically an SCI or Tata Steel universal beam designation, e.g., 254×102×28 UB) 4. Engineer issues a structural report or specification drawing 5. Contractor orders the steel to the specification from a steel stockholder or fabricator 6. Building Control reviews the structural specification as part of the Building Regulations application

**Steel grades**: structural steel in UK residential projects is typically grade S275 or S355 (designating minimum yield strength in N/mm²). The engineer specifies the grade as well as the section size.

**Fabrication and delivery**: standard UB sections are available from stock and can be delivered within a few days. For beams with welded plates, cranked sections, or non-standard configurations, fabrication takes 2–4 weeks.

**Structural engineer fees**: for a residential project with 1–3 steel beams, structural engineer fees are typically £500–£2,000 depending on the scope of the assessment and whether calculations or just a specification note is required.

• **Pre-installation**:
• •The contractor will confirm the delivery window with the steel stockholder
• •Temporary support ('propping') must be installed before any cuts are made to the existing wall — timber props or acrow props supporting the load while the new opening is formed
• •The proposed opening is cut out of the masonry — this is the noisiest part of the operation
• •Padstones (engineering brick or reinforced concrete bearing plates) are formed at the beam ends to distribute the point load into the masonry

• **Installation**:
• •The beam is lifted into position — for small beams (up to 6 metres, under 100kg), this can be done by two or three labourers by hand. For longer or heavier beams, a chain block and temporary support scaffold are used.
• •The beam is lowered onto the padstones and set to the correct level
• •The gap above the beam is filled with wet mortar, solidly packed, and the masonry is rebuilt up to the ceiling level
• •Props are removed only after the mortar has cured (typically 24–48 hours)
• •The beam is then integrated into the floor or ceiling finish — wrapped in plasterboard and finished to match the ceiling

• **Typical programme**:
• •Propping, cutting, and installing a single beam: 1–2 days
• •Propping, cutting, and installing two beams (e.g., chimney breast removal across two floors): 2–3 days
• •Loft conversion floor beam installation (multiple steel beams alongside existing joists): 2–4 days depending on extent

• **What can go wrong**:
• •Unexpected hidden services (gas pipes, electrical cables, drainage) behind the wall — a proper survey and building drawings reduce but do not eliminate this risk
• •The wall proves thicker than expected — changing the padstone design
• •The existing floor or ceiling support is inadequate, requiring additional propping or re-engineering
• •Steel deliveries delayed — a single late delivery can delay an entire project by a week or more

• Steel costs in residential projects depend on:
• •The weight and size of the beam (in kg/m — a 254×102×28 UB weighs 28kg per linear metre; a 406×178×67 UB weighs 67kg/m)
• •The length of the beam
• •Whether fabrication is needed (end plates, cranks)
• •The labour to install (complexity, height, access)

**Approximate costs (2025, London):**

• **Single RSJ/UB beam installation, including propping, cutting, installation, and padstones**:
• •Small opening (up to 1.8m): £1,200–£2,500
• •Medium opening (1.8–3.5m): £2,000–£4,000
• •Large opening (3.5–5m, e.g., full rear wall): £3,500–£7,000

**Chimney breast removal including steel at two levels**: £3,000–£6,000

**Loft conversion floor steels (replacing existing floor joists with steel to upgrade to habitable floor)**: £4,000–£8,000

**Ridge beam for hip-to-gable or mansard (supporting new roof structure)**: £3,000–£6,000

**Steel material costs are typically 20–30% of the total figure** — the remainder is labour, temporary propping, padstone formation, and associated making good.