RSJ Steel Beams in London Homes: What Homeowners Need to Know

RSJ stands for Rolled Steel Joist — a structural steel section with an I-shaped cross-section (two horizontal flanges connected by a vertical web). The shape gives it exceptional resistance to bending under load for its weight.

In everyday UK construction "RSJ" is used informally to mean any steel beam used in residential work. The correct term for modern beams is Universal Beam (UB), manufactured to BS EN 10365. RSJ is the term most homeowners and builders use.

Steel beams are used to carry the load of a structure above a new opening, span across an extension to carry upper floor or roof loads, replace a chimney breast that has been removed, carry floor or roof loads over a large column-free space, and support loads at steel frame connections in larger projects.

You need a structural steel beam whenever you are transferring a significant load over an opening or span that masonry or timber cannot adequately carry.

Common situations in London homes: removing a load-bearing internal wall between reception rooms (very common in Victorian terraces); creating a rear extension connection through the rear wall; knocking through to a side return; removing a chimney breast; building the floor structure for a loft conversion; creating large openings in single-storey rear extensions.

In Victorian and Edwardian terraces, internal walls between the front and back reception rooms are very frequently load-bearing. Do not assume a wall is not structural before getting professional advice.

Beam sizing is a structural engineering calculation. It cannot be done accurately by a builder, by rule of thumb, or by copying what a neighbour used. It must be done by a Chartered Structural Engineer (MIStructE or CEng).

The engineer calculates: the dead load (the weight of the structure above — walls, floors, roof, finishes) plus the imposed load (people, furniture, and other movable items); the span (distance the beam must cross between supports — longer spans need heavier sections); and the deflection limit (steel beams sag under load; Building Regulations require deflection within limits, typically span/360 for floors).

The output is a beam specification such as 203x102 UB23 or 254x146 UB31, expressed as depth x width x weight per metre. Using an undersized beam is dangerous. Using an oversized beam wastes material but is not unsafe.

A padstone is a block of dense, load-bearing material placed at each end of a steel beam to distribute the concentrated beam reaction force into the surrounding masonry. Without padstones, the point load from the beam end would crush the brickwork below.

Padstones are typically dense precast concrete (the most common type in residential work), engineering bricks where access is limited, or steel spreader plates for very high loads or tight spaces.

The padstone must be large enough to spread the load (sized by the engineer), made from material with sufficient compressive strength (engineer specifies minimum class), set in a full bed of Class II mortar before the beam is placed, and solidly embedded in the masonry with full contact.

Common mistake: using standard house bricks as padstones. Standard bricks are not dense or strong enough and can shatter under the concentrated beam load.

Stage 1 — Pre-works design and approval: the structural engineer completes the design, Building Control is notified, party wall notices are served where required.

Stage 2 — Temporary support: before any masonry is removed, the structure above the proposed opening must be temporarily supported with Acrow props and spreader boards. The support must carry the actual load path — not just be positioned nearby. This must be set up by an experienced site foreman.

Stage 3 — Creating the beam pocket: a pocket is cut into the masonry at each end of the proposed beam for the padstones. The pocket must be at the correct level per the engineer's drawings.

Stage 4 — Padstone installation: padstones are set in a full bed of Class II mortar, levelled, and left to achieve adequate strength (typically 24–48 hours) before the beam load is transferred.

Stage 5 — Beam installation: the beam is slid into position, set on the padstones, checked for level, and packed tight using steel shims or dry-pack mortar to ensure full contact.

Stage 6 — Masonry removal: only once the beam is properly seated can the masonry below be removed, progressively from the middle outward.

Stage 7 — Building Control inspection: the inspector must see the beam and padstones before they are covered by plasterboard. Do not board over the beam before the inspection.

Undersized beam: not specified by an engineer, or specified incorrectly. Visible as excessive deflection (ceiling above the beam sags), cracking of plasterwork above the beam, or in severe cases structural failure.

Incorrectly sized padstones: too small, concentrating bearing stress which can crush masonry below. Appears as stepped cracking in masonry below the beam end over time.

Poor temporary support: the most common cause of acute structural failure during construction. If temporary support collapses or is incorrectly positioned before the permanent beam is installed, partial collapse of the structure above can occur.

Beam not fully bearing on padstones: a gap between the beam flange and padstone means the beam is not properly seated. The beam will creep or deflect as load is applied.

No Building Control approval: discovered at resale, this can cause significant delays and costs to regularise or insure the works.

Costs for a typical London residential steel beam installation including supply and installation of beam and padstones, temporary support, and masonry removal:

Small opening, single beam up to 3m span: £1,500–£3,500. Standard knocked-through wall up to 5m span: £2,500–£5,500. Rear extension structural opening: £3,000–£7,000. Complex multi-beam arrangement: £6,000–£15,000+.

Additional costs: structural engineer design fee £350–£800 for a single beam; Building Control fee £200–£600; party wall surveyor where required £700–£1,500+ per neighbour.

Note: the steel beam itself is often a small fraction of the total cost — £200–£800 for the steel section. The cost is in the labour, temporary support, masonry work, and professional fees.