Structural Steel Beams in Home Extensions: What You Need to Know
Structural steel beams — typically referred to as RSJs (Rolled Steel Joists), UBs (Universal Beams), or simply "steels" on site — are a core structural element in most home extension and loft conversion projects. Wherever a load-bearing wall is opened up, or where a new structural opening is created, a steel beam is almost always required to carry the load above. Here's what you need to understand about how they're specified, procured, and installed.
Why Steels Are Needed
Walls in most UK domestic properties carry load from the floors, roof, and walls above. When you create an opening in a load-bearing wall — to knock through from the kitchen to the dining room, to create bifold door openings across the rear of the house, or to create the structural frame for an extension — the load that was previously carried by the wall needs to be transferred to the structure on either side of the new opening via a beam.
The size of the beam required depends on: the width of the opening (longer spans require larger or heavier beams); the load being carried (what's above the opening — one floor, two floors, a roof, or just a ceiling); the bearing condition at either end of the beam (what the beam is sitting on); and the material of the existing structure.
How Beams Are Specified
Beam sizes are specified by a structural engineer. For any significant structural opening, a structural engineer's design is required — both for safety and for Building Control compliance. The engineer calculates the load, determines the required beam section size (expressed as, for example, 203 x 133 x 25 UB, which describes the section dimensions and weight per metre), and specifies the bearing and connection details.
Do not accept a contractor who proposes to install structural beams without a structural engineer's specification. Building Control will require engineer's calculations at inspection.
Temporary Works During Installation
Before a load-bearing wall can be opened up and a beam installed, the loads above must be temporarily supported — usually with adjustable steel props and timber needles (horizontal timbers passing through the wall). This temporary works requirement is a significant element of the programme and cost of any structural opening.
The temporary works must themselves be designed and managed safely. On a complex project, a temporary works coordinator may be required to design and sign off the temporary support arrangement.
What Steels Cost in a Home Extension
The cost of a steel beam for a domestic project depends on: the size and weight of the section (heavier sections cost more for both the steel and the installation); the difficulty of access (getting a 6-metre steel beam through an occupied house is a logistical challenge — it often requires manoeuvring through narrow corridors, cutting the beam to manageable lengths, or using specialist lifting equipment); and the extent of temporary works required.
A single standard 3–4 metre beam for a kitchen-dining opening can cost £800–£2,500 including supply and installation. A more complex arrangement involving multiple beams for a full-width rear extension can cost £3,000–£8,000+. These figures are for the steel and structural works element specifically — not the associated opening-up, making-good, and finishing.
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