Steel Beam (RSJ) Sizing & Installation

Removing a load-bearing wall is one of the most common structural alterations in UK domestic renovation, creating open-plan kitchen-diners, widening hallways, or building out into a garage. In almost every case the load previously carried by the wall must be transferred to a steel beam spanning between new or upgraded supports. Getting the specification wrong risks immediate structural failure or long-term settlement that can be costly to remediate.

No beam sizing in a load-bearing context should be attempted without a structural engineer’s calculations. This guide explains the process, the terminology, the typical specifications, and the costs so you can manage the project with confidence.

Terminology: RSJ, UC, UB and What They Mean

RSJ (Rolled Steel Joist) is the colloquial term most homeowners and tradespeople use for any structural steel beam in a residential context. Technically, the RSJ section with its tapered flanges has largely been superseded in modern practice by:¹

• UB (Universal Beam), a parallel-flanged I-section primarily designed to resist bending. This is what most domestic wall-removal beams are: the wide horizontal flanges carry tension and compression, and the vertical web resists shear. Sizes run from 127×76×13 UB (a small beam for modest spans) up to 1016×305×584 UB for heavy commercial loads.²
• UC (Universal Column), a square-proportioned I-section designed primarily for axial compression. Used as columns (posts) supporting a beam, or in portal frames, not typically as a spanning beam.

The section designation reads as depth × flange width × weight per metre. A 203×102×23 UB is 203mm deep, 102mm wide, and weighs 23 kg per metre run.³

When Is a Steel Beam Required?

A structural steel beam is needed whenever a load-bearing element is removed or interrupted and the load above must be re-routed. Common domestic situations include:

• Removing an internal load-bearing wall to create open-plan living space
• Creating a wider opening in a load-bearing wall (beyond what a standard steel lintel can span economically)
• Loft conversions where a new ridge beam or purlin is needed to convert a hip roof or to support new floor joists
• Garage conversions where the garage door opening needs to carry first-floor load above
• Basement excavations and underpinning where the ground floor structure must be temporarily or permanently supported

The Structural Engineer: Non-Negotiable

A structural engineer must size the beam for every load-bearing application.⁴ This is not optional and is not a task for the builder, no matter how experienced. The engineer calculates:

• The load path above the beam (floor loads, wall loads, roof loads)
• The required bending moment and shear capacity
• The required beam section (using standard load tables or software)
• The padstone specification (to spread the point load into the supporting masonry)
• Any lateral restraint requirements

Expect to provide the engineer with the wall location, the proposed opening span, the floors and roof above, and any unusual loads (water tanks, heavy stonework, etc.). Most engineers will want to visit the site for a domestic residential calc; some will work from drawings and photographs.

Building Regulations

All load-bearing structural alterations require Building Regulations approval under Approved Document A (Structure).⁴ You have two application routes:

Full Plans application: you submit structural drawings and engineer’s calculations in advance. The Building Control officer approves them before work starts.⁵ This is the preferred route for structural work, as it gives certainty before work starts.

Building Notice: you notify Building Control before work starts but do not submit drawings.⁵ An inspector visits at key stages. Faster to initiate but riskier, the inspector can require changes during construction that prove costly.

In either case, a completion certificate should be obtained once the work is passed.⁵ Without it, the alteration is hard to evidence when you sell, and the buyer’s solicitor is likely to ask for retrospective approval, called regularisation.⁵

Typical Beam Sizes for Domestic Spans

These are indicative examples only. Actual specification requires engineering calculations.

Padstones

A padstone distributes the concentrated point load at each end of the beam into the masonry wall beneath. Without a padstone, the beam bearing area is too small and the masonry can crush or split. Padstones are typically:

• Engineering brick: Staffordshire Blue or similar, usually Class B, laid in a strong cement mortar to the number of courses the engineer specifies; cost-effective and standard for most domestic applications⁶
• Precast concrete: a single unit sized by the engineer, often used where the pocket in the wall is too tight for brick courses

The engineer specifies the padstone dimensions based on the bearing stress calculation. A common misconception is that any hard brick will do; the engineer will normally call for engineering bricks (Class B is at least 75 N/mm², Class A at least 125 N/mm²) or a concrete padstone, not standard facing bricks.⁷

Temporary Propping

Before any load-bearing wall can be opened up, the load above must be transferred to temporary props and spreader beams (whaling boards). The propping sequence must be planned and supervised; collapsing a propped floor is a serious safety incident. Acrow props are cheap to hire: one 2026 hire price guide charges £7.67 a week per prop before VAT, and £11.49 for a Strongboy wall prop.⁸ A typical domestic job requires four to six props on two floors.

Propping must extend to a sound structural floor, propping onto a chipboard floor without spreaders will punch through. The propping engineer or an experienced builder familiar with structural alterations should design the temporary works.

Fire Protection

Steel starts to lose strength at about 300°C and loses it rapidly above 400°C.⁹ Building Regulations Approved Document B asks for a minimum 30-minute fire resistance for elements of structure in most houses, including three-storey houses, rising to 60 minutes for a compartment wall separating one house from another.¹⁰ In practice, a beam boxed in with fire-rated plasterboard to the board manufacturer’s tested specification can usually achieve 30 minutes; building control will want to see that specification. Where the beam is exposed or in an open-plan area, intumescent paint (a thin-film coating that swells into an insulating char when heated, applied to the dry film thickness the manufacturer specifies for the section and fire period) or board encasement must be used.⁹

Party Wall Act

If the beam bears on a wall that is shared with a neighbouring property, a party wall, the Party Wall etc. Act 1996 requires you to serve a Party Structure Notice at least two months before work begins.¹¹ Your neighbour can consent or appoint a party wall surveyor. The Act has no enforcement procedure for failing to serve notice, but the neighbour can seek a court injunction to stop the work or other legal redress.¹²

Costs in 2026

Building control charges are set by each council. Hillingdon, for example, charges £396 including VAT for forming a structural opening such as a simple through lounge, and £607.20 where the opening comes with other building work.¹³

The wide range reflects span, location, access, and the scale of making-good required. A straightforward ground-floor internal wall removal with a 3m beam in a standard semi-detached house usually sits in the lower half of that range when engaging a builder who handles the engineer engagement. High-end estimates apply where structural problems are discovered, where the beam is in a party wall situation requiring a surveyor, or where the floor finish above is expensive and must be perfectly reinstated.

Steel supply prices are influenced by global commodity markets and have fluctuated significantly in recent years; obtain quotes close to the time of ordering.

Post-Installation

Once the beam is in place and the temporary props removed, Building Control should inspect and sign off the structural element before it is enclosed. Keep the engineer’s calculations and the Building Regs completion certificate with the property deeds, they will be requested on any future sale or remortgage.

The rules in this note are those that apply in England. Scotland, Wales and Northern Ireland have their own building standards and permitted development rights.