RSJ beams — or Rolled Steel Joists — are the backbone of countless modern construction projects. These I-shaped steel sections deliver exceptional strength-to-weight ratios, making them ideal for creating open spaces and supporting innovative designs. As bespoke metalworkers, we’ve fitted RSJs in everything from home extensions to commercial fit-outs. Here’s a look at 10 smart ways they’re used today, with real-world examples to show their versatility.
1. Loft Conversions
RSJs excel in loft conversions, removing the need for multiple supporting walls. A single 203×133 UB beam can span 4-6 metres, opening up attic spaces for bedrooms, offices or en-suites.
We recently installed one in a Victorian terrace, supporting the entire first floor while creating a vaulted ceiling. Powder-coated to match the interior, it blends strength with style — no bulky steel posts in sight.
2. Glass Extensions
Full-height glass extensions demand slim, powerful supports. RSJs frame the structure, allowing uninterrupted views and maximum light. Grade S355 steel handles wind loads effortlessly, even on large bifold setups.
For a modern kitchen diner, we used parallel 254×146 beams to carry a glass roof. The result? A bright, airy space that feels bigger than it is, with the steel providing reliable load transfer to foundation pads.
3. Open-Plan Layouts
Knocking through load-bearing walls? RSJs sit neatly above new openings, creating seamless living areas. They distribute weight evenly to pad foundations, avoiding complex steelwork.
In a bungalow renovation, a 305×165 beam spanned an entire room, linking kitchen and lounge. Minimal disruption, quick install — and the exposed beam now anchors downlights and shelving.
4. Rooftop Terraces
Urban rooftops turn into usable spaces with RSJ supports. Cantilevered or framed decks handle planters, decking and foot traffic without overloading existing structures.
A city flat conversion used 356×171 beams to support a green roof with integrated seating. Galvanised for weather resistance, they ensure long-term stability even under snow loads.
5. Cantilevered Features
RSJs enable dramatic cantilevers for balconies, staircases and overhangs. Buried within walls, they project outwards with minimal visual bulk.
We’ve built 3m cantilevered decks using 305×165 sections, complete with glass balustrades. Perfect for modern homes wanting that “floating” aesthetic without compromising safety.
6. Exposed Industrial Interiors
Leave RSJs visible for a raw, factory-inspired look. Primed, painted or waxed, they become design features in lofts, studios and restaurants.
A workshop-to-studio conversion featured exposed 254×146 beams supporting mezzanine levels. Rust-patinated finish adds character, while integrated lighting highlights the steel grain.
7. Modular Construction
Prefabricated buildings rely on RSJs for rapid assembly. Bolt-together frames ship flat-packed, reducing site time and waste.
For a garden office range, we use S275 grade RSJs in modular kits. Precise fabrication means everything slots together — strong enough for full insulation and PV solar arrays.
8. Elevated Walkways and Bridges
Connect buildings or span features like ponds with RSJ walkways. Parallel beams with timber or steel decking create safe, maintenance-free paths.
A private garden bridge used twin 356×171 beams over water, with stainless handrails. Wide enough for wheelchairs, it’s become a daily feature for the homeowners.
9. Retail and Commercial Shelving
Heavy-duty RSJs form the spine of retail mezzanines and display systems. They support racking, counters and walkways without sagging.
In a showroom fit-out, 610×229 beams created a second-floor display gallery. Fireproofed and clad, they doubled the usable space without major structural changes.
10. Sustainable Retrofits
Upgrade older buildings with RSJs for low-carbon strengthening. They replace timber joists or reinforce masonry, extending life without demolition.
A heritage barn conversion used RSJs to support new oak floors. Hidden plates tie into existing walls, blending old and new while meeting modern loading codes.
RSJ Sizes and Specifications
Choose the right RSJ for your span and load:
| Size (Depth x Width) | Typical Span | Common Uses |
| 127×76 UB | Up to 3m | Loft props, light openings |
| 254×146 UB | 4-6m | Extensions, room spans |
| 356×171 UB | 6-9m | Terraces, walkways |
| 610×229 UB | 10m+ | Commercial, heavy loads |
Grades: S275 for general use; S355 for longer spans or dynamic loads. Always galvanise or paint for outdoor exposure.
Why RSJs Work So Well
RSJs offer high section modulus for bending resistance, low deflection and easy connections via end plates or cleats. Lighter than concrete alternatives, they speed up builds and cut foundation sizes.
In our workshop, we cut RSJs to precise lengths, pre-drill for bolts and apply finishes like intumescent coating for fire protection. This means faster site installs and fewer surprises.
Need RSJs for your project? From loft beams to cantilevered terraces, we supply bespoke metalwork solutions that stand the test of time. Get in touch for a no-obligation span calculation.