RSJ Steel Beam Size Guide: Indicative Sizes, Weights & Planning Advice
RSJ Steel Beam Size Guide: Indicative Sizes, Weights & Planning Advice
Whether you're removing a load-bearing wall, forming a new opening for bi-fold doors or planning a loft conversion, choosing the right RSJ size is the most critical structural decision you'll make. Choose too small and you risk deflection or structural failure. Go too large and you waste money on steel, delivery and installation.
This guide covers every standard UK RSJ and Universal Beam (UB) section — dimensions, weights, typical spans and common applications. Use the interactive calculators below to get an indicative beam size and weight estimate before discussing with your structural engineer.
What Is an RSJ? UBs, UCs and RSJs Explained
The term RSJ (Rolled Steel Joist) is used across the UK building trade to describe the steel I-beam or H-beam sections used for structural support. Technically, RSJ refers to an older joist profile with tapered flanges, now largely replaced by the modern Universal Beam (UB) — but in practice, builders, structural engineers and suppliers use RSJ, UB and steel beam interchangeably.
There are three main section types you'll encounter on a typical residential project:
- Universal Beam (UB) — the depth is significantly greater than the flange width. The standard choice for spanning horizontal openings. Most common for residential work.
- Universal Column (UC) — near-equal depth and width. Optimised to carry vertical compressive loads. Used as structural columns and posts, or where headroom is very limited.
- Parallel Flange Channel (PFC) — C-shaped section. Used for trimmer beams, edge beams and where a flat back face is needed against a wall.
All sections are specified by three numbers: depth × flange width × weight per metre. A 203×133×25 UB is 203mm deep, 133mm wide, and weighs 25kg per metre of length. These three numbers are always shown in full when ordering to avoid confusion between similar sections.
Indicative RSJ Section Starting Points
All sections below are supplied in S355 grade steel to BS EN 10025-2. S355 is commonly specified for UK structural steel beams and is widely used for residential and commercial structural steelwork.
The indicative size ranges shown are based on typical domestic floor loading assumptions only and are intended as a planning starting point. Heavier loads, continuous spans, or beams carrying multiple floors will require a larger or deeper section. Always verify with your structural engineer before ordering.
| Section (D×W mm) | Weight (kg/m) | Depth (mm) | Flange Width (mm) | Typical Span Range | Category |
|---|---|---|---|---|---|
| 127 × 76 | 13 | 127 | 76 | Up to 2.5m | Light |
| 152 × 89 | 16 | 152 | 89 | 2.5m – 3.0m | Light |
| 178 × 102 | 19 | 178 | 102 | 3.0m – 4.0m | Common |
| 203 × 102 | 23 | 203 | 102 | 4.0m – 5.0m | Common |
| 203 × 133 | 25 | 203 | 133 | 3.5m – 4.5m | Common |
| 203 × 133 | 30 | 207 | 134 | 4.0m – 5.0m | Common |
| 254 × 102 | 22 | 254 | 102 | 5.0m – 5.5m | Common |
| 254 × 102 | 25 | 257 | 102 | 5.0m – 6.0m | Common |
| 254 × 102 | 28 | 260 | 102 | 5.5m – 6.5m | Common |
| 254 × 146 | 31 | 251 | 146 | 5.0m – 6.0m | Common |
| 254 × 146 | 37 | 256 | 146 | 5.5m – 7.0m | Common |
| 305 × 102 | 28 | 309 | 102 | 6.0m – 7.0m | Heavy |
| 305 × 165 | 40 | 303 | 165 | 6.5m – 8.0m | Heavy |
| 356 × 171 | 51 | 355 | 172 | 7.0m – 9.0m | Heavy |
Universal Column (UC) Sizes for Structural Posts
Universal Columns are used where a section must resist vertical compressive loads — as posts, padstone supports or where a beam is installed in a near-vertical orientation. Their near-square profile makes them far more efficient than a UB when carrying axial load. All are supplied in S355 grade.
| Section (D×W mm) | Weight (kg/m) | Depth (mm) | Flange Width (mm) | Typical Use |
|---|---|---|---|---|
| 152 × 152 | 23 | 152 | 152 | Light column / post |
| 152 × 152 | 30 | 158 | 153 | Residential column |
| 152 × 152 | 37 | 162 | 154 | Residential column |
| 203 × 203 | 46 | 203 | 203 | Medium column |
| 203 × 203 | 52 | 206 | 204 | Medium column |
| 254 × 254 | 73 | 254 | 254 | Heavy column |
🔧 RSJ Beam Calculators
Three tools in one. All results are indicative only — confirm every beam size with a qualified structural engineer before ordering.
Enter your opening width and load type to get an indicative beam section. Results must be confirmed by a structural engineer before ordering.
Calculate total beam weight for delivery planning and lifting assessment. Based on nominal section mass.
Work out your total order length including bearing allowances. Many sections are available in standard 6m and longer lengths. For larger openings or cut lengths over 6m, contact us for a custom quote — we can advise on the best option for your project.
How to Read RSJ Beam Dimensions
When you see a section described as 203 × 133 × 25 UB, the three numbers mean:
- 203 — nominal depth of the section in millimetres (height of the web)
- 133 — nominal flange width in millimetres
- 25 — weight per metre in kilograms (kg/m)
Actual dimensions vary slightly from the nominal — a 203×133×25 UB has an actual depth of 203.2mm. Always use the full section designation (e.g. 203×133×25 UB) when ordering or providing to your structural engineer to avoid confusion between similar sections.
For Universal Columns (UCs), the depth and flange width are similar or equal. A 203×203×46 UC is nearly square in profile, making it far stiffer against column buckling than a UB of similar weight but much less efficient at spanning horizontal loads.
What Size RSJ Do You Need? The Four Key Factors
Beam selection is determined by four variables. Your structural engineer will assess all four when preparing calculations — but understanding them helps you have a more informed conversation about your project.
1. Clear Span
The horizontal distance the beam must bridge between its bearing points. Add the bearing length (typically 150–200mm each end onto masonry) to get your total order length. A 3.6m clear opening requires approximately 3.9m of steel ordered.
2. Total Load
The combined dead load (the weight of the structure above — roof, floors, walls) plus live load (occupants, furniture, snow). A beam over a single-storey timber-frame extension carries far less than one spanning a knocked-through ground floor with two floors of masonry above. Getting this wrong is the most common reason beams are under-specified.
3. Deflection Limit
Structural steel deflects under load. Building Regulations typically require deflection under working load to be no more than span ÷ 360 for floors (approximately 10mm over a 3.6m span). For roof beams, span ÷ 200 is often acceptable. At longer spans, deflection — not strength — typically governs which section is needed, which is why beam depth increases significantly for spans above 5m.
4. Available Depth
Headroom constraints may prevent using the most structurally efficient section. A 203×203×46 UC and a 254×146×31 UB have similar bending resistance but the UC is 50mm shallower. Where ceiling height is tight, a wider, shallower section may be specified at higher weight and cost. Always discuss depth constraints with your structural engineer early.
Do I Need a Structural Engineer?
Yes — for any structural alteration subject to Building Regulations. This includes virtually all load-bearing wall removals, extensions, loft conversions and significant structural openings in England and Wales.
Your building control officer will require a structural calculation as part of your Building Regulations application. Without it, your project will not receive sign-off. A structural engineer typically costs £200–£600 for a residential beam calculation — money well spent given the consequences of an under-specified beam.
RSJ Beam Delivery: What to Expect
Universal Beams and RSJs are delivered by specialist steel transport with a lead time of 5–10 working days from Rackerman. Beams are heavy — a 6m 254×146×37 UB weighs 222kg — and offloading assistance is required at point of delivery. You are responsible for ensuring suitable lifting equipment is available when your order arrives.
Plan your site access before ordering:
- Deliveries are made by specialist steel transport — check for access restrictions, low bridges or tight turns on your route before ordering
- Have a telehandler, forklift, or suitable chain hoist on site and ready for unloading on delivery day
- Long beams (6m+) cannot safely be carried by hand — mechanical lifting assistance is required
- For orders over £3,000, free HIAB offload is available — contact us for a quote
Ready to Order Your RSJ Beams?
All sections supplied in S355 grade, cut to size, and delivered by specialist steel transport to mainland UK. 5–10 working day lead time.
Shop RSJ Beams at Rackerman →Frequently Asked Questions
In everyday UK trade usage the terms are interchangeable. Technically, an RSJ (Rolled Steel Joist) is an older section profile with tapered flanges that has been largely superseded by the modern Universal Beam (UB), which has parallel flanges and improved structural properties. When a structural engineer or builder refers to an "RSJ", they almost always mean a Universal Beam to BS EN 10365.
For a 3m clear opening under light residential loading (single storey above), common starting-point sections discussed for light residential situations may include a 152 × 89 × 16 UB. Under medium loading (two floors above), sections such as a 178 × 102 × 19 UB or 203 × 133 × 25 UB are more commonly discussed as starting points. Your structural engineer will confirm the correct section based on the actual load calculation for your specific project.
For a 4m clear opening under medium domestic loading, common starting-point sections discussed for medium domestic loading include a 178 × 102 × 19 UB or 203 × 102 × 23 UB. Heavier loads — masonry above, or a beam carrying a floor — may require a 203 × 133 × 25 or larger section. Always confirm with a structural engineer before ordering. The span recommender above can give you an indicative size to discuss with your engineer.
Common starting-point sections discussed for a 5m span under typical residential loading may include a 203 × 102 × 23 UB for lighter loads or a 254 × 102 × 25 UB for medium loads. At 5m, deflection starts to govern beam selection rather than pure strength — section depth becomes increasingly important. Your structural engineer will run a full deflection check at this span length. Do not order without a structural calculation at 5m+.
Yes — for virtually all structural alterations subject to Building Regulations in England and Wales (including load-bearing wall removals, extensions and loft conversions), the beam size must be specified and signed off by a qualified structural engineer. Your local building control officer will require a structural calculation as part of your Building Regulations application. Never specify or order a beam based on a size guide or online calculator alone.
Mild steel RSJs will corrode if left unprotected and exposed to moisture. For beams within habitable space fully encased in fire-rated plasterboard or concrete, the encasement itself provides corrosion protection. Exposed beams — in garages, outbuildings, or external applications — should be primed with red oxide and finished with a suitable topcoat. In habitable spaces, intumescent paint or two layers of fire-rated plasterboard is required under Building Regulations for fire protection.
Yes. All RSJ and Universal Beam sections at Rackerman can be cut to your exact required length. Standard stock lengths are 6m. Use the cut-to-size calculator above to work out your total order length including bearing allowances. Cut-to-size items are non-returnable and excluded from our 30-day returns policy.
All Universal Beams and RSJ sections supplied by Rackerman are S355 grade to BS EN 10025-2. S355 has a minimum yield strength of 355 N/mm² — approximately 25% stronger than S275 — and is commonly specified for UK residential and commercial structural steelwork. Mill certificates are available on request.
RSJ and Universal Beam orders are delivered within 5–10 working days to mainland UK addresses. Delivery is by specialist steel transport. Offloading assistance is required at point of delivery — ensure appropriate lifting equipment is available on site. For orders over £3,000, free HIAB offload is available on request. Contact us to arrange.