Steel Beam Sizes and Weights: A Guide for Builders
Guide to structural steel beam sizes and weights for domestic work, covering RSJs, universal beams, extension projects and essential site checks.
Steel Beams in Domestic Construction
Steel beams turn up on nearly every extension, loft conversion, and knock-through job. They carry the loads that walls used to carry, opening up spaces and supporting floors and roofs above new openings. As a builder, you don't need to be a structural engineer, but you do need to understand the basics: what sizes exist, how much they weigh, how to handle them safely, and what building control expects.
What Is an RSJ?
RSJ stands for Rolled Steel Joist, though the term is used loosely on site to describe any structural steel beam. Technically, the sections used in modern domestic construction are Universal Beams (UBs) and Universal Columns (UCs), both manufactured to BS EN 10025. The Institution of Structural Engineers provides guidance on structural steel design for buildings.
| Section Type | Profile | Typical Use |
|---|---|---|
| Universal Beam (UB) | Deep and narrow (I-shaped) | Spanning openings, floor beams |
| Universal Column (UC) | Squarer profile | Posts, short spans with heavy loads |
| Parallel Flange Channel (PFC) | C-shaped | Lintels, wall plates, edge beams |
When someone on site says "RSJ," they almost always mean a Universal Beam.
Common Steel Beam Sizes for Domestic Work
Structural engineers specify beam sizes based on the load and span. Here are the sections you'll see most often on domestic projects:
Universal Beams (UB) — Most Common
| Section | Depth (mm) | Width (mm) | Weight (kg/m) | Typical Domestic Use |
|---|---|---|---|---|
| 152 × 89 × 16 UB | 152 | 89 | 16 | Small openings, lintels up to 2m |
| 178 × 102 × 19 UB | 178 | 102 | 19 | Window and door openings |
| 203 × 102 × 23 UB | 203 | 102 | 23 | Standard knock-through beams, 2–3m spans |
| 203 × 133 × 25 UB | 203 | 133 | 25 | Wider openings, light floor loads |
| 254 × 102 × 25 UB | 254 | 102 | 25 | Extension openings, 3–4m spans |
| 254 × 146 × 31 UB | 254 | 146 | 31 | Floor beams, loft conversions |
| 305 × 102 × 25 UB | 305 | 102 | 25 | Longer spans with lighter loads |
| 305 × 127 × 37 UB | 305 | 127 | 37 | Extension beams, 4–5m spans |
| 305 × 165 × 40 UB | 305 | 165 | 40 | Heavier floor loads, wider spans |
| 356 × 171 × 51 UB | 356 | 171 | 51 | Large extensions, 5–6m spans |
| 406 × 178 × 54 UB | 406 | 178 | 54 | Major structural beams, 6m+ spans |
The naming convention tells you everything: 254 × 146 × 31 UB means the beam is approximately 254mm deep, 146mm wide across the flanges, and weighs 31kg per metre.
Working Out Beam Weight
The weight per metre figure is crucial for planning the lift:
Total beam weight (kg) = Weight per metre × Beam length
A 4m length of 254 × 146 × 31 UB:
31 × 4 = 124kg
That's a two-person lift minimum with proper manual handling technique, or more realistically a three-person lift to get it into position at height. For anything over about 100kg, consider mechanical lifting (chain block, engine crane, or telehandler).
The Steel Calculator gives you beam weights for any section size and length, which is useful when planning deliveries and working out whether you need lifting gear.
What Determines the Beam Size?
The structural engineer considers:
- Span — the clear distance between supports
- Loading — the weight the beam must carry (floors, walls, roof above)
- Deflection limit — the maximum amount the beam can bend under load (typically span/360 for floors)
- Support conditions — are the walls either side strong enough to carry the beam reactions?
Rule of Thumb (Not a Substitute for Engineering)
As a very rough guide for typical domestic loads, beam depth in mm is approximately span in mm ÷ 15 to 20. So a 4m span might need a beam around 200–270mm deep. But this is only a starting point — always get the engineer's calculation.
Bearing and Support
Padstones
Every steel beam needs to sit on a padstone at each end. The padstone spreads the concentrated beam load over a wider area of wall.
| Padstone Material | Typical Size | When to Use |
|---|---|---|
| Dense concrete (50 N/mm²) | 225 × 225 × 100mm minimum | Standard domestic bearings |
| Engineering brick (Class A) | 2–3 courses | Where concrete padstone is awkward |
| Concrete cast in situ | As specified by engineer | Heavy loads or weak walls |
The bearing length (how far the beam sits on the wall) is specified by the engineer but is typically 150mm minimum for domestic work. Check the structural calculation — some beams need 200mm or more.
Packing and Levelling
The beam must sit level on its padstones. Use steel shims (plate washers) to level — never use timber packing under a steel beam. Timber crushes and rots over time, causing the beam to drop.
Fire Protection
Building Regulations Part B requires structural steel to maintain its integrity for a specified period in a fire:
| Situation | Fire Resistance Required |
|---|---|
| Beam in a loft conversion floor | 30 minutes |
| Beam between ground and first floor | 30 minutes |
| Beam in a flat or HMO | 60 minutes |
| Beam above an integral garage | 30 minutes |
Protection Methods
| Method | Fire Rating | Notes |
|---|---|---|
| Plasterboard encasement (2 × 12.5mm) | 30 minutes | Most common domestic method |
| Plasterboard encasement (2 × 15mm fire-rated) | 60 minutes | For flats and higher-risk situations |
| Intumescent paint | 30–60 minutes | Leaves the beam exposed — used for feature steels |
| Board systems (Supalux, Masterboard) | 30–60 minutes | Thinner profile than plasterboard |
Plasterboard encasement is the cheapest and most common option. The board is fixed to timber battens around the beam, creating a box. Make sure there are no gaps — fire will exploit any weak point.
Intumescent paint is popular for feature beams where the customer wants the steel exposed. It looks like normal paint but expands in a fire to form an insulating char layer. Expect to pay £30–£60 per linear metre for supply and application.
Handling and Installation
Delivery
Steel is heavy and awkward. A 5m beam at 37 kg/m weighs 185kg. Make sure:
- The delivery driver has a means of offloading (most steel suppliers use a HIAB crane truck)
- There's clear access from the drop point to the installation position
- The steelwork is delivered before the scaffold goes up if it needs to go in from outside
Manual Handling
Any beam over about 50kg (a 3m length of 152 × 89 × 16 UB) needs at least two people. For heavier beams:
- Chain blocks hung from the floor joists above (if they're strong enough) work well for loft conversions
- Acrow props either side of the opening support the beam temporarily while it's manoeuvred into position
- Telehandlers or mini cranes for beams going in from outside through an opening
Never carry a steel beam on a ladder. Plan the lift before the beam arrives.
Propping
While the beam is being installed, the existing structure needs temporary support:
- Strongboy props through the wall for knock-throughs — one prop either side of each supporting point, typically at 600mm centres
- Acrow props under the floor above while the wall is being removed
- Needle beams through the wall if you're removing a long section
Leave temporary propping in place until the beam is fully bearing, the padstones have cured (if using wet concrete), and the masonry above has been rebuilt.
What to Check on the Structural Calculation
When the engineer sends you the calculation, check that it includes:
- Beam size and grade (usually S275 or S355 steel)
- Required bearing length at each end
- Padstone specification — size and material
- Any lateral restraint requirements (restraint straps tying the beam to the wall or floor)
- Fire protection specification
- Connection details if two beams meet at a post
Keep a copy of the structural calculation on site for the building control inspection. They'll want to see it and compare it to what's been installed.
If you need to estimate the weight of other building materials alongside the steelwork, the Weight Calculator handles common construction materials.
Common Mistakes
- Not getting a structural calculation. Find a structural engineer through the Institution of Structural Engineers directory or ICE. Never specify a beam size yourself — even if you've done a similar job before, every situation is different. A structural engineer's calculation costs £150–£400 and protects you legally.
- Wrong grade of steel. S275 and S355 look identical but have different yield strengths. Make sure the beam delivered matches the specification. Check the mill certificate or markings on the steel.
- Insufficient propping during installation. The wall above the opening can collapse during removal if it's not properly supported. Take propping seriously.
- Skipping the padstone. A steel beam sitting directly on blockwork concentrates the load and can crush the blocks. Always use padstones.
- No fire protection. Building control will pick this up at final inspection. Budget for it from the start.
- Not checking beam depth against floor buildup. If the beam is deeper than the floor zone (joists + decking + ceiling), it'll hang below the ceiling line. Discuss this with the engineer early — there may be a shallower, wider beam option.
Quick Summary
- Always get a structural engineer's calculation — never guess the beam size
- Specify padstones at each bearing point
- Plan the lift — check weight, access, and handling method before delivery
- Support the existing structure with temporary propping during installation
- Fire-protect the beam to Building Regulations Part B
- Keep the structural calculation on site for building control
The Steel Calculator gives you beam weights and section properties for common structural steel sizes — useful for planning deliveries and checking what lifting gear you'll need.