What Size Steel Beam Do I Need? An Engineer's Guide
If you are knocking through a wall, opening up a kitchen, or extending — the question almost everyone asks first is "what size steel beam do I need?"
There is no single answer. Beam sizing depends on the span, the load above the beam, the bearing at each end, and the deflection limit Building Control will accept. But there are typical sizes engineers reach for again and again on London terraces, semis and Essex 1930s houses, and this guide walks you through them.
⚠️ Important: The sizes below are indicative. You cannot use them as a build specification. Building Control will only accept beams sized by structural calculations specific to your property. We can do those for you — see our steel beam calculations service.
Quick Reference: Typical RSJ / UB Sizes by Span
| Clear span | Single-storey load (floor + roof) | Two-storey load (floor + wall + roof) |
|---|---|---|
| Up to 2.4m | 127x76 UB13 | 152x89 UB16 |
| 3.0m | 152x89 UB16 | 178x102 UB19 |
| 4.0m | 178x102 UB19 | 203x133 UB25 |
| 5.0m | 203x133 UB25 | 254x146 UB31 |
| 6.0m | 254x146 UB31 | 305x165 UB40 |
These figures are typical for a standard London or Essex residential property with regular timber floor joists at 400mm centres, 215mm masonry walls above, and standard pitched roofs. Real properties vary, especially older Victorian terraces with wider joists, heavier slate roofs, or chimneys still in place.
What Actually Determines the Beam Size
A structural engineer is solving four things at once when sizing a beam:
1. The clear span
This is the distance the beam has to bridge between supports — not the wall length, the clear opening. Bigger spans need stiffer (deeper) beams.
2. The load on the beam
This is the most misunderstood part. The beam doesn't just carry the wall above it — it carries everything that wall used to support:
- Floor above (joists may run parallel or perpendicular to the wall — it makes a huge difference)
- Wall above (single-leaf, cavity, solid, with or without windows)
- Roof above (pitched, flat, slated, tiled)
- Any point loads — a steel from a previous loft conversion, a chimney breast still standing upstairs, a stud wall on the floor above
This is why you can't copy a neighbour's beam size. If their floor joists run the other way, their beam carries half the load yours does.
3. The deflection limit
Even if a beam is strong enough, it must not sag visibly. Building Control require deflection to be limited (usually span/360 for plaster ceilings). On long spans this often pushes the beam up a size — strength isn't the issue, stiffness is.
4. The bearing and padstone
Each end of the beam needs to sit on something solid — usually a concrete or steel padstone built into the existing masonry. The padstone size and the bearing length are part of the calculation. Get this wrong and the wall under the beam crushes locally, even if the beam itself is correctly sized.
RSJ vs UB vs UC — Which Should I Use?
These terms are often used interchangeably on site, but they're not the same:
- RSJ (Rolled Steel Joist) — the historic British term. On most London building sites, "RSJ" just means "a steel beam, please". The actual section is almost always a UB.
- UB (Universal Beam) — deeper than it is wide. Designed to resist bending. The default choice for load-bearing wall removals. ~95% of residential beams are UBs.
- UC (Universal Column) — squarer cross-section, similar depth and width. Designed for vertical loads. Used as posts, or as short heavily-loaded beams where headroom is tight (a 152 UC can replace a 203 UB and gain you 50mm of head height).
- PFC (Parallel Flange Channel) — C-shaped. Sometimes used in pairs either side of a wall for "goalpost" arrangements where you want to avoid dropping a ceiling.
For a typical load-bearing wall removal, expect a UB. For an extension steel-frame "goalpost" with limited headroom, you may see UCs or paired PFCs.
Worked Example: 4m Kitchen Wall Removal in an Ilford Terrace
A common job we quote in Ilford: opening up between a kitchen and dining room in a 1930s mid-terrace.
- Clear span: 3.8m
- Floor above: joists run perpendicular to the wall (worst case — wall takes full half-bay)
- Wall above: single-leaf 100mm masonry stud wall + 215mm cavity at first floor gable
- Roof: standard pitched, no chimney
- Result: 178x102 UB19 with 215x215x215 concrete padstones each end, propped during install
Cost for the calculations and detail: £500 fixed fee. Turnaround: 5 working days.
If the same house had a chimney breast still standing on the first floor over that wall, the load roughly doubles and the beam jumps to a 203x133 UB25 — same span, very different beam. (Removing the chimney first is sometimes a better answer; see our chimney breast removal guide.)
Why You Cannot Skip the Calculations
Three reasons:
- Building Control will reject it. No calcs, no completion certificate. No certificate, problems when you sell.
- Insurance. If anything moves later — sticking doors, ceiling cracks — and you have no structural calculations on file, your buildings insurer can refuse a claim.
- It's actually cheap. A fixed-fee calculation is £400–£800. Tearing out and replacing a wrongly-sized beam is £8,000+.
For full pricing on calculations and surveys see how much does a structural engineer cost in London.
What We Need From You to Quote
To turn around a steel beam calculation in 5 working days, send us:
- A photo of both sides of the wall
- Approximate clear span (tape-measure accuracy is fine)
- Floor plan if you have one (architect's drawing or hand-sketch)
- Whether floors above run parallel or perpendicular (or "not sure")
- Postcode
We'll come back with a fixed fee and a turnaround date the same day in most cases.
Get Steel Beam Calculations from a Local Structural Engineer
SBS is based in Ilford and we cover Ilford, Redbridge, Newham, East London, Tilbury, Grays, and Thurrock. Every calculation is fixed fee, signed off by a qualified structural engineer, and includes the structural detail and Building Control support.
- Service page: Steel Beam Calculations →
- Related: Load-Bearing Wall Removal Guide →
- Chimney support: Chimney Breast Removal Guide →
- Pricing: Structural Engineer Cost in London →
📞 Call 07401 650600 or email us with a photo of your wall and we'll quote the same day.
