RSJ Beams: What They Are
An RSJ (Rolled Steel Joist) is a steel beam with an "I" or "H" shaped cross-section. It carries load across an opening — for example where a wall has been removed, above a wide window or bi-fold doors, or under a chimney breast that has had its support taken out.
You will also see them called metal beams for building, steel joists, or simply "steelwork". The term RSJ is the one most UK homeowners search for, but it is now an informal name.
RSJ Meaning vs Modern Terms
| Term | What it means | Typical use |
|---|---|---|
| RSJ | Rolled Steel Joist (older UK name) | General term for a steel beam |
| UB | Universal Beam | Most wall removals, openings, extensions |
| UC | Universal Column | Posts and heavily loaded short members |
| PFC | Parallel Flange Channel | Edge beams, tight-depth situations |
| Flitch beam | Steel plate bolted between timbers | Slim floor and roof beams |
When we write a calculation for a London home it will name a UB (for example 203x133 UB25) rather than "an RSJ" — but they describe the same idea.
Where RSJ Beams Are Used in Homes
- Load-bearing wall removal — opening up a kitchen and dining room
- Rear and side return extensions — spanning the opening between old and new
- Loft conversions — supporting new floors, dormers and the staircase opening
- Chimney breast removal — supporting the chimney stack above
- Bi-fold and wide window openings — replacing masonry that used to span the gap
Steel Reinforcement vs RSJ Beams
People often search for "steel reinforcement" when they mean a beam. The two are different:
- Steel reinforcement (rebar, mesh) sits inside concrete foundations, slabs and lintels to take tension.
- RSJ / UB beams are rigid sections that span an opening by themselves and bear on padstones or masonry at each end.
A job such as a rear extension normally uses both: reinforced concrete foundations and slab, plus steel beams where the extension meets the house.
Why You Cannot Guess an RSJ Size
The right beam depends on the span, what sits above it (floors, roof, a chimney, masonry), the bearing length at each end, and the strength of the supporting walls. Two similar-looking houses can need very different beams. See our guide to RSJ sizes in the UK and what size steel beam you need.
What Building Control Expects
Building Control will ask for:
- Structural calculations signed off by a qualified engineer
- A structural detail showing the beam, bearings, padstones and any posts
- Inspection of the beam before it is covered up
Get RSJ Beam Calculations in London and Essex
SBS is based in Ilford and provides fixed-fee steel beam calculations across Ilford, Redbridge, Newham, East London, Tilbury, Grays and Thurrock.
- Service: Steel Beam Calculations →
- Related: Load-Bearing Wall Removal Guide →
- Pricing: Structural Engineer Cost in London →
📞 Call 07401 650600 or email us with a photo of your wall and we will quote the same day.
