RSJ (Rolled Steel Joist)

A steel beam — typically I-shaped — used to provide structural support when load-bearing walls are removed during a renovation, allowing open-plan layouts.

An RSJ — which stands for Rolled Steel Joist — is a structural steel beam used to support the weight above when a load-bearing wall is removed. The name comes from the manufacturing process: the beam is formed by rolling hot steel into shape, typically creating an I-shaped or H-shaped cross-section. You may also hear it called a “steel” or simply a “beam.”

When you need an RSJ

RSJs are most commonly needed when homeowners want to:

  • Create open-plan living spaces — knocking through between a kitchen and dining room or living room
  • Widen door openings — making a standard doorway into a wide opening
  • Remove chimney breasts — where structural support above needs to be maintained
  • Convert lofts — where existing roof structures need reinforcement

How an RSJ is specified

You cannot simply buy an RSJ off the shelf and install it. A structural engineer must:

  1. Survey the property to understand what loads the wall currently carries
  2. Calculate the required beam size — the length, depth, and weight of steel needed
  3. Specify the bearings — how the beam sits on the remaining walls at each end (called padstones)
  4. Produce engineering drawings for the contractor and building control inspector

The structural engineer’s calculations are a legal requirement and must be submitted as part of your building regulations application.

Installation process

Installing an RSJ is a critical structural operation:

  • Temporary support (Acrow props) holds up the structure while the wall is removed
  • The steel beam is lifted into position (some beams are extremely heavy)
  • The beam is set on padstones or steel bearing plates at each end
  • The area around the beam is made good — plastered and finished

Cost considerations

The RSJ itself is often not the biggest cost — the labour, temporary support, structural engineer’s fees, building regulations application, and making good afterwards all add up. Always get a structural engineer’s specification before asking your contractor for a price, and make sure the work is included in your scope of work.