RSJs Explained: What a Steel Beam Does in Your Renovation
In short
An RSJ steel beam (rolled steel joist) is used to carry the weight of the structure above when part of a load bearing wall is removed. It is sized by a structural engineer, rests on solid bearings at each end, is inspected by building control and is then fire protected and hidden behind plaster.
An RSJ, short for rolled steel joist, is the steel beam that makes open plan living possible in older houses. When a load bearing wall is taken out, the floors, walls and roof above still need support. The RSJ picks up that weight and carries it to the walls or columns at either end. Builders use “RSJ” for almost any steel beam, although most beams specified today are technically universal beams or universal columns.
Where you will find RSJs in a London renovation
- Knocking a kitchen through to a dining or reception room
- Opening the rear wall of a terrace into a new single storey extension or side return
- Wraparound and L shaped extensions, where steel often forms a frame at the corner
- Loft conversions, where new steels carry the floor and the dormer or mansard above
- Removing chimney breasts, where the remaining stack must be supported
How an RSJ is sized
A structural engineer calculates the load from everything above the opening and specifies the beam size, its bearings and anything else it needs, such as padstones, columns or new foundations. The steel is not chosen by eye. Its depth and weight depend on the span, what it carries and how much deflection is acceptable, which matters for avoiding cracks in plaster and tiles later.
How it is installed
- The structure above is propped with temporary steel props and needles.
- The wall is cut out to form the structural opening.
- Padstones are set at each end to spread the load into the masonry.
- The beam is lifted in, levelled and packed tight to the structure above.
- Building control inspects the steel and bearings before it is covered.
- The beam is fire protected, either boxed in with fire rated board or coated with intumescent paint, and finished.
Hiding the steel
A well planned beam disappears. Depending on the joist direction, it can sometimes be set within the ceiling depth so there is no downstand at all. Where a downstand is unavoidable, it is boxed in neatly and lined up with doors, glazing or kitchen units so it reads as part of the design. In contemporary schemes, some clients choose to leave the steel exposed and painted as a feature, which needs a fire protection finish suitable for that.
How we handle steel beams on London’s Prestige projects
We arrange the structural engineer, order the steel to the engineer’s calculations, prop and install it with our own team, and book building control inspections before anything is closed up. We also plan the beam position with the finished room in mind, so ceiling lines, glazing and joinery align. The glazed corner on our Meadway Close wraparound extension is a good example of steel designed to disappear. Find out more about our extension service, or book a site visit anywhere in London to discuss what your project needs.
Common questions
What does RSJ stand for?
Rolled steel joist. It describes an older beam profile, but builders and homeowners still use RSJ for most steel beams. The engineer's drawing will give the exact section size.
Does an RSJ need to be fire protected?
Usually, yes. Building regulations normally require structural steel in a house to have fire protection, typically fire rated plasterboard or intumescent paint. Building control will confirm what is needed.
Can a steel beam be hidden in the ceiling?
Often, depending on the joist direction and the beam depth. Where it cannot be fully hidden, a neat boxed downstand lined up with doors or glazing keeps it looking deliberate.