RSJ Beam: What It Is, When You Need One and How It Goes In
Updated: 2 days ago
An RSJ (rolled steel joist) is a steel beam, usually I-shaped in section, that carries the weight of the structure above when a load-bearing wall is taken out or a large opening is formed. In most homes it's what makes an open-plan kitchen diner, a knock-through into an extension or a wide set of bifold doors possible. The size of the beam is never guessed: a structural engineer calculates it, and building control inspects the work.
What does an RSJ beam actually do?
A load-bearing wall carries weight down to the foundations: floor joists, the walls above, sometimes part of the roof. Remove it and that weight needs a new route down. The RSJ spans the gap and hands the load to a bearing at each end, which in turn passes it down through the wall or a column to the ground.
Strictly, "RSJ" describes an older rolled section. Most beams specified today are universal beams (UB) or universal columns (UC), and some openings use a box section or a steel "goalpost" frame. Builders and homeowners still call all of them RSJs, so we do too.
When do you need an RSJ?
You need a steel, or another engineered beam, whenever you remove or open up a wall that carries load. The common cases in a home:
Knocking through from the house into a rear or side extension. The original back wall almost always carries the floor and wall above.
Joining two rooms, such as a kitchen and dining room, where the dividing wall supports the first-floor joists.
Wide openings for bifold or sliding doors in an external wall.
Loft conversions, where new steels carry the new floor and the dormer or hip-to-gable structure.
Removing chimney breasts on the ground floor while keeping the stack above, which needs support such as steel or gallows brackets, depending on the engineer's design.
Not every internal wall is load-bearing. Whether a wall carries load depends on the joist direction, what sits above it and how the house was built, so it needs checking by someone qualified rather than judged by tapping it.
Who decides the size of the beam?
A structural engineer. They look at the span, the loads coming down from above, the bearings at each end and what those bearings sit on, then produce calculations and a specification: the beam section, the padstones, any columns and the temporary support needed while the wall comes out.
Those calculations go to building control with the building regulations application or building notice. Removing a load-bearing wall is building work under the Building Regulations whether or not it needs planning permission, and the structural part is checked against the requirements for structure. We explain how the engineer fits into a bigger project in our post on whether you need a structural engineer for an extension.
How is an RSJ installed?
A typical installation follows a set order. The details vary with the engineer's design, but the principles don't.
Temporary support. Adjustable props and strongboys (or needles through the wall) take the load before any masonry is removed. The props sit on something solid, not a suspended floor.
Forming the opening. The wall is taken out carefully above the line of the new beam, then below it.
Bearings and padstones. Each end of the beam sits on a padstone (a dense concrete block or engineering brick bearing) that spreads the point load into the wall. The engineer specifies the minimum bearing length.
Lifting the beam in. Steel is heavy, so this is planned: how it gets through the house, how many people, whether a lifting aid is needed.
Packing and making good. The gap between the top of the beam and the masonry above is packed tight with slate or steel shims and mortar, so the load transfers without the wall above settling.
Fire protection. Exposed steel usually needs fire protection, typically boarding or an intumescent coating, to meet the fire safety part of the regulations.
Inspection. Building control inspects at the stages they require, commonly with the beam in and the bearings visible before it's boxed in.
Does an RSJ need a party wall notice?
Often, on semi-detached and terraced houses. If one end of the beam bears in the party wall, cutting the pocket for it is work to a party wall, which needs at least two months' notice. Our guide to the party wall agreement sets out the process and the timings.
What affects the cost of steel work?
The main drivers are the span (longer spans need deeper, heavier sections), whether columns or a goalpost frame are needed, access for getting the steel in, how much wall comes out and what's above it, the engineer's fees and the building control fees, plus the making good afterwards: plastering, flooring and redecorating where the wall was. A quote should list the steel, the temporary works, the fire protection and the making good as separate items, so you can compare like with like. Our kitchen extension cost guide covers how the knock-through fits into a wider budget.
RSJ beam questions, answered
Can you live in the house while a steel goes in?
Usually, yes. The noisy, dusty part (props in, wall out, beam in) takes a few days for a typical knock-through, and the rest of the house can be screened off. Expect the room itself to be out of use until it's made good.
Is a timber or concrete lintel an alternative?
For small openings, yes. A standard lintel may be enough over a doorway, and engineered timber beams suit some spans. The engineer decides based on the load and the span; steel is chosen when the load or the span is too much for the alternatives.
Do I need building control sign-off for an RSJ?
Yes. Removing a load-bearing wall is controlled building work. Keep the completion certificate, because a buyer's solicitor will ask for it when you sell.
Opening up the back of the house is part of most of the home extensions in Kettering we build, and of our house extensions in Daventry, and our own team handles the propping, the steel and the making good, working to the engineer's design. You can see a finished rear extension in our single storey extension case study, or contact us to talk through the wall you want to take out.
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