If you're creating a new structural opening in your home, whether through building an extension or creating a more open-plan interior, you'll likely hear the term RSJ costs.
An RSJ (rolled steel joist) is a steel beam that is essential to the structural integrity of your home. Designed to support any load from above – be this wall, ceiling or another storey – it has an I or H-shaped cross section.
However, unlike some elements of your build, where you can estimate costs online, looking for RSJ costs on the internet is certainly not the advised route to take. While it's easy to assume you know the length of the opening and can therefore find a beam that fits, it's a far more complex calculation that can lead to serious consequences if you get it wrong.
What is the correct term for an RSJ?
In case you're wondering if this article applies to you when your builder or architect discuss steel beams, it's worth understanding that despite "RSJ" being a popular term in the trade, it's not actually the correct one.
"Many homeowners still refer to steel beams as “RSJs”, but while the term is widely used, it’s actually outdated," explains James Atkinson, associate at Design4Structures. "In the industry, beams are now more accurately referred to as Universal Beams (UBs), Universal Columns (UCs) or hollow sections, depending on their shape and structural performance."
If this was news to you, as it was me, then don't worry, you'll still likely hear all three acronyms being used, so just be sure whoever is pricing them up understands what it is you need.
James Atkinson as an Associate at Design4Structures and works with homeowners and property developers.
What factors influence RSJ costs?
While it's easy to head to a builder's merchant and get prices for other building materials used in ceilings and walls, such as timber and types of plasterboard – most of which will be priced based on size, why can't you do the same with RSJ costs once you know the length and width of beam that you need?
Dave Bendon, founder and director of D.Bendon Construction explains.
"If you are removing a load-bearing wall, opening up, or creating a new structural opening that needs an RSJ to bear the load from above, you will need the involvement of a building inspector and a structural engineer," he explains.
"The calculations are complex and not something even an experienced DIYer can work out," he warns. "While an experienced or competent builder may get a feel for what's needed, it's never something you should specify without professional involvement."
"Sizing a steel beam is not a matter of approximation," agrees James Atkinson. "When a wall is removed, the loads it was carrying - from floors, roofs or walls above, must be safely redirected. That requires proper structural calculations, an understanding of how loads travel through a building (known as load paths), and compliance with Building Regulations and current design codes.
"The beam must not only be strong enough to carry the weight, but also stable and suitable for the wider structure of the property," he continues. "It’s about ensuring long-term safety and performance, not just making the opening work on day one."
"Several key factors influence the size of a steel beam. The most obvious is what it is supporting," expands James. "A heavy masonry wall or additional storey generates far greater load than a lightweight timber floor. The span, meaning the clear distance the beam must bridge without support, is also critical. Generally, the longer the span, the larger or deeper the beam required.
"Strength is only part of the equation though," he warns. "Beams must also be checked for serviceability, in other words, how much they will bend (deflection) and whether vibration could be noticeable. Even if a beam won’t fail structurally, excessive movement can lead to cracked plaster, uneven floors or long-term maintenance issues.
"In UK domestic projects, the most commonly used sections are Universal Beams (UBs), which are deeper and efficient at reducing deflections; Universal Columns (UCs), which are more compact in depth and can sometimes sit within a floor zone; and closed Rectangular or Square Hollow Sections (RHS and SHS), which perform better where twisting forces may occur," James says. "Each has advantages depending on the structural demands and practical constraints of the space."
What impacts RSJ costs the most?
"Span and load are usually the primary cost drivers," explains James. "As a general rule, the longer the beam and the more weight it needs to support, the larger and heavier the section required, which increases material cost.
"However, other constraints can influence price. If the beam must fit within a restricted depth, for example, to achieve a flush ceiling, a heavier or alternative section may be needed, increasing cost.
"Fabrication requirements, such as welded plates, stiffeners or connection details to support floors and walls, also add to the overall price. Availability is another factor," he adds, "as standard stock sections are typically more economical than uncommon or bespoke sizes.
"Ultimately, the most cost-effective solution is one that balances structural performance, practicality and buildability from the outset," says James.
Additional build costs
However, it's not just an RSJ cost that you need to account for when pricing up this element of the build.
"There are also all the additional works that need to take place to install the RSJ," explains Dave Bendon. "These include, but aren't limited to, the opening up of the wall, padstones for the RSJ to sit on, the labour involved to get the beam in place and then the reinstatement of the wall around the new RSJ."
Add to this structural engineer fees and the potential cost of an independent building inspector, and your RSJ costs will be far more than just the cost of manufacturing steel.
Having experienced RSJ installation myself when renovating and extending, I can also vouch for the fact that it can be a labour and machinery intensive job, often requiring a crane to hoist the beams into place. All of which can see costs escalate.
Or, like me, you may need extra hands on deck when the RSJs are installed. On two occasions when I needed them, there wasn't sufficient access for a crane, so it involved an army of strong welshmen to get them into place. A nerve-wrecking experience I don't think any of us were keen to repeat again.
Dave Bendon has over 30 years experience in construction, working on a variety of commercial and residential projects, including listed buildings, new builds, conversions and extensions.
The breadth of RSJ costs
In order to explain why it's impossible to give accurate figures for RSJ costs without understanding all of the calculation, experienced quantity surveyor Tim Phillips says as the breadth of pricing can vary so much, it's impossible to put an average price per metre on the cost of an RSJ.
"You can expect to pay anything from £100 to £300 per metre for the steel, between £500 to £2,000 for labour, and as the install increases in complexity, you could end up with the cost of a single RSJ reaching £5,000," says Tim.
The lesson? Don't rely on prices online that haven't been calculated using structural engineer calculations, building inspector involvement and the advice of an experienced builder who has installed them before.
RSJ online calculators — Yes or no?
So what of the online RSJ cost calculators you may have seen during your research into how much an extension costs, or even how much a house costs. Are these an accurate representation of RSJ costs?
"Online calculators can appear helpful because they simplify the process," says James Atkinson, "however, they typically only consider basic inputs such as span and assumed loads. Structural alterations are rarely that straightforward.
"Removing a wall affects more than just the opening. The supporting walls must be capable of taking new concentrated loads. The foundations beneath them must be suitable for the revised load paths. The overall stability of the building must not be compromised," he expands.
"Connections and detailing also play a significant role. These wider considerations cannot be properly assessed by a simple online tool. Relying solely on a calculator, without a full structural review, introduces unnecessary risk."
It's simply not a risk worth taking, agrees Dave Bendon. "Installing the wrong RSJ is a safety hazard, and if you don't follow the recommended advice for getting the beams calculated correctly, you could find yourself facing problems with your house insurance if something does go wrong in the future and you need to make a claim."
If you're planning on knocking down internal walls, it always pays to get professionals involved. Or, if you're thinking about adding, rather than removing walls, find out when a internal stud or solid wall is the better option, and how to build a stud wall with a door.
