If you are planning an extension, open-plan knock-through or loft conversion, you will almost certainly hear your builder and engineer talking about RSJs or structural steel beams. Understanding what these beams do, and how they are installed, will help you feel far more confident about the work on your home.
What an RSJ beam actually does
RSJ stands for rolled steel joist, which is a common type of structural steel beam. In domestic projects, people often use RSJ to mean any steel beam that carries loads above an opening.
The basic idea is simple. When you remove or alter something that used to carry weight, such as a wall or section of roof, the RSJ becomes the new support. It transfers loads safely into the surrounding structure, so your home remains stable and compliant with Building Regulations.
Common situations where steel beams are needed
Most modern home improvements rely on steel at some point. You are likely to need an RSJ in any of these scenarios:
Removing a load-bearing wall to create an open-plan kitchen, diner or family room
Forming a wide rear opening for bi-fold or sliding doors in a rear or side extension
Loft conversions where new floor joists, the ridge, or party walls need extra support
Supporting chimney breast removals or large openings between rooms
In all these cases, the beam takes the place of brickwork, timber or old support that is being taken out or has been altered. Getting this right structurally is just as important as how the finished space looks.
Open-plan knock-throughs and kitchen diners
When a load-bearing wall comes out between, say, a kitchen and dining room, an RSJ is installed across the opening. The floor joists and any walls above are then supported by this beam instead of the removed masonry.
Often the steel is recessed up into the ceiling and boxed in with plasterboard so that, once finished, you only see a shallow bulkhead or even a flat ceiling line. Your builder and engineer will decide the best approach based on structure and services such as pipes and cables.
Rear openings for bi-fold or sliding doors
Creating a wide opening in an existing rear wall for large doors removes a lot of support from the back of your house. A structural steel beam across the top of the new opening carries the brickwork above and transfers the load into the sides.
This beam is often hidden within the new extension ceiling or boxed in internally. Externally, the brickwork or cladding is built around it, so all you notice is the clean opening and glazed doors.
Loft conversions and roof support
With a loft conversion, steel beams are commonly used to support the new floor and sometimes the roof. These beams may sit in pockets cut into the party walls or gable walls, spreading the load safely.
Additional steels can support the ridge line of the roof or pick up loads from dormers. The engineer will design a framework of beams and new timbers that work together so the new space feels solid and meets Building Control requirements.
How RSJs interact with padstones and existing walls
An RSJ does not just sit on bare brick. The loads at each end of the beam are concentrated, so they need to be spread safely into the supporting walls. This is where padstones come in.
Padstones are small, very strong concrete or engineering brick blocks that sit under the beam ends. They spread the load into the masonry below, reducing the risk of cracking or crushing. Your builder will cut pockets into the existing wall, install padstones, and then bed the beam onto them.
The engineer specifies padstone size and position, as well as any extra strengthening required to existing walls. This ensures the old and new parts of your home work together structurally.
Who does what: engineer, Building Control and builder
Structural engineer: They calculate the size, type and position of each steel beam, as well as padstones and connections. They provide drawings and calculations for Building Control and the builder.
Building Control: Whether through the local authority or an approved inspector, Building Control check that the proposals meet Building Regulations. They will usually inspect key stages, including structural supports.
Builder: The builder follows the engineer’s design, arranges steel fabrication and delivery, installs temporary supports, fits the beams and makes good afterwards. Good communication between all three is essential.
Typical process for installing an RSJ in your home
1. Design and approval
The process starts with an engineer’s visit and measurements. They design the beam and issue calculations and drawings.
These are submitted to Building Control as part of the overall project. Once any queries are answered, you have the green light to proceed on site.
2. Delivery and access planning
Steel beams can be very heavy and awkward, so your builder will plan how to get them into position. This may involve delivering to the front drive and carrying or hoisting into place.
Occasionally, access through neighbouring properties or over fences needs to be agreed in advance. Your builder should talk you through the plan before delivery day.
3. Propping and temporary supports
Before any load-bearing wall is removed, the area is propped. Adjustable steel props and strongboys or needles are used to hold up the floors and walls above while the opening is formed.
This stage can look alarming, but when done correctly it is a well-established, safe method. No major masonry is taken out until adequate temporary support is in place.
4. Installation, making good and inspections
With the area propped, the builder forms padstone pockets, installs the padstones and lifts the beam into place. The beam is packed, levelled and secured according to the engineer’s details.
Once Building Control are satisfied, props can be removed and the opening finished. Brickwork is patched, ceilings and walls are boarded and plastered, and any boxing-in is built so the steel is hidden.
Safety, dust and disruption in your home
Structural work is noisy, dusty and disruptive, but a well-organised builder will minimise the impact. Expect sheeting to seal off areas, floor protection and regular sweeping and vacuuming.
You may lose use of certain rooms for a while, and there will be vibrations from drilling and cutting. Children and pets should be kept away from the work area, and you may be asked to stay out during lifting operations for safety.
Short FAQ on structural steel in extensions
Will my house be safe while work is done?
Yes, provided the work is carried out by competent professionals following an engineer’s design. Proper temporary propping is installed before any structural wall is removed, and Building Control inspect key stages.
Do I always need a structural engineer?
For any change to load-bearing walls, roofs or floors, you should have an engineer’s design. Building Control will usually require calculations for RSJs and other structural elements, and your builder relies on these details.
How long does installation take?
The time on site depends on access, beam size and how much masonry is involved. As a rough guide, a simple single opening might be completed over a few days including making good, while more complex jobs form part of a longer extension or loft programme.
Next steps for your own extension or loft project
If you are considering an extension, knock-through or loft conversion and want clear advice on structural steel, it helps to involve an experienced contractor early on. They can liaise with your engineer, plan access and sequencing, and keep disruption to a minimum.
JW Carpentry & Build handle structural work, extensions and loft conversions across a wide range of homes, working closely with engineers and Building Control to keep projects safe and compliant. To arrange a structural assessment and talk through your ideas, call JW Carpentry & Build on 07710890538 or explore the services for Structural Work, Extensions and Loft Conversions.
