An RSJ — rolled steel joist — is the beam that holds the upstairs up when you take a wall out downstairs. If you are knocking through to create an open-plan kitchen, removing a chimney breast, or opening up a hallway, a structural beam will almost certainly be involved. Getting it specified, installed and signed off correctly is what separates a safe alteration from one that causes cracking, sagging or worse a few years down the line.

We install RSJs every month across Widnes, Liverpool, Runcorn, Warrington and St Helens. This guide is the straight-talking version of what is involved: when you need a beam, how it is sized, how it goes in, and what building control will check before they sign it off.

When you actually need an RSJ

Any wall that supports a floor, a roof, or another wall above is load-bearing, and removing it without a beam will cause structural movement. The signs of a load-bearing wall are usually obvious: it runs at right angles to the joists upstairs, it sits over the footings rather than on top of floorboards, and the wall above lines up with it. If there is any doubt, we get a structural engineer in to look before we quote, because guessing here is not an option. The deeper background on the role of RSJs in structural support goes further into the signs that point to a load-bearing wall.

The two most common RSJ jobs we do are opening up between a kitchen and dining room to create an open-plan living space, and removing the wall between a kitchen and a back addition to form a kitchen-diner. Both change how the ground floor lives. Both need a properly sized beam supported on engineered padstones at each end.

Chimney breast removals are the other regular ask. The chimney is usually load-bearing for the upstairs sections, so removing the ground-floor section without a beam — or removing both floors without supporting the stack above — causes serious problems. We always go beam-and-gallows or full-removal with a proper beam, never a quick patch.

How an RSJ is specified

A structural engineer calculates the beam size from three things: the span between supports, the load the beam will carry, and the deflection criteria. The output is a size and grade — for example, a 203 x 133 x 25 UB in S275 steel — plus the padstone size, the bearing length, and any lateral restraint required. This calculation goes to building control with the Full Plans application and is checked before work starts.

We do not pick beam sizes off rule of thumb. A beam that is too small fails. A beam that is too big is heavier and harder to install, and the padstones underneath need to be sized accordingly. We use independent structural engineers we have worked with for years, because the calculations need to be right first time.

Fire protection of the steel is a separate decision. In most domestic situations, plasterboard cased around the beam provides the required fire resistance. In some cases, intumescent paint or specialist board is needed. The engineer specifies which, and building control checks it.

How the installation actually goes

Day one is usually props. We install acrow props under the floor above to take the load before any wall comes down. Once the area is supported, the wall is opened up under the beam line, the bearings are cut out, the padstones are bedded in, and the beam is craned or carried into position. Steel beams for typical domestic spans weigh anywhere from 80 to 250 kg, and they go in on muscle, lifting gear or a small mobile lift, depending on the room.

Once the beam is bedded, the props come down in stages, the wall is removed cleanly, and the area is made good. The bearings get checked by building control before the plasterboard goes up. We never close in a beam until the inspector has signed off, because the day after plastering is too late to fix a bearing problem. A homeowner-friendly walk-through of the steps is in our simple guide to installing an RSJ.

Total time on site for a typical single-room knock-through is one to two weeks: opening, beam in, made good, plastered, decorated. We protect the rest of the house with proper dust sheets and sealed barriers, and we are honest about the noisy days and the dust-heavy days at quote stage.

What building control checks

Building control inspects the bearings before plasterboard goes up. They will check that the padstones match the spec, the bearing length is correct, the beam is properly seated, and any restraint is in place. They will also check that the fire protection plan is correct. If the spec was right and the install is clean, the inspection takes 20 minutes and you have a signed completion certificate at the end.

The certificate matters. It is the paper trail that proves the structural alteration was done properly, and it is what a buyer’s solicitor will want to see when you sell. We deliver every project with a clean compliance pack: structural calculations, building control approval, inspection notes, and completion certificate. That is part of our full project management service, not an extra.

Frequently Asked Questions

Do I need planning permission for an RSJ installation?

Generally no, because the work is internal and does not change the external appearance of the property. Building regulations approval is required in every case because the work is structural. We submit a Full Plans application as part of every RSJ project we run.

How much does an RSJ installation cost?

For a typical single-beam knock-through in a domestic property, expect £3,500 to £7,500 fully fitted including engineering, beam, padstones, installation, making good and building control. Larger spans, double beams, or chimney breast removals run higher. We provide fixed-price quotes after a site visit.

How long does it take?

A typical knock-through with a single RSJ takes one to two weeks on site, including making good. Larger jobs with two beams or significant masonry removal can take three to four weeks. Plastering and decoration adds a further week or so depending on size.

Can I live in the house during the work?

In most cases yes. The work is contained to the affected area, and the rest of the house remains usable. We protect floors, doorways and adjacent rooms with dust barriers. The main disruption is the day of beam installation, which is noisy and dusty.

What if my house is old and has unusual walls?

Older properties in the North West often have solid brick walls, timber framing or unusual construction. The structural engineer accounts for this in the calculation, and we adjust the install method accordingly. We have done this on Victorian terraces, 1930s semis and post-war builds across the region.

Why not just hire a builder who can fit a beam?

A reputable builder will not install an RSJ without an engineer-signed calculation and a building control approval. If a tradesperson offers to skip the paperwork, walk away. The cost of getting this wrong is not measured in pounds; it is measured in structural failure, refused insurance, and an unsellable house.

Structural work is not the time to economise on the paperwork or the people. If you are planning an open-plan kitchen, a chimney breast removal or any internal knock-through, book a site visit and we will walk through what is involved with the property in front of us.