Knock-Throughs and RSJs Explained: What Homeowners Need to Know

Knocking through a wall to create an open-plan space is one of the most popular home improvements we do. It can completely transform how a house feels — turning two cramped rooms into one bright, sociable space. But it's not as simple as hiring a sledgehammer. If the wall is load-bearing (and most internal walls between your main rooms are), you need a steel beam to take over the structural role. Here's everything you need to know before you swing that hammer.

How to Tell If a Wall Is Load-Bearing

This is the first and most important question. Get it wrong, and you risk serious structural damage — cracks in walls, sagging floors, or in the worst case, partial collapse. There are some indicators you can check yourself, but a structural engineer should always make the final call.

Signs a Wall Is Likely Load-Bearing

• It runs perpendicular to the floor joists above. Go into the loft or lift a floorboard upstairs and check which direction the joists run. A wall that runs at right angles to the joists is probably supporting them.

• It's in the centre of the house. Walls running down the middle of a property typically carry the first floor and sometimes the roof.

• It's thick. Load-bearing walls are usually full brick (225mm) or block (100mm+). Thin stud walls (timber frame with plasterboard) are rarely load-bearing, though there are exceptions.

• It continues down to the ground floor. If the wall continues from the ground floor up through the first floor, it's likely structural.

Signs a Wall Is Probably Non-Load-Bearing

• It runs parallel to the floor joists above

• It's a thin timber stud wall

• It was clearly added later (different brick type, doesn't bond into the external walls)

Important:these are guidelines, not guarantees. I've seen stud walls that turned out to be supporting joists, and thick walls that were non-structural. Always get a structural engineer to confirm before any demolition. Their inspection costs £200–£400 and could save you from a disaster.

What Is an RSJ?

RSJ stands for Rolled Steel Joist, though the correct modern term is Universal Beam (UB). It's a steel beam (typically an I-shaped or H-shaped cross-section) that spans the opening where the wall used to be. The beam carries the load from above — the floor, walls, or roof — and transfers it to the walls at either side of the opening.

Think of it as a bridge. The load-bearing wall was like a dam, supporting everything above it continuously. Remove the wall, and you need a bridge (the RSJ) to carry that load across the gap, supported at each end by solid bearing points.

The Structural Calculation Process

Your structural engineer will calculate the size of beam required based on:

• The span — how wide the opening is. Longer spans need bigger beams.

• The load above — what's sitting on top of the wall? Just a floor? A floor plus a wall above? The roof?

• Bearing points — the beam needs solid support at each end. The engineer specifies the minimum bearing length (typically 150mm) and may require a concrete padstone to spread the load.

For a typical knock-through in a 3-bed semi (opening up the living room into the dining room), the engineer will usually specify something like a 203 x 133 x 25 UB or similar. The calculation takes 1–2 weeks and costs £300–£500.

How the Installation Works

Installing an RSJ is a precise process. Here's the typical sequence:

The whole process takes 1–2 days for a straightforward single-beam installation. More complex jobs (multiple openings, stacked beams, or deep renovations) take longer.

How Much Does an RSJ Installation Cost?

For a standard knock-through with RSJ in the Christchurch area:

• Simple knock-through (one beam, standard span): £1,500–£2,500

• Wider opening or heavier loads: £2,500–£3,000+

• The steel beam itself: £200–£600 depending on size

• Structural engineer's calculation: £300–£500

The total including the engineer, beam supply, installation, making good, and building regs is typically £2,000–£3,500 all-in.

Building Regulations

Any structural alteration to a load-bearing wall requires building regulations approval. This isn't optional. A building control officer will inspect the work at key stages — typically before the beam is concealed behind plasterboard — to confirm it matches the structural engineer's specification.

You'll receive a completion certificate when the work is signed off. Keep this safe — your solicitor will need it when you sell the property.

Removing a non-load-bearing wall doesn't technically require building regs, but if there's any doubt about whether it's structural, get it checked. The consequences of getting it wrong are too serious to gamble on.

Thinking About Opening Up Your Home?

A knock-through is one of the most satisfying home improvements you can do — the change in how a house feels is immediate and dramatic. If you're considering it, start with a structural engineer's assessment. We work with several excellent engineers in Dorset and can coordinate the whole process from initial assessment through to a finished, plastered opening.