RSJ Steel Beams and Structural Knock-Throughs: What You Need to Know

Creating an open-plan space by removing a wall between a kitchen and a dining room, or knocking through into a rear extension, is one of the most popular structural alterations in the UK. When done correctly, it transforms how a house functions. When done incorrectly, it is dangerous.

This article explains how structural knock-throughs work, what an RSJ steel beam is, and why the process must involve a structural engineer.

What Is a Structural Wall?

Not every internal wall is structural. Partition walls — lightweight studwork or blockwork — carry no structural load. Structural (load-bearing) walls carry the weight of floors, walls, or roof above, transferring those loads to the foundations.

Removing a partition wall is straightforward. Removing a structural wall requires a steel beam to carry the loads the wall was transferring. Before any work begins, a structural engineer must confirm whether the wall is structural, calculate the loads, design a beam of appropriate size, specify the padstone arrangements, and confirm the foundations can carry the new concentrated loads.

What Is an RSJ?

RSJ stands for Rolled Steel Joist — a structural steel section hot-rolled into an I-shaped cross-section. Two horizontal flanges connected by a vertical web give the beam its strength in bending.

A typical domestic knock-through might use a 152×152 UC or a 203×133 UB — but the correct size is always determined by structural calculation, not convention. Using an undersized beam is dangerous; an oversized beam wastes money. The term "RSJ" is used colloquially for any structural steel beam; in practice it may be a Universal Beam, Universal Column, or parallel flange channel.

The Knock-Through Process

Stage 1 — Structural engineer's design. Drawings and calculations specifying beam size, padstones, temporary support, and any foundation check.

Stage 2 — Building Control notification. A Building Notice is submitted before work begins; the officer inspects at key stages.

Stage 3 — Temporary support. Acrow props and spreader boards support the structure above before the wall is removed. This is not optional — inadequate propping is the most common cause of structural accidents.

Stage 4 — Forming the opening. The wall is carefully removed within the temporary support zone.

Stage 5 — Padstones. Dense engineering brick or concrete blocks at each bearing position distribute the concentrated load and prevent local crushing.

Stage 6 — Beam installation. The beam is lifted in, bedded onto the padstones, checked for level, and grouted.

Stage 7 — Making good. Props are removed once the mortar has cured; the soffit and reveals are made good — or left as an exposed steel beam if preferred.

Stage 8 — Building Control sign-off. The officer inspects and issues a completion certificate.

Common Mistakes to Avoid

• No structural engineer. The beam size must be calculated — attempting a knock-through without one is dangerous and non-compliant.
• Inadequate temporary support. The single most common cause of structural accidents during alterations.
• No Building Control notification. Missing documentation will be picked up by a buyer's surveyor.
• Wrong padstone specification. Can lead to local wall failure over time.

What Dancy Building Contractors Does

We carry out structural knock-throughs and RSJ beam installations across West Sussex and Surrey. We work alongside the client's structural engineer (or can recommend one for smaller projects), manage Building Control, and deliver the work to a high standard.