When Do You Need an RSJ? A Simple Guide for Builders & Property Owners
- southwesttradestee
- Jan 21
- 3 min read
RSJs (Rolled Steel Joists) are a critical structural component in many construction and renovation projects. If you’re removing walls, opening up spaces, or altering the structure of a building, chances are an RSJ will be required.
This guide explains when you need an RSJ, why they’re used, and what to consider before ordering one — written for builders, developers and property owners across the UK.
What Is an RSJ?
An RSJ (Rolled Steel Joist) is a structural steel beam used to support loads where walls or structural elements have been removed or altered.
RSJs are commonly used to:
Support floors
Carry roof loads
Replace load-bearing walls
Create large open-plan spaces
They are designed to safely transfer weight to suitable load-bearing points, such as walls or columns.
Common Situations Where an RSJ Is Required
1️⃣ Removing a Load-Bearing Wall
This is the most common reason an RSJ is needed.
If a wall supports:
A floor above
A roof structure
Another structural element
An RSJ is required to replace the load path once the wall is removed.
2️⃣ Creating Open-Plan Spaces
Open-plan kitchen-diners, extensions and knock-throughs almost always require steel beams to support the structure once walls are removed.
The larger the opening, the larger the beam required.
3️⃣ Loft Conversions
Loft conversions frequently use RSJs to:
Support new floor joists
Strengthen existing structures
Redistribute roof loads
RSJs are often hidden within ceilings or floors once installed.
4️⃣ Extensions & Structural Alterations
RSJs are commonly used where:
Existing walls are opened up into extensions
New structural openings are created
Loads need to be transferred to new foundations
How Do You Know If a Wall Is Load-Bearing?
Some indicators include:
The wall runs perpendicular to floor joists
It supports beams or joists above
It is located centrally within the property
However, visual checks are not enough. A structural engineer should always confirm whether a wall is load-bearing and specify the correct steel beam.
Do You Need Building Control Approval for an RSJ?
Yes — in almost all cases.
RSJ installations usually require:
Structural calculations
Building Control approval
Inspection during installation
This ensures the beam is correctly sized, installed and supported.
Who Sizes an RSJ?
RSJs must be sized by a structural engineer, not guessed on site.
Sizing depends on:
Load above
Span length
Building type
Support conditions
Using an incorrectly sized RSJ can cause structural issues and will almost certainly fail Building Control.
Supply Only vs Supply & Install RSJs
RSJs can be supplied:
Supply only (for builders)
Supply and install (turnkey solution)
Builders often prefer supply-only, while homeowners typically opt for supply and install to ensure compliance and correct fitting.
How Much Does an RSJ Cost?
RSJ costs vary depending on:
Beam size and weight
Length
Steel grade
Whether installation is included
As a rough guide:
Smaller RSJs may cost a few hundred pounds
Larger or bespoke beams can run into thousands
Installation, temporary supports, padstones and labour all need to be factored into total project cost.
Common RSJ Mistakes to Avoid
Guessing beam sizes
Not allowing for padstones or bearings
Poor temporary support during installation
Ordering steel before engineer approval
These mistakes cause delays, extra cost and failed inspections.
RSJ Supply & Installation in the South West
If you’re a builder, developer or property owner working in Somerset, Devon, Dorset or the wider South West, using a reliable steel supplier helps keep projects moving.
Correct sizing, clear lead times and proper coordination with Building Control are essential for smooth installations.
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