What Are Steel Mullions and When Do You Need Them?

Steel mullions are an important component in modern construction, providing support, separation and structural reinforcement where required. They are commonly used around window and door openings, in commercial buildings and in certain structural applications where a suitably designed steel section is needed.

But what exactly is a steel mullion, which size should you choose, and when do you need one?

In this guide, we explain everything you need to know about steel mullions, their common applications and what to consider before buying.

What Is a Steel Mullion?

A steel mullion is a vertical steel member used to divide an opening, support a frame or provide structural support as part of a building design.

Mullions are often associated with windows and doors, where they can separate individual window panels or form part of a larger glazing system. Depending on the application and engineering design, steel mullions can also be used as intermediate supports in certain building structures.

Steel mullions are available in different shapes, sizes and thicknesses to suit different requirements.

At Archeva, our range includes square steel mullions and corner mullions designed for construction applications.

What Are Steel Mullions Used For?

Steel mullions have several practical applications in residential, commercial and industrial construction.

1. Window and Door Frames

One of the most common applications is providing a vertical member within a window or door frame.

Mullions can divide a large opening into smaller sections, allowing multiple window panels or glazing units to be incorporated into a single design.

Steel is particularly useful where a strong, durable frame component is required, provided the selected section is suitable for the loads involved.

2. Commercial and Industrial Buildings

Factories, warehouses, offices and other commercial buildings often incorporate large openings that require carefully designed framing systems.

Steel mullions may form part of these systems, helping to support frames and divide openings according to the building design.

The appropriate section size, wall thickness and fixing method will depend on the specific application.

3. Intermediate Structural Supports

Certain steel mullions can also be designed for use as intermediate supports in building structures.

For example, some applications may require a steel member to help transfer loads between structural elements. However, a mullion should only be used to support a concrete slab, beam or other structural component when it has been specifically designed and approved for that purpose.

Always obtain the appropriate structural engineer's specifications before using a mullion for load-bearing applications.

4. Corner Window and Structural Junctions

Corner mullions are used where two sections meet at a corner or where a specially configured support is required.

Their design differs from a standard straight mullion because the top plate or connection arrangement may need to accommodate a corner configuration.

Choosing the correct corner mullion from the beginning can help simplify installation and ensure that the component matches the intended design.

What Size Steel Mullion Do You Need?

Choosing the correct size is one of the most important parts of specifying a steel mullion.

A larger steel section is not automatically the correct choice. The required size depends on the opening dimensions, loads, fixing arrangements, material thickness and intended application.

Archeva offers several square steel mullion sizes, including:

• 76 mm square mullions – for suitable applications where the specified section meets the design requirements.

• 100 mm square mullions – an option for projects requiring a larger square section.

• 120 mm square mullions – for applications requiring a different section size or capacity.

• 150 mm square mullions – a larger section for projects where the design calls for it.

These dimensions refer to the nominal outside size of the square section. They do not, on their own, indicate how much load the mullion can safely carry.

Wall thickness, steel grade, member length, connections and support conditions all affect performance.

If you are unsure which size to order, confirm the specification with your architect, engineer or building professional before purchasing.

Square Mullions vs Corner Mullions

Although both products are used in construction, they are designed for different configurations.

Square steel mullions

These use a square steel section and are generally suited to applications requiring a straight vertical member.

Corner steel mullions

These incorporate a corner-style plate or connection arrangement for applications where a standard straight configuration may not be suitable.

Before ordering, check the dimensions of the steel section, plate sizes, hole positions and overall height against your construction drawings.

What Should You Check Before Buying Steel Mullions?

Before placing an order, make sure you have the following information available.

1. The required dimensions

Confirm the specified section size and overall length in millimetres.

2. Steel thickness

The wall thickness of the steel section affects its properties and suitability for the intended application.

3. Baseplate and top plate requirements

Check the specified plate dimensions, thickness and hole positions. These must match the intended connection and supporting structure.

4. Fixing requirements

Confirm the correct anchors, threaded rods, chemical mortar or other connection components for the particular installation. The fixing system must be suitable for the supporting material and the loads involved.

5. Surface finish

Steel mullions may require a suitable protective coating to help reduce corrosion. Consider the installation environment and the specified finishing requirements.

6. Engineering approval

If the mullion forms part of a load-bearing system, confirm that the complete assembly has been designed for its intended loads. Do not select a section based on appearance or dimensions alone.

Why Choose Steel Mullions from Archeva?

Archeva supplies steel construction products for residential, commercial and industrial projects across South Africa.

Our online range includes square steel mullions and corner mullions in several sizes, making it easier to find a product that matches your project's specified requirements.

You can browse the available options online, review the listed product specifications and order directly through our website.

If your project requires a particular configuration, plate arrangement or specification that you cannot find in the standard range, contact Archeva to discuss your requirements.

Browse Archeva's Steel Mullions

Whether you are working on a window installation, a commercial building project or another construction application, choosing the right steel component starts with understanding the requirements of the job.

Explore our available products here:

Shop Steel Mullions: https://archeva.co.za/collections/mullions

Explore Construction Steel Products: https://archeva.co.za/collections/construction-steel

Need help finding the right product for your project? Visit https://archeva.co.za/ and contact Archeva with your required dimensions and specifications.

Important: Product dimensions and descriptions do not constitute structural design approval. Always confirm suitability, load capacity, connections and installation requirements with the responsible design professional before using a steel mullion in a structural application.