Purlin vs Structural Steel: What’s Right for Your Shed?

Planning a new shed but not sure which structure to choose? You’re not alone. One of the most common questions we get is the difference between purlin sheds and structural steel sheds and which one actually delivers better value.
Here’s what you need to know before you design your shed.

What Is a Purlin?

A purlin is a cold-formed steel section, manufactured by rolling flat sheet steel through a series of rollers to create its shape.

The two most common types are:

  • C purlins (C-shaped)

  • Z purlins (Z-shaped)

Purlins are typically made from thinner steel (around 1mm–3mm thick) and are commonly used in purlin sheds, where the main portal frames are built from these rolled sections.

What Is Structural Steel?

Structural steel is hot-rolled, meaning the steel is heated and formed into much thicker, stronger sections—typically ranging from 5mm to 20mm.
In a structural steel shed, the main portal frames and rafters are fabricated from heavy-duty steel members, then welded and bolted together for maximum strength.

Common structural steel components include:

  • Universal Beams (UB / I-Beams / H-Beams)

  • Welded Web Trusses – engineered for strength, efficiency, and open, usable space

Purlins and girts are still used, but only to support the roof and wall cladding, which is what they’re designed for.

Purlin Sheds: Pros & Cons

Purlin sheds can be a practical solution, but only in the right application.

Where they work well:

  • Smaller sheds (generally under 12m span)

  • Lower upfront material cost

  • Simple, bolt-together construction

Where they fall short:

  • Labour-heavy installation on larger sheds (higher build costs)

  • Thinner steel is more prone to damage (e.g. forklifts or machinery impact)

  • Limited span capacity without adding extra bracing

  • Internal braces (knee braces, apex ties) reduce usable space

For larger sheds, what looks cheaper upfront often becomes more expensive once installation time and complexity are factored in.

How Far Can C Purlins Span?

C purlins are best suited to smaller spans.

While some designs push up to 24 metres, this is typically:

  • At the upper limit of performance
  • Dependent on significant additional bracing
  • Less capable in high wind conditions

To compensate, designs often double up purlins (back-to-back sections), which increases both material use and installation time, sometimes by 3–4x.

Structural Steel Sheds: Pros & Cons

Structural steel is built for performance, especially in larger or more demanding environments.

Key advantages:

  • Much stronger and more impact-resistant
  • Ideal for machinery sheds, warehouses, and industrial use
  • Fully customisable (including large spans and curved roofs)
  • Faster installation due to prefabricated components
  • More cost-effective for large-scale builds

Because components are engineered, cut, and welded off-site, installation is faster and more precise—saving time and money.

How Far Can Structural Steel Span?

With structural steel, spans are engineered to suit the project.

That means:

  • Larger clear spans are achievable
  • Fewer internal obstructions
  • Designs tailored to your exact site and use

When combined with lower installation costs, structural steel becomes the clear choice for larger sheds.

Our #1 Tip for Building a Shed

No matter which option you choose, site-specific engineering is critical.

Wind ratings, terrain, and usage all impact your shed design. A structure built for flat ground may not perform the same on a hill or in an exposed area.

At TechSpan, every shed is engineered for its exact location and purpose, this means no guesswork.

The Bottom Line
  • Purlin sheds are a cost-effective option for smaller spans (under 12m)
  • Structural steel sheds are stronger, more flexible, and better value for larger builds

If you’re building a large shed, structural steel isn’t just the stronger option, it’s often the smarter investment.

Have a project in mind? Let’s chat.

Try our 3D Shed Builder