Timber vs Steel Purlins: Choosing the Right Roof Purlin System for Your Shed

When planning a shed project in New Zealand, one detail that’s easy to overlook is the type of purlins used in your design. Purlins form the backbone of your shed’s roof structure, so choosing the right system is crucial for long-term strength, stability, and ease of maintenance.

In this guide, we explain what a purlin is and compare the most common types: Timber purlins, steel purlins, and C-section purlins, so you can make the best choice for your next build.

• What is a Purlin?
• Timber Purlins
• Steelbone Purlins (enclosed steel purlin)
• C Section Purlins
• Tophat Purlins
• Choosing the Right Purlins for Your Shed

What is a Purlin?

A purlin is a horizontal beam that supports your roofing iron and helps transfer loads to the main rafters. In simple terms, purlins and rafters work together to hold up the roof, with purlins running parallel to the ridge line.

Whether you’re constructing a farm shed, aircraft hangar, or storage building, the roof purlin plays a vital role in overall performance. Most New Zealand sheds use either timber purlins or steel purlins, each with its own advantages.

Understanding the differences between purlin types and how each contributes to roof strength will help you choose the right system for your structure.

Timber Purlins

Timber purlins are widely used across New Zealand, particularly in pole sheds and mixed timber-and-steel designs. They’re valued for their natural insulation and acoustic benefits.

Advantages of Timber Purlins

• Noise reduction: Timber absorbs sound better than steel, reducing echo and machinery noise inside the shed.
• Temperature control: As a natural insulator, timber helps regulate internal temperature and reduce condensation.
• Ease of installation: Timber purlins make it simpler to add wiring or lining after construction.
• A tidy finish:  At Alpine, all our timber is pre-dried before use, ensuring straight, stable purlins that are easy to handle.
• Bird-proof design: Combined with our steel rafters, the system leaves no room for birds to perch or nest.

Disadvantages of Timber Purlins:

• Limited to about 6m spans, as that’s the maximum standard timber length.
• Non-dried timber can warp or move after installation.

For workshops or storage sheds where noise control and warmth are important, timber purlins remain a dependable and proven option.

Steelbone Purlins

Steelbone purlins are fully enclosed, V-shaped sections made from high-tensile galvanised steel. They’re designed for strength and durability, commonly used in agricultural and industrial applications.

Advantages of Steelbone Purlins

• Bird-proof: The closed profile leaves no ledges for birds to perch.
• Low maintenance: Smooth surfaces minimise dust buildup and make cleaning easier.
• Fast installation: Pre-formed profiles allow efficient on-site assembly.

Disadvantages of Steelbone Purlins:

• Maximum spans are generally up to 10 m, sometimes less in high-snow zones.
• The closed section uses more steel per metre, increasing material costs slightly.

Steelbone purlins are ideal for wide-bay agricultural sheds that need maximum internal clearance and minimal maintenance.

C Section Purlins

C-section steel purlins are one of the most common systems used in all-steel and commercial sheds. Available in a variety of profiles and thicknesses, they offer flexibility for different spans.

Advantages of C Section Purlins:

• Broad range of depths and gauges for optimised design.
• Lightweight yet strong - perfect for industrial applications.
• Cost-effective for large projects where bird proofing isn’t a concern.

Disadvantages of C Section Purlins:

• The open shape provides surfaces where birds can perch and dust can accumulate.
• Additional brackets or braces are often required, increasing labour and reducing aesthetics.
  
  

C-section purlins are best suited to large industrial or commercial buildings, rather than sheds where tidiness or low noise are priorities.

Tophat Purlins

Commonly found in lightweight roll-form sheds, tophat purlins are cost-effective but less robust than enclosed systems.

Pros of tophat purlins:

• Lightweight and inexpensive per metre.

Cons of tophat purlins:

• Positioned on top of rafters, offering many places for birds to perch.
• Rely heavily on cladding for rigidity, which can pose issues in high wind or snow regions.
• Easily bent or dented due to thin steel.
  
  

Choosing the Right Purlin System for You

When comparing timber purlins vs steel purlins, consider the structure’s purpose, environment, and desired finish. For a quiet, insulated workshop, timber purlins are a great fit. For large, open agricultural buildings, steel purlins - particularly enclosed designs - provide superior strength and bird protection.

It’s also worth thinking about installation access, wiring needs, and long-term maintenance. Your purlin choice affects not only strength but also usability and comfort inside the shed.

Talk to the Experts

At Alpine Buildings, we’ve worked with every purlin design system in New Zealand. Our clearspan sheds are engineered to optimise space, performance, and durability - whether that means using pre-dried timber purlins or enclosed steel purlins for extra strength.

If you’re unsure which roof purlin system is right for your project, our team can help you work out what system is best for you.

Start your shed design with Alpine Buildings today and build a structure that lasts a lifetime. Or two.