Purlins are a vital yet often misunderstood component in steel buildings. While large beams and columns steal the spotlight, purlins quietly carry the load of the roof cladding, making them crucial to the structure’s stability.
In my experience, many miss the key distinction between purlins and rafters, leading to design errors such as over-spanning or improper lapping. These mistakes can compromise the entire building’s integrity.
This guide will explain what purlins are, how to select the right type, and common mistakes to avoid, ensuring your steel building stands strong and reliable.
What Are Purlins?
Purlins are horizontal members that run along the length of a roof structure, providing support for the roof cladding. They may not look like much, but they are essential in transferring the weight of the roof to the main frame. In simple terms, purlins are what hold the roof panels up and ensure the load is evenly distributed.
The most common confusion arises when people mix purlins up with rafters. While rafters form the main skeletal structure of a roof, purlins run perpendicular to them and support the roof sheeting. You could think of rafters as the framework, and purlins as the horizontal supports that keep everything in place.
In Australia, steel purlins dominate commercial construction because of their strength, durability, and versatility. Unlike timber or concrete, steel purlins are immune to termites, warping, and rot, making them the preferred choice for long-lasting and low-maintenance buildings.
Types Of Purlins
Purlins come in a few different types, each suited for specific needs and spanning behaviours. The most common are C purlins and Z purlins. Here’s a breakdown of their differences:
| Purlin Type | Shape | Best For | Spanning Behaviour | Lapping |
| C Purlins | C-shaped | Roof framing for most buildings | Works well for shorter spans, commonly used in domestic and commercial buildings | Can be lapped for continuous runs |
| Z Purlins | Z-shaped | Long-span roofs, commercial use | Offers better spanning capacity, ideal for large roofs and industrial buildings | Usually used with bolted connections, lapping increases stability |
C purlins are typically used in smaller buildings or where the spans are relatively short. They’re more common in residential or low-rise commercial buildings. Z purlins, on the other hand, are used in larger structures because they offer superior strength, making them the go-to choice for industrial buildings or long-span roofs.
When selecting the right purlin for your project, it’s essential to consider both the span and load requirements. While C purlins are great for smaller applications, Z purlins should be the go-to when dealing with heavy roofs or large spans.
Purlins Vs Girts
Purlins and girts are both essential elements in steel-framed buildings, but they serve different purposes. Understanding their roles will help clarify why both are needed in a typical portal frame structure.
- Purlins: These support the roof cladding and run horizontally along the roof structure. They are responsible for transferring the weight of the roof panels to the main frame.
- Girts: In contrast, girts support the wall cladding. They are located horizontally around the exterior walls of the building and help carry the weight of the wall panels.
The key difference lies in the direction of the loads they carry. Purlins handle the load of the roof, while girts handle the load of the walls. Both are critical for the structural integrity of the building. In a portal frame system, purlins and girts work together to ensure that the entire structure remains balanced and stable.
Example Scenario:
Imagine you’re constructing a large industrial shed. The purlins will bear the load of the roof panels, while the girts will support the external wall cladding. If either is overlooked or miscalculated, the weight distribution could be uneven, leading to structural issues like sagging roofs or wall instability.
Purlin Sizing And Span Tables
When selecting purlins for a project, one of the most crucial factors to consider is the span – the distance between the supports. Understanding how to read purlin span tables and choose the right size and spacing is essential for ensuring the roof structure is both efficient and safe.
How To Read Span Tables?
Span tables provide a guideline on the maximum distance a purlin can span without sagging or becoming unstable. These tables consider factors such as:
- Purlin Type (C or Z)
- Steel Grade
- Roof Load (including the weight of cladding, snow, and wind load)
- Spacing Between Purlins (typically measured in 900mm to 1,200mm centres)
A span table typically lists the maximum span for different purlin sizes at various spacing intervals. For example, a C purlin with a certain steel grade may span 6 meters when spaced at 1,000mm but only 4 meters when spaced at 1,500mm. This variation helps balance the cost of materials and the building’s structural requirements.
Understanding Gauge And Depth Selection
- Gauge refers to the thickness of the purlin. A thicker gauge (e.g., 1.5mm) is stronger but heavier.
- Depth is the height of the purlin section. A deeper section increases the purlin’s load-bearing capacity.
The larger the span, the larger (and often thicker) the purlin needs to be to safely carry the load. Span tables will specify the maximum allowable span for each combination of gauge, depth, and load.
Spacing
The typical spacing for purlins in Australian roofs is between 900mm and 1,200mm centres. Spacing purlins too far apart can compromise the roof’s ability to bear weight, whereas placing them too close together can lead to unnecessary material costs.
Purlin Lapping Systems
When it comes to long-span roofs, purlin lapping systems become crucial. Lapping refers to the method of joining two purlins to create a continuous run across the roof. This process is necessary when the required span exceeds the length of a single purlin, which is typical in larger buildings or sheds.
Types Of Lapping Systems
There are two main types of lapping systems commonly used in steel framing:
- Sleeved Lapping
This system involves overlapping two purlins, then inserting a sleeve or splice plate over the connection. The sleeve is bolted or welded to secure the purlins together. Sleeved lapping provides a solid, strong joint, allowing for greater spanning capacity without the need for additional supports. - Swaged Lapping
In this system, one purlin end is swaged (compressed) to fit into the other, creating a tight, seamless connection. The swaged joint is then bolted to ensure it holds under load. Swaged lapping is commonly used in more advanced framing systems, offering excellent strength and stability for long, continuous roof spans.
Why Lapping Increases Structural Efficiency?
Lapping is important because it allows for a continuous purlin system, reducing the need for additional supports in the middle of the span. The structural efficiency gained by using lapped purlins is significant, as it distributes the load across a longer span without weakening the roof’s integrity.
However, proper installation is key. If not executed correctly, the connection between lapped purlins could become a weak point, potentially leading to deflection, sagging, or failure. That’s why careful attention to detail during the lapping process — whether sleeved or swaged — is crucial.
Purlin Bridging And Fly Bracing
Bridging and fly bracing are essential techniques used to increase the structural stability of purlins in steel buildings, especially when dealing with long spans or high roof loads. These systems prevent purlins from experiencing lateral-torsional buckling, which can cause the entire roof structure to fail.
What Is Purlin Bridging?
Purlin bridging involves connecting multiple purlins together with cross braces, usually at regular intervals along the span. These cross braces help prevent the purlins from twisting or buckling under pressure. The purpose of bridging is to ensure that the purlins remain straight and level, even under load.
In a warehouse with a large, open roof space, purlins could be spaced 1.2 meters apart. Without bridging, these purlins would be at risk of twisting, especially when subjected to wind loads or heavy roof cladding. Bridging keeps them aligned, distributing the load evenly across the entire roof.
Fly Bracing
Fly bracing works similarly to purlin bridging, but it is designed to support the entire frame rather than just the purlins. Fly bracing is usually installed between the top chords of the frame, creating a bracing system that resists lateral forces. This system is often used in portal frame buildings where the horizontal forces (like wind) are significant.
Fly bracing is often required by Australian standards, particularly in areas with high wind ratings or seismic activity, as it adds extra resistance to forces that could cause the frame to sway or distort.
Australian Standards For Bridging And Bracing
The AS 4600 standard outlines the requirements for purlin bridging and fly bracing in steel-framed buildings. This standard ensures that the design and implementation of these systems meet the required strength and safety levels.
For instance, in regions with high wind speeds, the spacing of purlins might need to be closer together, and additional bridging may be required to prevent buckling. Always consult local standards and guidelines to ensure your building complies with the necessary regulations.
Steel Grades For Purlins
The steel grade used for purlins plays a critical role in their performance, particularly when it comes to strength, deflection, and weight. In the Australian market, two common steel grades used for purlins are G450 and G500. The choice of grade depends on the specific requirements of the project, such as the load the purlins will carry and the environmental conditions.
Common Steel Grades For Purlins
| Steel Grade | Strength | Common Uses | Advantages |
| G450 | 450 MPa | Residential and light commercial buildings | Provides an excellent strength-to-weight ratio; commonly used for general construction |
| G500 | 500 MPa | Industrial, commercial, and high-load buildings | Higher strength, ideal for larger spans and heavy roofing loads |
Why Choose The Right Steel Grade?
- G450 Steel: This grade is widely used because of its good balance between strength and weight. It’s often chosen for residential and small-scale commercial buildings, where load requirements aren’t as extreme. G450 provides sufficient strength while remaining cost-effective.
- G500 Steel: If you’re dealing with large industrial structures, long-span roofs, or buildings in extreme conditions, G500 is the better choice. Its higher yield strength (500 MPa) means it can support heavier loads over longer spans without excessive deflection.
In a commercial warehouse with a roof span of 18 meters, G500 purlins would be selected to ensure the purlins can handle the heavy roof cladding and high wind loads. In contrast, a smaller, single-storey shed might only require G450 purlins, as the loads and spans are not as demanding.
Choosing the right steel grade ensures the purlins can handle the loads placed on them without excessive deflection or failure. It also impacts the overall weight and cost of the framing system.
Cost Of Purlins In Australia
The cost of purlins varies depending on several factors such as the type of steel, size, span, and steel grade used. It’s important to have a clear understanding of how purlin prices are calculated to estimate project costs accurately. Below, we’ll look at the typical price ranges for C and Z purlins, as well as how to estimate the number of purlins you’ll need for your project.
Price Per Lineal Metre
The price of purlins is usually quoted per lineal metre. For standard C purlins and Z purlins, you can expect the following general pricing:
| Purlin Type | Price per Lineal Metre | Typical Uses |
| C Purlins | $2.50 to $5.00 | Small to medium residential and commercial roofs |
| Z Purlins | $4.00 to $7.00 | Large span commercial and industrial roofs |
Estimating Quantities
To calculate the number of purlins required for your project, start by measuring the total length of the roof and determining the spacing between purlins (typically 900mm–1,200mm). Here’s a simple formula to estimate how many purlins you’ll need:
- Measure the total length of the roof (in metres).
- Decide on the purlin spacing (e.g., 1,000mm).
- Divide the total roof length by the purlin spacing (converted to metres). For example, for a 20m roof with 1,000mm (1m) purlin spacing, you’d need approximately 20 purlins.
This estimate doesn’t account for the overlap of purlins or any lapping, which may slightly increase the number of purlins required.
Factors That Affect Purlin Costs
- Steel Grade: Higher-strength steel grades (like G500) cost more due to their enhanced load-bearing capacity.
- Purlin Size and Depth: Larger purlins that can span greater distances will be priced higher due to the increased material used.
- Transport Costs: For projects in regional areas, delivery costs can add up, especially when ordering large quantities of steel purlins.
For a small warehouse with a roof span of 20 metres and a Z purlin system spaced at 1,200mm centres, the cost per lineal metre may range from $4.00 to $7.00, depending on the size and grade. If you need 20 purlins for the span, your total material cost would be approximately $80 to $140 per metre of purlin, excluding any installation or delivery charges.
Purlins may not be the most glamorous component of a steel building, but they are vital to the structure’s overall integrity and longevity. By understanding the different types of purlins, how to properly size and space them, and the critical role of bridging and lapping systems, you can avoid costly mistakes that compromise your roof’s stability.
Selecting the right type of purlin, consulting span tables, and ensuring proper compatibility with your roof cladding will all contribute to a more secure and efficient steel building.
Additionally, always account for local conditions, such as wind loads and snow loads, and never overlook the importance of proper bridging to prevent purlin buckling. With these considerations in mind, you can confidently specify and install purlins that will keep your building safe, durable, and cost-effective for years to come.