Purlins play a critical role In the world of construction, especially when it comes to steel structures and roofing systems. In a nutshell, these horizontal structural members run across the roof and wall frame, which helps support loads from the roof deck or sheeting.

Fair enough, something like a metal purlin might not be considered to be the most glamorous element of a building’s infrastructure, but make no mistake, purlins are absolutely essential for structural integrity and long-term performance.

A good starting point would be to understand that not all purlins are created equal. Which one you use is highly dependent on the nature of your project. Whether it’s a warehouse, commercial building, agricultural shed, or industrial facility, the type of purlin you choose will prove highly influential when it comes to cost, ease of installation, and overall durability.

Let’s look at five common types of purlins. There is also a breakdown of their pros and cons to help you make the right choice for your next build.

C Purlins

C purlins are also known as C-sections. They get their name from their distinctive C-shaped cross-section. You will find that these are among the most commonly used purlins in construction as a result of their high structural efficiency and ease of installation.

Pros: C purlins are incredibly versatile and lightweight, which makes them easy to handle and transport. Their shape allows for direct bolt or screw connections, which simplifies the installation process. They’re especially effective in supporting medium-span roofs and walls. The open profile of these purlins also allows wiring or plumbing to be routed through without additional drilling or modifications.

Cons: Despite their strength, C purlins are generally less suitable for heavy load-bearing compared to other types like Z purlins. They also can’t easily be lapped, which means being overlapped for added strength or length. However, this limits their structural continuity. In some applications, this could mean that you will need more supports to get the job done, which increases material and labor costs.

Z Purlins

Z purlins get their name from their Z-shaped cross-section. This type of purlin is regularly used in large-scale construction where greater spans and load capacity are required.

Pros: The main advantage of Z purlins is their ability to overlap at joints. This provides increased strength and stability over long spans. This attribute makes them a go-to choice for industrial buildings, large sheds, and warehouses. Their shape allows better load distribution, especially on sloped roofs, and this design means they can reduce the number of required structural frames.

Cons: Z purlins are more complex to install, particularly in tight spaces or on irregular roof shapes. The lapping feature requires precise alignment and additional fastening, which can certainly slow down construction. Z-purlins also tend to be more expensive than C purlins, both in material cost and installation labor.

Sigma Purlins

Without question, sigma purlins are less commonly used than C and Z types. This type of purlin has a distinctly unique, thicker profile that resembles the Greek letter Sigma. They’re primarily designed for heavy-duty performance.

Pros: Sigma purlins offer excellent strength-to-weight ratios, making them highly efficient for supporting heavy roofs. They also work well for supporting solar panel arrays, or coping with snow loads in colder climates. Their design minimizes flex under stress and offers greater rigidity. In some cases, Sigma purlins can allow for wider spans with fewer supports.

Cons: Their complex shape can make manufacturing and cutting more expensive. Sigma purlins are also less widely available than standard C or Z sections, which could cause delays in sourcing them. Installation may require specialized tools or fasteners, which is a factor that drives up labor costs due to this level of complexity.

Box Purlins

Box purlins are typically made by combining two C purlins back-to-back to form a closed rectangular section. These are often used in buildings where high structural strength is critical.

Pros: Box purlins are extremely strong and stable. The closed section resists twisting and bending better than open profiles, making them ideal for buildings in high-wind conditions or seismic zones. They are also useful in architectural applications where visible purlins contribute to the building’s aesthetic.

Cons: The biggest downside to using box purlins is weight. They are heavier than most other purlin types, which increases handling difficulty. This also impacts shipping costs, and adds to labor costs. They also use more steel, so your material costs will be higher. All of this means that your installation may require cranes or lifts, depending on the project scale.

Hat Purlins

Hat purlins, also known as top-hat sections. When you see their shape you can see where the description comes from. They’re more commonly used in lightweight framing, ceilings, or where insulation or finishing materials are installed below the roofing deck.

Pros: Hat purlins are lightweight and easy to install. They work well in low-load applications and are often used in smaller buildings, garages, or interior structures. Their shape provides a natural cavity for insulation or wiring. They’re also cost-effective and can be manufactured in various custom lengths.

Cons: Hat purlins are definitely not suited for heavy structural use. Their limited load-bearing capacity makes them inappropriate for large spans or commercial roofing systems. If used in the wrong application, they can sag or warp under pressure, leading to structural issues down the line.

Choosing the Right Purlin: Key Considerations

While the differences between these purlin types might seem purely technical, they matter greatly when it comes to real-world applications.

Choosing the right purlin type depends on several key factors. These mainly include the size and span of your roof, local climate conditions, the building’s purpose, and, naturally enough, what sort of budget you are working with for your project.

If you’re building a standard commercial or industrial facility with large spans and roof loads, Z or Sigma purlins are often the way to go. If it’s a smaller project or something with a tight budget, C purlins may prove to be a better fit. For architectural builds or areas with demanding environmental conditions, box purlins provide superior strength, though at a higher cost.

Your decision might also be influenced by availability and lead time. The bottom line is that choosing the right type isn’t just a design decision, it’s also about structural safety, cost-efficiency, and long-term performance.

The best approach is to talk through your options with your purlin supplier.