Have you ever looked up at the interior of a steel structure and wondered: “what are those bars supporting the roof called?” Well, if you have, then you’ve come to the right article to find your answer.
The answer to this question, of course, is the title of this blog: purlins. A crucial component of most building structures, purlins are what – literally – keep the roof over your head. They are an important part of most building structures and are a mark of quality when it comes to steel building structures.
This article will help you understand the role purlins play in the integrity of your steel building structure, so you can make a more informed purchasing decision for your next building. Proper structural integrity and load support is key to the longevity of a building, and to the safety of a building’s occupants. Understanding whether your building requires purlins, what material they should be made of, and what shape they should take, are all questions you should have the answer to before building a new structure – so let’s dive in!
What are Purlins?
Found in most large structures, purlins are the horizontal beams that provide structural support to a building by helping to pass roof loads to other major structural elements. They carry the weight of the roof sheeting, panels, and any additional loads — such as snow, rainwater, or roof accessories — and transfer that weight to the major structural elements of the building. In other words, they’re what keep your roof safely in place.
By diffusing the load across rafters, walls, or metal support beams, purlins ensure that no single part of the structure bears the entire weight of the roof. How many purlins a roof requires, as well as their size and thickness, will always depend on the specific design loads of the building.
Purlins can be classified by their shape, material, and intended use. Among all options, steel purlins are the industry standard. Unlike wood, steel will not rot, warp, or weaken under moisture, and its expansion and contraction in varying climates is minimal. That’s why steel purlins are the reliable choice for long-term structural performance.
Most metal purlins are made from cold-formed steel — thin steel sheets rolled and pressed into shape. This process produces components that are strong, lightweight, and easy to work with. To further protect against corrosion, the finished purlins are galvanized, coating the steel in zinc for lasting durability. Galvanized steel purlins can often be recognized by their distinctive crystal-like “spangle” pattern.
Purlins, Girts, and the Secondary Framing System: A Necessary Partnership
When talking about purlins in construction, it is important to remember that they are not working alone. Purlins are part of what is called the secondary framing system — and their horizontal counterparts, girts, play an equally important role.
Together, steel purlins and girts form the grid of support that ties the entire building envelope back to the primary framing. Purlins carry the roof loads and transfer them to the rafters, while girts support the wall panels and connect them to the columns. Both components also provide convenient attachment points for cladding, insulation, and accessories, making them critical to the overall building envelope.
This partnership is what gives metal building purlins their importance beyond the roof itself. By working together, purlins and girts ensure the structure can handle snow accumulation, wind loads, and other forces while maintaining rigidity and safety. Without a properly engineered secondary framing system, a steel building would lack the load paths it needs to perform as designed.
If you would like to explore the history and other key components of a pre-engineered steel building, take a look at our blog Everything You Should Know About Pre-Engineered Metal Buildings.
That is why quality purlins and girts are always engineered with the primary frame in mind, creating a complete system where every element contributes to the building’s long-term strength and reliability.
Why Purlins Are Used in Metal Buildings
While we now understand what purlins are, we want to reinforce the reasons why they are used. Purlins add to the structural integrity of a building’s roof by running perpendicular to the rafter beams (see adjacent image).
So why are purlins used in metal buildings? There are 3 structural functions provided by purlins for metal roofing.
- Purlins tie the rafters together, which stabilizes and reinforces the frame.
- Purlins provide additional support between the spans of the separate framing bays,
- Purlins improve the roof’s resistance to excessive rain, snowfall, and any other temporary loads that might be exerted on the roof.
What Are the Different Types of Purlins?
We know that steel as a material is a better pick when it comes to important structural elements of a building, like purlins, but let’s also examine the different options that exist for purlin shape.
C-Shaped Purlins:
As the name implies, C-shaped purlins take the shape of the letter C and are used to reinforce the structural integrity of the roofing system. In some construction projects, C-shaped purlins can be used as support beams in the flooring system, but for the purposes of pre-engineered steel building constructions they are exclusively used in the roofing system.
C-shaped purlins come in a variety of lengths, and thickness, so they can really be designed to fit any requirement you might need. However, C-shaped purlins aren’t quite as sturdy as the next kind of purlin we’ll look at. As a result, C-shaped purlins are typically used in smaller structures and building projects.
Z-Shaped Purlins:
Thanks to their shape, Z-shaped purlins are stronger than C-shaped purlins. The Z-shape of these purlins allows them to overlap at the joint, which provides more support than what the C-shaped purlins are capable of.
Z-purlins are used to support the roofing of larger buildings because of their strength and support. This type of purlin sits between the building and the roofing sheets, behaving like the sheet support to make sure it’s attached firmly and securely.
Like C-shaped purlins, Z-shaped purlins can be manufactured to meet nearly any length or thickness requirement. C- and Z-shaped purlins also come pre-drilled and ready to use, making them even easier to install.
Strength and Load-Bearing Capacity: The Lapping Advantage of Z-Purlins
When it comes to strength, Z-purlins stand out for their unique ability to overlap at the joints. This method of connecting one Z-purlin to the next is called lapping, and it creates a continuous structural line across the roof.
By lapping, Z-purlins are able to share the load more efficiently, which reduces stress on any single section of the roof. This overlapping connection not only increases the effective span length but also helps stabilize the building under heavy snow or high winds.
In practice, this makes Z-purlins the go-to choice for projects with longer spans and higher load requirements. If you are considering a large warehouse or industrial facility, lapped Z-purlins offer superior load-bearing capacity compared to a standard c purlin system.
C-Purlin vs. Z-Purlin: A Side-by-Side Comparison
Still wondering what is purlin design best for your project? The chart below highlights the practical differences between C-purlins and Z-purlins, making it easier to see which option fits your application.
| Feature | C-Purlin | Z-Purlin | Implication for Your Project |
|---|---|---|---|
|
Cross-Sectional Shape |
C shape, uniform |
Z shape, angled
|
Impacts strength and overlap ability |
|
Connection Method |
Placed end-to-end |
Lapped over one another |
Affects span length and stability |
|
Structural Behaviour |
Good for smaller spans |
Superior for longer spans |
Guides choice by building size |
|
Strength |
Lighter to moderate loads |
Higher load capacity |
Determines performance under heavy loads |
|
Ideal Span |
Short to medium |
Long spans |
Match span type to building design |
|
Roof Slope |
Limited adaptability |
Handles sloped roofs better |
Important for pitched roofs |
|
Installation |
Simple, fast |
Requires lap installation |
Balances ease vs. performance |
By reviewing these differences, you can better match your choice of metal building purlins to the needs of your structure.
Application and Best Use-Cases: Choosing the Right Purlin for the Job
Not every project has the same requirements, which is why different types of metal purlins exist. Choosing the right one depends on the building’s size, loads, and intended use.
Z-Purlins are the preferred choice for:
- Large-span metal roofs where strength is critical
- Industrial buildings with heavy snow or wind loads
- Structures requiring continuous support across multiple bays
- Steel roof purlins on sloped or wide-span designs
C-Purlins are ideal for:
- Smaller to mid-size steel buildings
- Agricultural and light commercial projects
- Cost-conscious builds where loads are moderate
- Straightforward installations where overlapping is not required
Both options are efficient, durable, and long-lasting, but knowing whether a c purlin or a z purlin best fits your project will help you get the most cost-effective and structurally sound result.
Metal Purlin Finishes
Beyond shape and application, purlins also come with different protective finishes. Choosing the right finish can make a big difference in long-term performance. Two of the most common finishes for metal purlins are galvanized and red oxide, let’s look at how they differ:
Galvanized Purlins
Galvanized purlins are coated with a layer of zinc, which acts as a barrier against corrosion. This finish is particularly valuable in environments where moisture or humidity may be present. Steel roof purlins with a galvanized finish are ideal for long-term durability, as the protective coating ensures that the steel maintains its strength for decades with very little maintenance.
Red Oxide Purlins
Red oxide purlins are coated with a primer that gives them their distinctive reddish colour. This primer provides a degree of protection during storage and transport, and many builders choose to apply additional paint over top once the building is assembled. While not as corrosion-resistant as galvanizing, red oxide metal roof purlins are a cost-effective option for projects where budget is a driving factor.
Spacing, Spans and Laps
Once you know which type of purlin and finish are right for your project, it’s also important to understand how they are measured and installed. There are three terms you should familiarize yourself with: purlin spacing, purlin span, and purlin laps. Let’s define and examine each of these below:
- Purlin spacing: this refers to the amount of space between each purlin as you move down the side of the roof. Purlin sections assume the slope of the roof they are supporting, but the number of purlins and space between each one will be determined by the size, weight, and other factors of your specific building.
- Purlin span: the span is the distance between the centre of the bolts on each end of each individual purlin. Importantly, the span does not refer to the length from one end of the roof to the other, but the length between ends of a single purlin. Almost certainly, the width of your roof will require multiple purlins (and multiple spans) to fully support, which is where laps come in.
- Purlin laps: this refers to the overlap between individual purlins. When a structure is very wide, it may be necessary for multiple purlins to overlap one another. The general recommendation for purlin laps is that they should make up 15% of the span. At the midway point between each lap (i.e., 7.5% of the span) the two purlins should be bolted to the primary structure.
Understanding these three terms can help you to better understand how purlins should be properly installed on the roof of your steel building.
Handling and Storage
Before your building arrives it’s important to consider where on-site your purlins will be stored. Purlins should be stored in a dry place to prevent water from sitting on them. Water getting in between closely stacked purlins could potentially lead to premature corrosion and delay building schedules.
Once you are ready to start attaching your purlins it’s crucial to remember not to cut or change the purlins during the assembly of your structure. Purlins are made specifically with the structure and potential load bearing of the structure in mind. Cutting or changing the purlins could compromise the structural integrity of them and decrease the functionality of the secondary framing system.
Purlins are installed horizontally under metal roofs and are installed much like metal roofing. Purlins should be bolted on as welding is not recommended in the assembly of Purlins. Welding can affect the cold-formed steel, add stress connection, and damage the protective coating on the steel leading to premature corrosion and the diminished durability of the steel.
5 Strengths of Steel Purlin Roofing
By now you should have a clear picture of what purlins are and why they matter. Let’s finish by looking at the top five reasons steel purlins are such a smart choice for your next project:
1. Versatile for Different Structures
Steel purlins can be used in almost any building design. In pre-engineered kits, they can be pre-cut and pre-drilled, arriving at your site ready to install — a big advantage for both professionals and DIY builders.
2. Reliable Roofing Support
Purlins quite literally keep the roof over your head. They provide the strength a roof needs to protect against weather, impacts, and moisture — ensuring a building that is safe, insulated, and built to last.
3. Fire-Resistant
Unlike wood, steel is non-combustible. In the event of a fire, steel purlins will not burn, and instead help slow the spread of flames — an important safety factor in any building.
4. Lightweight but Strong
Cold-formed steel makes purlins both thin and light, without sacrificing strength. Their efficient design means less material is needed, which translates into easier handling during assembly and cost savings for the project.
5. Long-Lasting
Steel is known for its exceptional lifespan, often exceeding 50 years with minimal upkeep. Because purlins are not directly exposed to the elements, they require very little maintenance while continuing to perform for decades.
Keep the Roof Over Your Head!
Well, there you have it – you’re a purlin expert! By now you should understand what purlins are, why they’re crucial to the structural integrity of a building, and some of the most common forms that purlins take.
If you’re considering a new building construction, you’ll want a construction partner in your corner. Norsteel Buildings has been providing quality metal steel buildings to our clients for nearly thirty years. When you partner with us, you are assigned a steel building expert who acts as your personal building consultant and resource.
If you have any questions about a steel building project, don’t hesitate to reach out to us, give us a call at 1-866-822-4022!
FAQs
What is a steel purlin and what is its primary function?
A steel purlin is a horizontal structural member that supports roof and wall panels in a pre-engineered steel building. Its primary function is to transfer loads from the panels to the main structural frame, ensuring stability and strength throughout the system.
What materials are commonly used for purlins?
The most common material for purlins is cold-formed steel due to its high strength-to-weight ratio, durability, and cost-effectiveness. While wood and concrete have historically been used in traditional construction, steel remains the preferred option for modern pre-engineered metal buildings.
By meeting your specific area’s building codes, we not only adhere to their rules, but ensure the safety and integrity of your building, as well as the people and property inside it.
What are the specifications of purlins?
Purlin specifications typically include shape (C-shaped or Z-shaped), depth, thickness (gauge), length, and coating type (such as galvanized or prime-painted). These specifications are determined by engineering requirements like building span, load conditions, and local codes.
How far can steel purlins span?
Steel purlins can generally span 10 to 25 feet, depending on their size, shape, and the load requirements of the building. Z-purlins often span further than C-purlins due to their ability to overlap and create continuous lengths.
How far should a z purlin overlap?
When overlapping Z-purlins, the typical overlap is 6 to 12 inches, though exact requirements may vary based on design loads and building codes. Overlapping ensures continuity and helps the purlins act as a single, stronger structural member.
How long do purlins last?
Steel purlins can last 40 years or more when properly coated and maintained. Galvanized and prime-painted finishes provide resistance to corrosion, significantly extending their service life compared to untreated materials.
What are the disadvantages of steel purlins?
Steel purlins require careful engineering and professional installation to avoid issues like sagging or corrosion in harsh environments. They can also conduct heat and cold, meaning insulation is important in temperature-sensitive applications. However, when properly specified, their advantages outweigh these considerations.
How do I choose the right size of purlin?
Choosing the right purlin size depends on the span, load requirements, and building design. Engineers typically calculate the required depth and gauge based on structural codes and usage. It is always recommended to work with an experienced steel building provider to ensure the correct specification.
Can you walk on a metal roof with purlins?
It is not recommended to walk directly on purlins, as they are designed to support panels and loads, not concentrated foot traffic. Always step on the areas directly supported by purlins (i.e., the roofing panels fastened to them), and use safety equipment to protect both workers and the integrity of the roof system.
