Steel Purlins: What You Should Know

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Steel Purlins: What You Should Know

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. In other words, they support the weight of the metal sheeting, plyboard, wooden panels, and other materials that comprise the surface of a roof, and anything else that might be built up on top of a roof (e.g., snow, water, building accessories, etc.).

While it can vary from structure to structure, purlins typically help spread the load of a roof to other structural elements like building walls, rafters, or metal support beams. This helps to diffuse the weight of a building’s roof across the entire surface and to multiple components of the building, rather than having the majority of the roof’s load be concentrated on just the walls, just the rafters, etc. Purlins are essentially necessary components for a quality building; regardless of whether your structure is made from wood, steel, or some other material, purlins are a critical part of any structure’s roof. How many purlins a roof has, and the thickness or depth varies depending on the loadings for that building.

Purlins take on a number of forms and are typically classified according to their shape, the material they are made of, and their intended use.

We’ll discuss the different purlin shapes in a moment, but when it comes to material there’s no question that steel purlins are the preferred choice by the construction industry. Unlike wood, steel purlins will not rot, they are not affected by water and moisture, and their expansion and contraction in different climates is minimal. These are all weaknesses of wooden purlins, and ones which can seriously hinder the structural integrity of the roof and, indeed, of the structure itself.

Steel purlins are typically made from cold-formed steel. As opposed to working with hot metal, cold-formed steel is made by rolling and pressing thin sheets of steel into the desired shape. Cold-formed steel is stronger than hot-formed steel and easier to work with. After the forming of the steel it is then dipped in molten zinc in a process known as galvanizing. Galvanizing coats the steel in zinc which protects the steel from harsh environments and corrosion. Galvanized steel can often be identified by its ‘spangle’, a crystalized pattern on its surface.

Purlins make up the secondary support system in the framing along with their horizontal counter parts, girts. Girts and purlins run in between the primary framing much like cross beams in a wooden structure. The purpose of the secondary structural supports is to distribute loads onto the primary framing and foundation. The secondary framing is also used to attach wall cladding and the roof.

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).
Building schematic showing the placement of rafters vs purlins

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.

Spacing, Spans and Laps

Before installing purlins, 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:

Diagram showing the location of purlin spans, spacing, and slope

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.

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.

Profile of a C-Shaped Purlin

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.
Profile of a Z-Shaped Purlin

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.

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 as to prevent water from sitting on in 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 its 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 deminished durability of the steel.

5 Strengths of Steel Purlin Roofing

By now you should have a clear understanding of what purlins are, and why they’re so integral to a well-designed and built structure. Before you go though, let’s quickly examine the top 5 reasons steel purlins are a no-brainer for your next building project:

Ideal for Different Structures

You already know that purlins can be used for all types of structures, meaning they are appropriate for almost any building project. For example, pre-engineered buildings are incredibly popular options for both professionals and amateur do-it-yourself builders. Steel purlins can easily be pre-cut, and pre-drilled before they arrive at your build site, making them an ideal choice for customers looking to buy a prefab steel building kit. Steel purlins are easy to prepare ahead of time, so your steel building kit will come with purlins ready to be installed!

Provide Roofing Support

One more time – purlins literally help keep the roof over your head. Every building needs a roof, and purlins are an integral component of a quality roofing build. A quality roof keeps the building safe from weather conditions, impacts, improves insulation and prevents moisture from entering the building. A high-quality structure requires proper support and steel purlins provide support that lasts for decades.


Unlike wooden structural components, steel components – including purlins – are not combustible. Steel does not ignite or burn, which gives it a massive leg-up in terms of safety over similar wooden components. God forbid, if there were ever an emergency concerning a fire in or around the building, steel purlins (along with other steel building components) would not catch fire and would instead help contain or slow down the blaze.


Steel purlins’ light weight makes them incredibly easy to handle and assemble. The innovative design of purlins (particularly in their Z-shaped form) means that the purlins themselves can be very thin and lightweight, without sacrificing on the strength of the purlin support itself. In addition, because these components are thin, it means less steel material is required to form them, which translates to cost savings for the end customer.


As mentioned earlier, steel purlins can last for decades and decades. As a construction material, steel has an incredibly long lifespan, easily lasting upwards of 50 years with occasional maintenance. Steel purlins are no different, and because they aren’t directly exposed to the elements, very little maintenance of the purlins themselves is required.

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 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 using the form here, or give us a call at 1-866-971-7575!

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One of the most popular materials used in construction today, steel plays a vitalrole in controlling construction costs while helping the industry achieve itssustainability goals. Steel offers a number of unique benefits and significantadvantages over many other construction materials!

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