If you have ever looked at a steel building or metal roof and wondered what supports the roofing sheets underneath, the answer is usually a purlin.
Purlins are an important part of modern construction, but many people are unsure what they do, where they are used, or which type works best for a project.
In this guide, you will learn what a purlin is in construction, the different types of purlins, common materials, installation basics, and where they are most commonly used. We will also explain how to choose the right purlin for better roof strength and long-term performance.
Our construction research and practical building knowledge help us create simple, reliable guides homeowners and builders can trust.
Why Purlins Are Important
Purlins do more than just hold up a roof. They play a key role in keeping a building safe and stable over time.
They carry the weight of the roof covering and pass that load to the main frames or rafters below. Without them, the roof sheets would have nothing to rest on.
A good purlin system also spreads weight evenly across the structure. This prevents any single point from carrying too much stress, which could lead to damage or failure.
On top of that, purlins add overall rigidity to a building. They help the frame resist wind pressure, rain loads, and other forces that act on the roof and walls every day.
Common Types of Purlins
There are two main types of steel purlins used in construction today: C purlins and Z purlins.
Each has its own shape and best use case.
C Purlins
C purlins are shaped like the letter "C" when viewed from the end. They have a flat web in the middle with flanges on the top and bottom.
Features:
- Simple, clean shape
- Easy to cut and fix on site
- Available in various sizes and thicknesses
Common Uses: C purlins work well as wall rails, floor joists, and roof purlins in smaller or mid-size buildings. They are a popular choice for portal frame buildings and steel sheds.
Z Purlins
Z purlins are shaped like the letter "Z." One flange points one way while the other points the opposite direction.
Features:
- Can be lapped over each other at supports
- Handles longer spans well
- Offers good load capacity for its weight
Common Uses: Z purlins are often used in larger roof systems where spans between frames are greater. The overlapping feature makes them a smart choice for continuous roof runs in commercial and industrial buildings.
Common Uses of Purlins in Construction
Purlins show up in many types of buildings across different sectors.
Roofing Systems
This is the most common use. Purlins are fixed across the top of roof frames to support metal or other roofing sheets. They keep the roof surface flat and properly supported.
Wall Framing
Purlins also serve as horizontal rails in wall panels. They hold the wall cladding in place and transfer wind loads back into the main structure.
Warehouses and Steel Buildings
Large industrial buildings depend heavily on purlins. In warehouses, purlins run across wide spans to carry heavy roof loads, including insulation, HVAC units, and snow loads in some regions.
Residential and Commercial Structures
Purlins are used in home extensions, garages, and commercial buildings. They help support lightweight metal roofing and are often used with timber rafters in residential builds.
Benefits of Using Steel Purlins
Steel purlins have become the go-to option in modern construction for several solid reasons.
Lightweight Design
Despite being strong, steel purlins are relatively light. This reduces the overall load on the building frame and makes handling on site easier.
Easy Installation
Steel purlins come pre-cut and ready to fix. They slot into place quickly, which saves time during the build and reduces labor costs.
Durability and Corrosion Resistance
Most steel purlins are galvanized or coated to resist rust. This makes them suitable for buildings in humid or coastal areas where moisture is a concern.
Cost Savings
Because steel purlins are quick to install and long-lasting, they help keep both build costs and maintenance costs low over the life of the structure.
How Purlins Are Installed
Purlins are installed horizontally across roof rafters or steel frames to support the roofing panels above. They are usually spaced evenly based on the roof load, span length, and type of roofing material being used.
Steel purlins are commonly attached using bolts or screws, while wooden purlins may be nailed or fastened directly to rafters.
Proper alignment and spacing are important because uneven installation can weaken the roof structure and create stress points over time.
Tips for Choosing the Right Purlin
- Choose Z purlins for large buildings and long roof spans, while C purlins are better for smaller structures.
- Calculate roof load requirements carefully, including wind, snow, equipment, and roofing material weight.
- Select durable materials like galvanized steel for better strength, corrosion resistance, and long-term performance.
- Follow proper purlin spacing guidelines to improve roof stability and prevent structural stress issues.
- Check local building codes and project specifications before finalizing the purlin size, type, and installation method.
Conclusion
I have worked around enough construction projects to know that small components often make the biggest difference. Purlins are one of those. They are not flashy, but without them, the roof would not stay up for long.
If you are planning a steel building or roof system, take time to choose the right purlin type and size. It is a small decision that has a big impact on safety and cost.
Have questions or a project coming up? Drop a comment below. I would love to help.
Frequently Asked Questions
What is the purpose of a purlin?
A purlin supports the roof or wall covering of a building. It transfers loads from the cladding to the main structural frame.
What is the difference between a beam and a purlin?
A beam is a primary structural member that carries large loads. A purlin is a secondary member that supports the roof or wall surface between main frames.
Which is better, C purlin or Z purlin?
It depends on the project. Z purlins are better for longer spans, while C purlins are simpler and more suitable for shorter spans and wall applications.
What materials are used for purlins?
Steel is the most common material. Wood and aluminum are also used depending on the building type and local requirements.
How far apart should purlins be spaced?
Spacing typically ranges from 1.2 to 2 meters. The right spacing depends on the roof load, span between frames, and the type of cladding being used.
