C vs Z Purlin: Key Differences and How to Choose
C purlin and Z purlin are the two most common cold formed steel sections for roof and wall framing in steel structure buildings. Both are made on a purlin roll forming machine, but their cross-section shape changes how they lap, stack, and transfer load. This guide explains the differences and how to choose the right profile for your project.
What Is a C Purlin?
A C purlin (Cee purlin) has a vertical web with two flanges that both point the same direction, forming a C shape when viewed from the end. Because the section is open on one side, C purlins are usually connected with bolts through the web or attached with cleats. They are simple to fit, easy to brace, and are the standard choice for roof purlins and wall girts in most factory and warehouse construction.
Typical Cee profiles from ZTRFM lines range from 4 inch Cee (about 100 mm web) upward, formed from galvanized or PPGI coil in the 1.9–2.6 mm range on standard machines, and up to 8 mm on heavy duty lines.
What Is a Z Purlin?
A Z purlin (Zee purlin) has flanges that point in opposite directions, so the profile looks like a Z. The key advantage is that Z sections can be nested inside each other, which reduces shipping volume, and they can be lapped end to end at supports, which lets a continuous member carry load across two spans. For long-span roofs and higher load requirements, Z purlins are usually the economical choice.
ZTRFM builds Zee lines including the 4 inch Zee purlin machine in US style, which forms US standard Zee sections for export projects.
Key Differences: C vs Z Purlin
| Factor | C Purlin | Z Purlin |
|---|---|---|
| Section shape | Open C, flanges same direction | Flanges opposite, Z shape |
| End connection | Bolted through web or cleat | Lapped and bolted through flange |
| Span behaviour | Single span typical | Continuous over laps, longer effective spans |
| Shipping | No nesting, more space | Nestable, up to 30% less volume |
| Common use | Wall girts, short roof spans, simple frames | Long roof spans, high load, large industrial roofs |
| Solar mounting | C strut 41x21 rails | Z rails for PV ground mount |
Which One Should I Choose?
- Choose C purlin for wall girts, short spans (up to about 6–7 m), simple bolted connections and projects where the same section is used everywhere.
- Choose Z purlin for long roof spans, heavy snow or wind loads, and where lap splicing at supports lets you use a lighter section than a single-span C would need.
- For projects that use both, a combined C/Z line (automatic C & Z purlin machine) switches profiles on one machine, so one investment covers both sections.
- For heavy steel structures, the heavy duty C & Z line forms plate up to 8 mm.
How C and Z Purlins Are Made
Both profiles are cold roll formed from coil on the same machine family. The coil passes through the decoiler and leveler, is punched (optional pre-punch or servo tracking punch), then formed through 20 roller stations into the C or Z section, and cut to length with a flying shear. Length and quantity are set on the PLC; on the automatic line the HMI switches the profile configuration between C and Z.
FAQ
Q: Which is stronger, C or Z purlin?
A: For the same steel weight, a Z purlin with lapped connections carries longer spans because the lap creates continuity over the support. A C purlin is normally used as a simply supported member.
Q: Can one machine make both C and Z purlins?
A: Yes. A combined C/Z roll forming line changes the forming station inserts manually, semi-automatically, or fully automatically from the HMI. See the automatic C & Z purlin machine.
Q: What thickness of steel can purlin machines process?
A: Standard ZTRFM lines run 1.9–2.6 mm PPGI/PPGL or galvanized coil; heavy duty lines form plate up to 8 mm.
Q: Why are Z purlins cheaper to ship?
A: Z sections nest inside each other, reducing freight volume by up to about 30% compared with C sections of the same size.
Q: Do I need a separate machine for solar purlins?
A: Not necessarily. Small C strut 41x21 rails for PV mounting can be formed on a dedicated solar panel support purlin machine, or on a standard line with the right stations.
Q: How do I order a purlin line?
A: Send your profile drawings, thickness range and required line speed to steven@zt-rfm.com. ZTRFM supplies the pass schedule, line layout and quotation, with trial report and video inspection before delivery.