Key takeaways
- Purlins are cold-formed from 1.5–3.0 mm galvanised coil and sit between the rafters, carrying the cladding and the loads on it.
- C purlins suit single spans and small buildings; Z purlins lap to form continuous beams and typically halve the bending moment on long roofs.
- Sigma sections put more steel away from the neutral axis, carrying more load in the same depth envelope.
- Purlin capacity rises quickly with thickness, so the economic answer is usually the thinnest section the engineer's span table allows.
The short answer
A purlin is a horizontal cold-formed steel member fixed across the main roof structure — rafters, trusses or portal frames — to carry the roof cladding and whatever load sits on it: wind, snow, foot traffic during construction, and services. Wall purlins perform the same job on a vertical face and carry wind load instead.
Almost all purlins are cold-formed from galvanised steel coil between 1.5 mm and 3.0 mm thick. That is the same material and the same roll forming process used for roof sheets, which is why purlin machines and roofing machines sit in the same workshop.
C, Z and sigma: what the geometry does
The three sections differ in how they respond to load and how they can be connected.
A C purlin is symmetric. It behaves like a simple beam, so it is normally used as a single span from one rafter to the next. Its symmetry means you can nest C sections efficiently for transport, and its flanges are easy to bolt to.
A Z purlin is point-symmetric — rotate it 180° and it looks the same. That property lets adjacent Z sections lap over the rafter and nest together, turning a row of separate spans into a continuous beam. On a long roof, that lapping roughly halves the bending moment compared with a series of single spans, which is why Z purlins dominate long industrial roofs.
A sigma section adds stiffening folds in the web and flanges. Those folds put more material away from the neutral axis, so a sigma purlin carries more load than a C purlin of the same depth and thickness. It is the right answer when the structural zone is fixed — usually by the cladding system or available headroom — and you cannot go deeper.
| Section | How it is loaded | Typical use | Main advantage |
|---|---|---|---|
| C purlin | Single span, simple beam | Small buildings, end bays, columns, floor joists | Symmetric, easy to connect and nest |
| Z purlin | Continuous, lapped multi-span | Long industrial roofs and walls | Lapping halves bending moment versus single spans |
| Sigma (Σ) | Single or continuous | Cladding rails, restricted-depth roofs | Highest capacity for a given depth |
| Omega (Ω) | Cladding rail, face-fixed | Wall rails and long rail runs | Stiff in the direction of cladding wind load |
Thickness and span: where the money is
Purlin capacity rises faster than thickness. Going from 2.0 mm to 2.5 mm substantially increases section strength, because the extra material sits at the extremities of the section. But coil cost rises linearly with thickness, so over-specifying purlins is expensive.
The practical effect is that the cheapest compliant roof is usually the one with the thinnest purlin the engineer's span table allows, at the widest spacing the cladding tolerates. That is why purlin design is nearly always a joint decision between the structural engineer and the cladding supplier.
One caveat that catches people out: high-strength steel does not work the way most people assume. A G550 purlin is stronger per unit thickness, but it is also more brittle and less tolerant of over-forming, so the tooling has to be set up for it. Running G550 through a machine set up for G300 is a common cause of distorted purlins.
What to look for on a purlin machine
Three things separate a purlin machine that will still be producing accurately in five years from one that will not.
First, a real leveling unit. Purlin material retains a strong coil set at 3 mm, and unleveled strip becomes bow and camber that will not close on site — a failure that only shows up during erection, when it is most expensive.
Second, servo width adjustment if you intend to change between C and Z, or between sizes. A manual changeover takes 30–45 minutes per change; servo takes under 10 minutes, which is the difference between running one profile a shift and running three.
Third, an inline punch that can be programmed per run. Purlin connections need bolt holes at specific pitches, and punching them in a separate operation means handling every length twice.