Cold-formed C and Z steel purlins bundled in a workshop - galvanized steel roof framing

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C Purlin vs Z Purlin:
Which Should You Specify?

Both are cold-formed from galvanised coil and both are priced by weight. The difference in section shape decides your span, your steel tonnage and your fixing detail.

26 Oct 2026Technical Guide

Every steel structure warehouse, factory or poultry house drawing review reaches the same question: should the roof purlins be C sections or Z sections? The answer is not a matter of preference — it follows directly from your span, your bay spacing and your load case. This guide sets out the practical differences and gives a simple rule for choosing.

What is a C purlin?

A C purlin (also called a Cee, or C section) has a rectangular C profile with two equal flanges. It is roll-formed from galvanised steel strip, usually 1.5 mm to 3.0 mm thick, in depths from about 80 mm to 350 mm. Because the two flanges are identical, a C section is symmetrical: it is easy to stack, easy to nest for shipping, and simple to bolt or screw through the web.

That simplicity makes it the default for wall girts, eave members, edge purlins and short-span roof purlins. Two C sections usually butt together at a support with a cleat, so each span behaves independently.

What is a Z purlin?

A Z purlin has a Z-shaped profile with unequal flanges — one flange turns up, the other turns down. That asymmetry has one purpose: it lets adjacent Z purlins nest into each other and overlap at the supports. The overlap turns a row of independent simply-supported spans into a continuous beam.

Continuity is the whole point. Over an internal support, the bending moment reverses, so the lapped section shares the load between two members. The result is that a Z purlin of the same steel weight can carry a longer span than a C purlin — and on long runs, the saving in tonnage is real.

The four practical differences

C purlin vs Z purlin at a glance
Item	C purlin	Z purlin
Section	Symmetrical C, equal flanges	Asymmetrical Z, unequal flanges
Joint at support	Butt joint with cleat	Nests and laps — continuous beam
Best span	Up to about 6 m, single span	6–12 m with lapping
Typical use	Wall girts, eaves, edge purlins, short roof spans	Long-span roof runs, multi-bay buildings
Installation	Simple — no lap detailing	Needs lap length, stitch screws and correct overlap sequence
Steel efficiency	Good on short spans	Better on long spans — less tonnage per m²

Indicative span guidance

The table below is a starting point for early-stage estimating. It assumes a typical light roof (single skin or insulated panel), Q355 steel and normal wind loading.

Indicative clear span by section (Q355, roof purlins)
Section	Depth	Thickness	Purlin spacing	Indicative clear span
C	120 mm	2.0 mm	1.2 m	4.5 – 5.5 m
C	160 mm	2.0 mm	1.2 m	5.5 – 6.5 m
C	200 mm	2.5 mm	1.5 m	6.0 – 7.0 m
Z	150 mm	2.0 mm	1.2 m	7.0 – 8.0 m (with lapping)
Z	200 mm	2.5 mm	1.5 m	9.0 – 10.5 m (with lapping)
Z	250 mm	2.5 mm	1.5 m	10.0 – 12.0 m (with lapping)

Indicative only. Final sections must be verified by a structural engineer against your actual wind, snow and seismic loads, and against deflection limits — not just bending capacity.

How to choose — a simple rule

Span under about 6 m, single span → C purlin. Lapping saves nothing here.
Span over 6 m, or three or more bays → Z purlin with lapping. This is where the steel saving appears.
Wall girts and eave members → C purlin. Fixing is simpler and there is no lap to detail.
Long continuous roof runs → Z purlin. Continuity across every internal support.
Mixed buildings → Z purlins on the roof, C purlins on walls. This is the most common arrangement we ship.

What to send with your enquiry

To get a purlin schedule and price back quickly, send us:

Building length, width and eave height
Purlin spacing (or ask us to propose one)
Roof and wall cladding type and weight per m²
Wind load, snow load or the project location so we can apply the right code
Any crane, mezzanine or ceiling loads
Quantity — either a schedule or a total tonnage

Frequently asked questions

Yes, and it is common practice. Use Z sections where the roof run is long enough for lapping to pay off, and C sections for wall girts and eave members where a simple butt connection is cheaper and faster to install.

1.2 m to 1.5 m is normal for standard roofing sheets and insulated panels. Thinner sheets, heavy snow zones or fragile cladding need closer spacing, which is one reason we ask for the cladding type before quoting.

On spans over 6 m, yes. Lapping creates continuity that raises capacity per kilogram, so a lighter Z section can replace a heavier C section. On short single spans the benefit is small and C sections are usually the better buy.

Q235 or Q355 (S235 / S355) with Z275 galvanised coating is standard for export. Coastal sites, fertilizer plants and high-humidity climates should specify a heavier coating or an additional paint system, because purlins are the first members to show corrosion.