Z & C Purlins — Cold-Formed Structural Sections

Z & C Purlins — Cold-Formed Structural Sections for Roof and Wall Framing

Z and C purlins are the structural backbone of every pre-engineered and portal-frame building. These cold-formed, high-tensile steel sections transfer roof and wall cladding loads to the primary structural frame, delivering a strength-to-weight ratio that hot-rolled angles and channels cannot match at an equivalent cost per linear metre.

When a building envelope demands precise detailing, predictable deflection behaviour, and a finish that survives thirty years of monsoon, industrial atmosphere, and thermal cycling — cold-formed purlins in S350 or S450 grade are the engineered answer.

Z Purlin — Continuous-Span Roof Secondary Steelwork

The Z-section's asymmetric profile is its defining advantage. When adjacent bay Z purlins are lapped over a rafter, they nest into each other, creating a doubled cross-section precisely where bending moments peak at the support. This lap-joint continuity reduces the required section depth and material weight compared to a simply supported design.

Where Z Purlins Are Specified

• Roof secondary steelwork in industrial warehouses, logistics centres, and manufacturing plants
• Continuous-span designs where moment continuity across the rafter line reduces section depth
• Large-bay structures with rafter spacings of 6 m to 12 m
• Long-run roof systems where sheeting laps align with purlin laps for weather-tightness

Z Purlin Lap Design

The lap length is calculated as a proportion of the bay span — typically 10–15 % of span on each side of the rafter. A 7.5 m bay requires a 750 mm to 1 125 mm lap on each side. Pre-punched holes in the web at standard lap lengths allow bolted connections without site drilling, saving two to four minutes per purlin joint on a large roof.

C Purlin — Single-Span and Wall Girt Applications

The symmetric C-section does not nest, eliminating the lap-joint advantage. This is not a limitation — it is a design feature. In locations where lapping is geometrically or structurally inappropriate, the C purlin's symmetry becomes an advantage: it sits flush, fixes cleanly, and carries load without the torsional eccentricity introduced by lapping.

Where C Purlins Are Specified

• End-bay secondary steelwork where only a single purlin spans to the end wall
• Wall girts carrying vertical cladding on all four faces of a building
• Eave purlins, ridge purlins, and verge (barge) members where a single clean profile is required
• Shorter bay spacings up to 7.5 m under lighter-duty loading
• Situations where the roof pitch prevents Z purlin nesting from working correctly

Specification Table

Coating Options and Corrosion Performance

Hot-Dip Galvanized — Z275 Coating

The zinc coating is applied before the coil is roll-formed into a purlin. This means the galvanized layer is continuous across every formed face. At Z275 (275 g/m²), this coating provides a minimum zinc thickness of approximately 19 µm per face — sufficient for a 25-to-40-year service life in rural and semi-industrial atmospheric conditions (ISO corrosivity category C3).

For coastal or heavily industrial environments (C4/C5), specify Z350 coating or add a factory-applied organic top coat. Contact our technical team for corrosivity category mapping of your project location.

Pre-Painted — Polyester or PVDF Finish

Pre-painted purlins are formed from a pre-coated coil. The coating system — primer + topcoat applied by coil-coating before roll-forming — is uniform and factory-controlled. Pre-painted sections are suitable where the internal roof space is visible from within the building and a finished appearance is required without a separate painting contract.

Purlin Schedule — How We Size Your Project

Provide the following information with your inquiry and our structural team prepares a purlin schedule within 24 hours:

1. Bay spacing (distance between primary frames in metres)
2. Roof pitch (degrees or rise:run)
3. Roof dead load (self-weight of sheeting + insulation in kN/m²)
4. Design wind speed or wind pressure (from IS 875 Part 3 or your structural engineer's note)
5. Eave height and building length
6. Site location (for corrosivity category assessment)

The purlin schedule specifies section size, thickness, lap length, bolt size, and pre-punched hole layout — everything the erector needs without ambiguity on site.

Frequently Asked Questions

What is the difference between a Z purlin and a Z girt? The section shape is identical — the terminology changes with orientation. When the same cold-formed Z-section is used horizontally on a wall to support cladding, it is called a girt. When used on a roof to support sheeting, it is called a purlin.

Can Z and C purlins be mixed in the same building? Yes. The standard practice is to use C purlins in end bays (where lapping is not possible) and Z purlins in internal bays (where lapping provides continuity). The section depth and thickness should be matched so that the deflection profile is consistent across all bays.

What self-drilling screw size fixes sheeting to a 2.0 mm purlin? A 5.5 × 32 mm or 5.5 × 38 mm hex head self-drilling screw (point Type 2 or Type 3) drills through the sheeting and into the purlin flange in a single operation. See our Self Drilling Screws page for the full fixing schedule.

Are mill test certificates provided? Yes. Material Test Certificates confirming steel grade (S350/S450), coating weight (Z275), and dimensional conformance to relevant standards are supplied with every delivery.