Z & C Purlins for Industrial Roof & Wall Framing

In a lapped Z purlin system, the purlins are positioned so that adjacent sections overlap by a specified lap length at each primary frame support. At the lap, the two nested Z sections are bolted together — creating a moment splice that transfers the negative bending moment at the support into the lap zone, stiffening the system at the support and reducing the positive moment in the mid-span zone. The structural effect is that the lapped system behaves more like a continuous beam than a simply-supported span — achieving significantly greater load capacity and reduced deflection for the same section size, compared to single-span simply-supported purlins. This is the principal reason lapped Z purlin systems are the dominant secondary framing choice for multi-bay industrial and commercial buildings.

Z and C purlins are manufactured from high-tensile cold-formed steel — typically G350 (350 MPa yield strength) or G550 (550 MPa yield strength) to AS 1397 or equivalent standards. High-tensile steel enables thinner section walls to achieve the required structural performance, reducing section weight and material cost. Galvanised coating weight is typically Z275 (275 g/m² total zinc coating) as standard for most industrial and commercial applications — Z350 (350 g/m²) is available for higher corrosion exposure environments. The correct combination of steel grade and coating weight for a specific project depends on the structural design loads and the corrosion exposure category of the building environment.

A complete Z and C purlin system includes several structural accessories that are as important as the sections themselves. Cleats are formed steel brackets that connect the purlin to the primary frame flange — sized and drilled for the specified connection bolt pattern. Sleeves are short lengths of slightly larger section that fit over the purlin at lap positions to reinforce the lap joint and maintain section geometry under the bolted lap. Anti-sag rods are steel tie rods that run along the roof slope between purlins to prevent lateral displacement under asymmetric wind loads and during cladding installation. Bridging components stabilise the purlin bottom flange against lateral-torsional buckling under load. All accessories are specified as part of the purlin system design — and must be correctly installed for the system to achieve its designed structural capacity.

Purlin selection is a structural engineering calculation — not a rule-of-thumb. The variables are the purlin span (portal frame spacing), the cladding dead load, the design wind uplift and gravity load for the building's wind load zone, the required deflection limit, and the lapping configuration. Purlin manufacturers publish span tables that give the maximum allowable span for each section depth and thickness combination under standard loading conditions — the structural engineer uses these tables, adjusted for actual project loads, to select the minimum adequate section. Depths range typically from 150mm to 300mm in standard increments; thicknesses from 1.5mm to 3.5mm. We supply sections to the structural engineer's specification for any depth, thickness, and load combination.