Z/C Purlins

Both are cold-formed steel sections that carry the roof and wall sheets between the main frames. A C purlin is a plain channel and is normally used for wall girts, end bays and shorter runs. A Z purlin has offset flanges, so consecutive sections overlap at the rafter and the purlin line behaves as a continuous member across several bays. That continuity is why most PEB roofs use Z sections through the main bays and C sections at the eaves and end walls.

It depends on the rafter spacing, the roof sheet profile and thickness, the design wind speed at your site, and the imposed load. A building with 6 m bays and a light sheet needs a very different section from one with 8 m bays carrying services below the roof. Send us your frame drawing or the basic dimensions and our design team will confirm the section and the spacing.

Spacing is set by what the roof sheet can safely span between supports, then checked against wind uplift at the eaves and corners where suction is highest. Spacing is usually tightened in those zones rather than kept uniform across the whole roof. Anything hung from the purlins, such as lighting, cable trays or a false ceiling, has to be allowed for at the design stage.

Purlins bolt to cleats fixed on the top flange of the rafter. Where the span needs lateral restraint, sag rods or tie members are added between purlin lines to control deflection and stop the sections twisting during erection. On a Z purlin roof the lapped joint over the rafter is what gives the run its continuity, so lap length and bolting are part of the design and not a site decision.

Sections are rolled to suit the design, so depth and thickness are selected from the span, the spacing and the loading rather than picked from a fixed list. Tell us your bay size, sheet type and site location and we will confirm the section along with the coating suited to your environment.

Yes. Purlins are regularly supplied as a standalone item for a roof replacement, an extension to an existing shed, or a structure being erected by another contractor. Send the quantity, section and length required and we will quote on a supply basis.

The coating specification matters more than the section itself. Sites near the coast, or inside plants where the air carries chemical fumes, need a heavier protective coating and correctly matched fasteners, since ordinary fasteners will corrode long before the purlin does. Tell us where the building is and what is manufactured inside, and the specification is selected accordingly.