Light-gauge cold-formed steel roof purlins replace heavier hot-rolled steel angle and channel sections.

A purlin is a secondary structural member that spans between the primary/main steel frame to support the roof cladding. The purlin transfers load from the roof cladding to the primary steel rafters. Similarly, a side rail is a secondary structural member that spans between the primary/main steel frame to support the wall cladding and transfer load from the wall cladding to the primary steel columns. Both purlins and side rails are used to provide restraint to the primary steel frame members. Light gauge cold-formed steel roof purlins are used on a wide range of building types including retail and leisure, industrial, warehouses and distribution, healthcare and education.

Cold-formed steel roof purlins and side rails are manufactured from pre-galvanised steel strip, in the form of coils, which have been slit to the desired width from the parent or “wide coil” supplied from the mills.
The purlin is formed gradually by feeding the strip continuously through successive pairs of rolls, which act as male and female dies. Each pair of rolls progressively forms the strip until the finished profile is produced. For example, five to eight “stages” or “passes” will be needed to form a lipped Z purlin, depending on the thickness of the material. More complicated profiles will require 15 or more passes.
The strip is pre-pierced and cut to length in-line, prior to roll-forming.
The principal advantage of cold roll-forming, compared with other methods of fabrication, is the high production capacity achieved. Sections produced by cold roll-forming are essentially uniform in cross section and can be manufactured to very close dimensional tolerances.
In building uses, the most common and simplest of profiles are the lipped Z and C sections – both defined by a central vertical web joined to horizontal flanges at either end. Additional lips are included because unstiffened, wide, thin plates are unable to resist significant compression and the flanges become inefficient.
Z sections are usually installed with the top flange pointing up the roof slope so that the load is applied through the shear centre and serious twisting of the sections does not occur.
Cold-rolled steel purlins are manufactured from hot-dipped galvanised steel, generally with a yield strength of 450N/mm².

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