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    C/Z Purlins

    Between the main steel frame and the roof sheets sits a layer of framing that carries the whole envelope: C and Z purlins. These cold-formed steel sections are light, strong, and precisely shaped to span between rafters, providing the cladding with a fixing point and transferring wind and roof loads back into the building’s primary structure.

    What C/Z Purlins Are

    C and Z purlins are cold-formed steel sections that serve as the secondary framing of a steel building. They sit between the primary frame, the rafters and columns, and the outer skin of roof and wall cladding, doing the essential job of supporting the sheeting and carrying its loads back to the main structure. Rather than being hot-rolled like heavy beams, purlins are roll-formed from thin steel coil into a precise C or Z cross-section, then cut, punched, and marked ready for bolting on site. This cold forming makes them remarkably efficient: a thin, shaped section carries far more load per kilogram than its weight would suggest. On the roof, these members are called purlins; the same sections used on the walls to support cladding are called girts. Together they form the skeleton the building envelope is fixed to.

    The Difference Between C and Z Sections

    The two profiles look similar but behave differently, and each has its place. A C section, or channel, is shaped like the letter C, with both flanges facing the same direction. It is simple, symmetrical about one axis, and well suited to shorter spans, end bays, and wall girts. A Z section is point-symmetric, shaped like the letter Z, with its flanges pointing in opposite directions. That geometry gives the Z one decisive advantage: because the flanges are offset, two Z purlins can overlap each other at a support, nesting together to form a continuous member across several bays. C sections cannot lap this way. The Z also nests neatly for transport, bundling flat for efficient delivery. Choosing between them comes down to span, continuity, and connection detail, which is why a single building often uses both profiles in different locations.

    Why Z Lapping Makes Roofs Stronger

    The ability to lap Z purlins is one of the most useful tricks in steel building design. In a multi-bay roof, purlins could simply run as separate simply-supported lengths between each pair of rafters, but that wastes their potential. By overlapping two Z sections over the support and bolting them together, the purlins become structurally continuous across the whole run. This matters because a continuous beam carries loads far more efficiently than a series of separate spans, reducing the maximum bending moment the section must resist. Just as importantly, the overlap creates a double thickness of steel exactly where the bending forces peak, over the support, so the strongest part of the assembly sits where it is needed most. The result is a lighter, more economical roof that uses less steel for the same strength, which is a core saving in efficient PEB design.

    Where C/Z Purlins Fit in the Building

    Purlins and girts are the connective tissue of a steel building, and they appear wherever cladding meets frame. On the roof, purlins run horizontally across the rafters or portal frames, spaced to suit the span capacity of the roof sheeting above them. On the walls, girts run between the columns to support wall cladding in the same way. Because they are thin cold-formed steel, screws pass easily through the cladding into them, giving the sheets a firm fixing across the whole surface. They are typically supplied with punched holes and slots in standard patterns, so bolted connections and bracing can be assembled quickly on site. Depths generally range from 100 mm to 350 mm, and thicknesses from 1 mm to 3 mm, selected by the engineer to match the span and loads. Finished with a galvanised coating or red-oxide primer, they resist corrosion throughout the life of the building.

    Why C/Z Purlins Matter in Steel Building Design

    It is easy to focus on the big steel: the columns and rafters that give a building its shape. But those primary members would carry nothing to the roof without the secondary framing that spans between them. C and Z purlins are part of the framing, and though each section is light, collectively they determine how efficiently the whole envelope performs. Their value shows up in cost, speed, and structural logic.

    Efficiency Through Cold Forming

    The great advantage of a cold-formed purlin is the strength it delivers relative to its weight. By roll-forming thin steel coil into a shaped C or Z section, the material is placed exactly where it resists bending best, so a light member spans a surprising distance. This weight efficiency ripples through the whole building: lighter purlins mean less steel to buy, transport, and lift, and they impose a lower load on the primary frame beneath them, which can then be sized smaller. For a large roof carrying hundreds of purlins, that compounding saving is significant, and it is a central reason cold-formed secondary framing dominates modern steel building.

    Continuity That Saves Steel

    The lapping of Z sections turns a set of separate spans into a single continuous beam, and that changes the structural mathematics in the designer’s favour. A continuous purlin carries load more efficiently than a simple span, lowering the peak bending forces, while the doubled steel at the laps reinforces the exact point where those forces concentrate. This allows the engineer to specify a lighter section than a non-continuous system would require. It is a neat piece of structural economy, achieved with nothing more than an overlap and a few bolts, and it directly reduces the steel tonnage in the roof.

    Fast, Bolted Assembly

    Purlins arrive pre-punched with holes and slots in defined patterns, so erecting the secondary framing is largely a matter of lifting sections into place and bolting them to cleats on the primary frame. There is no site welding, no drilling, and no guesswork. This speed matters on a build programme, because the roof and wall cladding cannot go on until the purlins and girts are up. Getting the secondary steel erected quickly and accurately keeps the whole envelope on schedule and gives the cladding crews a clean, ready surface to fix to.

    The Foundation for the Cladding

    Ultimately, every roof sheet and wall panel on the building is fixed to a purlin or girt. The spacing, depth, and strength of these members determine how the cladding performs against wind uplift, how far the sheets can span, and how firmly the envelope is anchored to the structure. Get the secondary framing right, and the roof stays tight and secure for decades; get it wrong and no quality of sheeting can compensate. That is why C and Z purlins, quiet and unglamorous as they are, sit at the heart of a sound steel building.
     
    Together, weight efficiency, structural continuity, fast assembly, and their role as the cladding’s foundation make C and Z purlins essential to economical, durable steel construction. They are the light framing that lets a heavy building work.

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    FAQs

    1. What is the difference between a purlin and a girt?
    They are the same type of cold-formed section used in different places. On the roof, the members supporting the roof sheeting are called purlins. The same C or Z sections used on the walls to support wall cladding are called girts.
    Z sections can be lapped over supports because their flanges point in opposite directions, letting two purlins overlap into a continuous, stronger member across multiple bays. C sections cannot lap this way and are suited to shorter spans, end bays, and wall girts.
    Lapping makes the purlins structurally continuous across several bays, which carries load more efficiently and lowers peak bending forces. The overlap also doubles the steel thickness over the support, exactly where the bending stress is highest, allowing a lighter section overall.
    Depths typically range from about 100 mm to 350 mm, with steel thicknesses from roughly 1 mm to 3 mm. The engineer selects the depth and thickness to match the span between supports and the wind and roof loads the member must carry.
    Yes. They are usually supplied as galvanised steel or coated with a rust-inhibiting red-oxide primer, so they resist corrosion throughout the life of the building even though they sit hidden beneath the cladding.

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