Purlin sizing is one of those decisions that looks straightforward until a roof panel lifts in a storm or a rafter line deflects mid-span. Span is the starting point, not the finish line. Wind classification, purlin spacing, sheeting profile, lap arrangement and bridging all feed into whether a section performs as specified. This guide covers the practical differences between C and Z sections, how lapping changes structural behaviour, and why bridging is not optional.
C Sections vs Z Sections: What the Shape Means Structurally
Both C and Z purlins are cold-formed steel sections, typically produced from Zincalume or galvanised coil to AS 1397. The difference in shape is not cosmetic. It determines how sections can be connected and how load is transferred across supports.
A C section (also called a channel) has flanges that project from the same side of the web. It is symmetrical about its horizontal axis. When two C sections are placed back-to-back over a support, they sit flush and bolt together cleanly, but each span is essentially independent. Load does not transfer continuously from one span to the next through the section geometry.
A Z section has flanges that project from opposite sides of the web, giving it a Z profile when viewed from the end. This geometry means that when two Z purlins overlap at a common support, one nests partially over the other. The overlapping length creates a double-thickness zone that allows bending moment to be carried continuously across the support, rather than treating each bay as a simply supported beam.
That continuity matters. A simply supported beam carries maximum bending moment at mid-span. A continuous beam redistributes moment, reducing the peak at mid-span and picking up negative moment over the support. For the same span and load, a continuous Z purlin arrangement typically allows a lighter section than a simply supported C arrangement, or carries more load with the same section depth.
Where Each Section Type Is Typically Used
C sections are common in single-bay sheds, end bays, and situations where the purlin terminates at a wall or gable. They are also used where back-to-back pairs are specified to carry concentrated loads, or where the detailing is simpler and the spans are short enough that continuity is not needed to meet load requirements.
Z sections dominate multi-bay shed construction precisely because lapping is straightforward. The nested overlap at each rafter or portal frame allows the purlin run to behave as a continuous member, and the lap length can be adjusted to tune the moment distribution. Most shed kit manufacturers specify Z sections for intermediate bays as standard.
Some fabricators use C sections throughout and achieve continuity by sleeving or by bolting back-to-back pairs at supports. This works, but it adds material and labour. Z lapping is generally more efficient where spans and bay counts justify it.
Lap Length and Why It Is Not Arbitrary
Lap length is the distance each Z purlin extends past the centreline of the support before the next purlin begins. Manufacturer load tables specify minimum lap lengths, and these figures are derived from the moment distribution assumptions built into the table.
A common starting point for lap length is 10 percent of the span on each side of the support, giving a total overlap of approximately 20 percent of span. For a 6-metre bay, that means each purlin laps roughly 600 mm past the support centreline. However, this is a general guide, not a universal rule. Manufacturer tables vary, and some specify lap lengths by section size rather than as a percentage of span.
Shortening the lap reduces the effective continuity. If the lap is too short, the double-thickness zone does not extend far enough to carry the negative moment that the load table assumes, and the section is effectively weaker than the table suggests. Extending the lap beyond the specified minimum does not proportionally increase capacity and adds unnecessary material.
The bolting pattern within the lap also matters. Most manufacturers specify two bolts minimum through the overlapping flanges, positioned within the lap zone. The bolt size and spacing are part of the connection design, not an afterthought.
Span, Spacing and Wind Classification
Purlin selection starts with four inputs: span (centre-to-centre of supports), spacing (centre-to-centre of purlin lines), wind classification to AS 4055 or AS/NZS 1170.2, and the dead load from the sheeting and any insulation.
Wind classification in Australia ranges from N1 through N6 for non-cyclonic regions, and C1 through C4 for cyclonic regions. A shed in a coastal Queensland location classified C2 faces substantially higher wind pressures than the same shed footprint in an inland N2 location. The purlin that works for one will not necessarily work for the other, even if the span and spacing are identical.
Purlin spacing is a variable that shed builders sometimes adjust on site to suit sheet lengths or frame positions. Changing spacing changes the tributary area each purlin carries, which changes both the downward load and the uplift load. Wider spacing means more load per purlin. If spacing is increased beyond what the load table was used to select the section, the section may be undersized.
Most manufacturer load tables present allowable spans in a grid format: section size on one axis, purlin spacing on the other, with separate tables for different wind classifications and load cases. Reading the wrong table, or interpolating between wind classifications, produces incorrect results.
Uplift: Why Span Alone Is Not Enough
A purlin sized to carry gravity loads (dead load plus live load from roof access or maintenance) can still fail under wind uplift. Uplift reverses the bending direction. The bottom flange, which is in tension under gravity load, goes into compression under uplift, and the top flange, normally in compression, goes into tension.
This reversal matters because the compression flange under uplift is typically unrestrained. Under gravity loading, the sheeting bears down on the top flange and provides lateral restraint to that flange. Under uplift, the bottom flange is in compression and the sheeting is pulling away from it, not restraining it. The bottom flange can buckle laterally unless bridging is present.
Uplift pressures on shed roofs are not small. On a low-pitch roof in a high wind classification, net uplift can exceed the gravity load by a significant margin. AS/NZS 4600 governs the design of cold-formed steel members including purlins, and uplift load cases must be checked separately from gravity load cases. Manufacturer load tables for purlins typically include uplift capacity figures alongside gravity capacity figures. Both columns need to be satisfied.
Bridging and Lateral-Torsional Buckling
Lateral-torsional buckling (LTB) is the failure mode where a beam deflects sideways and twists before reaching its full bending capacity. For purlins, it occurs when the compression flange is not adequately restrained against lateral movement.
Bridging is the system of rods, angles or tubes that run perpendicular to the purlin span, connecting adjacent purlin lines and preventing them from moving laterally relative to each other. Bridging transfers lateral force from the compression flange of one purlin to the tension flange of the adjacent purlin, and ultimately to a point of fixed restraint such as a rafter or a braced bay.
The number and position of bridging lines depends on the purlin depth, the span, and the load case. A shallow section over a short span may need only one bridging line at mid-span. A deeper section over a longer span may need two or more lines. Manufacturer load tables specify bridging requirements as part of the span table data. A span that is listed as achievable with one bridging line is not achievable at the same load without that bridging line.
Bridging must be anchored. A row of bridging rods that are not fixed at each end to a rigid point simply allows the whole purlin group to drift together. Anchor bridging is typically bolted to a rafter flange or connected to a dedicated anchor cleat. The position of anchor points is part of the purlin system design.
The Role of Sheeting as Restraint
Profiled steel sheeting, whether Trimdek, Custom Orb, Klip-Lok or another profile, provides lateral restraint to the top flange of a purlin when it is fixed correctly. This restraint is real and is accounted for in manufacturer load tables, which typically assume the sheeting is fixed at every purlin line with the specified fastener pattern.
If sheeting is fixed at alternate purlins only, or if the fastener pattern is reduced, the restraint assumption breaks down and the table capacity no longer applies. Similarly, insulation blanket installed between sheeting and purlin can reduce the effectiveness of the restraint if it prevents the sheeting from bearing directly on the flange.
Under uplift, as noted above, the sheeting does not restrain the bottom flange. This is why bridging requirements under uplift are often more demanding than under gravity loading, and why some manufacturer tables list different bridging requirements for the two load cases.
Final Sizing Is an Engineering Decision
Manufacturer load tables are the governing reference for purlin selection, not rules of thumb or experience from previous jobs in different wind zones. Tables from Lysaght, Stramit and other cold-formed steel manufacturers are produced in accordance with AS/NZS 4600 and include the assumptions about continuity, lap length, bridging and restraint that must be met for the tabulated capacity to apply.
For sheds outside standard configurations, for unusual spans or spacings, for high wind classifications, or for structures requiring engineering certification, a structural engineer must be engaged. The engineer will either use the manufacturer tables within their documented scope or carry out a first-principles design to AS/NZS 4600. Either way, the purlin schedule that comes out of that process is the one to build to.
CSA supplies Zincalume and galvanised steel sections, roofing profiles from BlueScope, Lysaght and Stramit, and the fasteners to connect them. If you are working through a purlin schedule and need to confirm section availability or get a quote on a full roof package, visit constructionsupplies.group/au or contact the trade desk directly.