Cold-Formed Steel Purlin Types for Solar PV: Zed, Sigma, and Eaves Beam Capacity Explained

Cold-formed steel purlins are the secondary structural members that span between the main frames of a steel portal frame building, supporting the roof cladding and transmitting loads to the primary structure. They are the structural elements most directly affected by the installation of a rooftop solar PV array, the array dead load and wind uplift forces act on the cladding, which transfers them into the purlins, which transfer them into the primary frame.

Understanding cold-formed purlin types is directly relevant to desktop structural assessment for solar PV because the purlin section is the primary structural variable in the dead load and wind uplift calculations. The section modulus, cross-sectional area, and torsional properties of the purlin determine whether it can carry the proposed loading within its design capacity. Getting the purlin type right is what makes the structural assessment reliable.

Z-Section Purlins, Geometry, Properties, and Applications

The Z-section purlin is the most widely used cold-formed purlin type in UK commercial and industrial construction. It is named for the approximate Z-shape of its cross-section, which is formed by cold-rolling a flat steel strip through a series of rollers that create the section profile without cutting. The Z-section has a web (the vertical element), two flanges (the horizontal elements at the top and bottom), and lips (short returns at the ends of the flanges that improve the section's resistance to torsional buckling).

Z-section purlins are typically manufactured in depths of 120 mm to 300 mm, with flange widths of 50 mm to 80 mm and steel thicknesses of 1.5 mm to 3.0 mm. The steel grade is typically S350 or S450 to EN 10346, providing yield strengths of 350 N/mm² and 450 N/mm² respectively. The higher yield strength of S450 steel allows a smaller section to carry the same load as an S350 section, which is why S450 is increasingly used in modern portal frame buildings where minimising purlin weight is commercially advantageous.

Z-section purlins have a key structural characteristic that affects their design under combined bending and torsion: because the section is asymmetric about both axes, it deflects out-of-plane when loaded vertically. This out-of-plane deflection is controlled by the sleeve or cleat connections at the purlin ends, which provide a degree of rotational restraint. The Eurocode EN 1993-1-3 design procedures for Z-section purlins specifically address this behaviour through the effective section properties and restraint assumptions.

C-Section Purlins, Geometry, Properties, and Applications

C-section purlins (also called lipped channel sections) have a C-shaped profile: a web, two flanges of equal width projecting in the same direction, and return lips at the ends of the flanges. Unlike the Z-section, the C-section is symmetric about its web, which means it does not deflect out-of-plane under vertical loading in the same way as a Z-section.

C-sections are used as purlins in a smaller proportion of UK commercial buildings than Z-sections, but they are also used as side rails on the walls of portal frame buildings, as cold-formed columns in mezzanine structures, and as secondary members in flat-roofed buildings. For solar PV structural assessment, C-section purlins require the same dead load and wind uplift checks as Z-sections but are designed using different effective section properties under EN 1993-1-3.

Sigma Sections and Other Profiles

Sigma-section purlins (named for their approximate resemblance to the Greek letter sigma) are a more recent development in cold-formed purlin technology. The sigma profile introduces an additional fold in the web, creating a more efficient cross-sectional geometry that provides higher section modulus for the same steel weight compared to Z or C sections of equivalent depth. Sigma sections are used in modern industrial buildings where long spans require high-capacity secondary members with minimum self-weight.

Other cold-formed profiles encountered in UK commercial buildings include hat sections (used as cladding rails rather than purlins), omega sections, and proprietary profiles marketed by specific cladding and framing system manufacturers. Where a building uses a proprietary profile, the structural assessment may require manufacturer-specific design data rather than generic Eurocode section tables.

Identifying Purlin Type and Size from Remote Data

Desktop structural assessments for solar PV must identify the purlin type and size without the benefit of a site visit. The primary sources of this information are: structural drawings (which show purlin type and size directly); purlin markings visible in internal photographs (cold-formed purlins are typically embossed with the manufacturer's code and section size on the web); and typology benchmarks based on building age, span, and use class.

When structural drawings are not available and internal photographs do not show purlin markings clearly, the engineer applies typology benchmarks. For a steel portal frame warehouse constructed between 1985 and 2005 with a bay width of 6 to 7.5 metres and a frame span of 20 to 30 metres, the typical purlin section range is Z175 to Z225 in S350 or S450 steel, at 1.4 to 1.8 metre centres. Conservative selection of the lightest section within this range ensures that the structural assessment does not overestimate capacity.

How Purlin Type Affects the Solar PV Structural Assessment

The purlin type and section directly determine the outcomes of the dead load bending stress check and the wind uplift fixing adequacy check. A heavier purlin section has a larger section modulus, allowing it to carry a higher bending moment before reaching design stress. A lighter section has a smaller section modulus and a lower capacity that constrains the permissible array dead load.

For wind uplift, the purlin type determines the pull-out resistance of the hook bolt or purlin clamp fixing used in pitched roof installations. The pull-out resistance depends on the flange thickness of the purlin, the flange width available for the hook bolt head or clamp jaws, and the steel grade. A C-section with a wider, stiffer flange provides higher pull-out resistance per fixing than a Z-section of similar depth with a narrower flange, which may mean fewer fixings are required to resist the design uplift force.

The structural engineer's calculation demonstrates adequacy or identifies constraints for the specific purlin type and section in the specific building. Where the assessment determines that the existing purlins are inadequate for the proposed loading, the options are: reducing the array dead load (lighter panels or lighter racking); increasing the fixing density to distribute the wind uplift over more fixings; adding additional purlins (supplementary purlins can be added between existing ones); or specifying a maximum array coverage area that keeps the loading within the purlin capacity.

Sigma vs Zed vs Eaves Beam: Performance Under Combined PV Loads

Cold-formed steel roof purlin systems use several distinct section profiles, the most common being Zed (Z-section), Sigma (S-section), and channel (C-section), each with different structural efficiency characteristics under the combined dead load, wind uplift, and potentially snow loading that rooftop PV installations impose.

Zed sections (Z-purlins) are the most prevalent type in UK industrial construction from the 1970s to present. Their asymmetric geometry allows the bridging rod to pass through the web at the correct neutral axis position, and the anti-sag rod system they use in the span centre reduces effective buckling length and increases the allowable bending moment. Under PV dead load, Z-purlins perform predictably: the dead load increases the applied downward bending moment, which is compared to the section’s design resistance derived from its section properties and effective length. Wind uplift generates a reversed bending moment that must be checked separately against the upward loading resistance, which is typically lower than the downward resistance due to the reduced lateral restraint provided by the panel attachment compared to the roof sheeting’s diaphragm action.

Sigma sections were developed as an improvement over Z-sections in terms of structural efficiency. The sigma profile’s symmetric geometry about its neutral axis means it performs more consistently under both downward dead load and upward wind uplift than a Z-section. For PV applications on buildings with sigma purlins, the uplift check is typically less onerous than on Z-purlin equivalents of the same section depth. However, sigma purlins require a different bridging and anti-sag rod configuration from Z-purlins, and the structural assessment must use the appropriate capacity tables for the sigma section type rather than the Z-section tables that some assessors apply by default.

Eaves beams are a distinct structural element from span purlins, they are the longitudinal members at the eaves that carry the lowest purlin or sheeting rail and transfer lateral loads to the portal frame columns. For PV installations, eaves beam fixings are not typically used for racking attachment, but the eaves beam is the critical load path for the portal frame’s lateral stability under wind load. Where PV installations modify the effective lateral restraint provided to the portal frame columns by the roof cladding system, for example, by replacing a continuous diaphragm roof with an array of discrete PV panels, the lateral stability of the portal frame must be confirmed to remain adequate under the revised cladding configuration.

British vs European Purlin Standards: Assessing Imported Section Sizes

UK industrial buildings constructed before 2005 will generally have cold-formed steel purlins designed to BS 5950 Part 5, the British Standard for design of cold-formed steel sections. Buildings constructed or re-roofed after approximately 2005, and particularly after 2010, may have purlins designed to BS EN 1993-1-3, the Eurocode for cold-formed steel structures. This distinction matters for structural assessment because the design resistance values for the same section size differ between the two code approaches, and using the wrong code for the section age can produce either unnecessarily conservative or unconservatively optimistic capacity results.

BS 5950 Part 5 uses permissible stress design with effective section properties derived from element slenderness ratios. The permissible stresses incorporate implied safety factors that are higher than the explicit partial factors in the Eurocode approach, which means that BS 5950-designed sections typically have more conservative design capacities than the same section assessed to BS EN 1993-1-3 limit state methods. For structural assessment of existing BS 5950-era buildings, using BS EN 1993-1-3 to assess the capacity of BS 5950-designed sections is technically defensible, the Eurocode is the current standard and can be applied to any section, but the assessor should confirm that the Eurocode section classification and effective section properties are appropriate for the specific section geometry before applying limit state capacity values.

European-sourced purlins, increasingly common in UK construction following the growth of pan-European steel suppliers, may use section sizes that do not correspond directly to any UK catalogue section. The structural engineer assessing these buildings must use the manufacturer’s published section properties and load tables (typically provided to BS EN 1993-1-3) rather than applying standard UK catalogue values that may underestimate or overestimate the actual section capacity. Confirming the purlin manufacturer and section designation at instruction, from maintenance records, the original roof contractor’s documentation, or physical inspection of the section markings on the purlin flange, allows the correct assessment basis to be applied from the outset.

WHERE SOLAR SURVEYS ADDS VALUE

Section Classification and Effective Section Properties

Cold-formed steel sections are classified by their cross-sectional geometry into Classes 1 to 4 under Eurocode 3. Most commercially-available Zed and Sigma purlins are Class 4 sections, their thin webs and flanges can undergo local buckling before the full yield stress is reached, and their effective bending capacity is less than the gross section capacity.

The effective section properties for Class 4 cold-formed sections are calculated using the effective width method (BS EN 1993-1-5) or the reduced stress method. In practice, purlin manufacturers provide pre-calculated effective section properties in their technical data, and the structural engineer uses these values directly. However, where sections are non-standard (custom lengths, non-standard gauges, or modified sections) the structural engineer must calculate effective properties from first principles rather than relying on manufacturer tables.

This distinction is important for solar PV assessments because the age of many UK portal frame buildings means some purlins predate the current range of standard Zed and Sigma sections. A building constructed with bespoke fabricated cold-formed sections from a 1980s manufacturer who no longer exists cannot be assessed from current manufacturer tables, the effective properties must be calculated from measured section dimensions.

Purlin Continuity and Multi-Span Behaviour

Purlins in UK portal frame buildings are typically installed in multi-span configurations, each purlin length spans two or more bays between rafters, with sleeved lap joints at the rafter positions. The multi-span continuity increases the purlin's effective load-carrying capacity compared to a simply-supported single span: the negative moment over the intermediate rafter reduces the positive mid-span moment, allowing the purlin to carry higher loads than its section capacity under simply-supported conditions.

The structural assessment for solar PV must use the correct multi-span capacity, not the simply-supported capacity. Under-estimating purlin capacity by using the simply-supported model is conservative, it may lead to a conclusion that the purlin cannot carry the solar array load when in fact it can. On buildings where structural capacity is marginal, using the correct multi-span model rather than the conservative single-span model may be the difference between an adequate and an inadequate structural assessment result.

The multi-span capacity is affected by the lap joint arrangement at the intermediate rafters. Purlin manufacturers specify the lap length and connection arrangement (typically 2 × M12 or 2 × M16 bolts through the laps) that provides the continuity assumed in the multi-span capacity tables. Where lap arrangements have been modified (e.g., shortened laps due to rafter spacing changes), the actual available continuity may be less than assumed, and the capacity must be recalculated accordingly.

Sigma Sections and Thermal Performance

Sigma-section purlins are increasingly used in modern commercial buildings as a thermal break alternative to conventional Zed sections. The Sigma profile has a web geometry that reduces thermal bridging compared to the flat web of a Zed section, which is relevant for buildings with high-performance insulation requirements. From a structural perspective, the Sigma section has different bending capacity characteristics from the Zed section for the same overall dimensions and gauge, they cannot be treated as interchangeable in structural calculations.

For solar PV structural assessments on buildings with Sigma purlins, the structural engineer must confirm that the capacity calculation uses Sigma-specific section properties, not the Zed section data from the same manufacturer's range. Using Zed section capacity tables for a Sigma purlin may underestimate or overestimate capacity depending on section geometry and the specific loading combination being checked. This is a subtle but important accuracy point for assessments where section capacity is close to the design load.