Australian Steel Roof Design — Purlins, Bridging & Lysaght/Zalutek Profiles

Comprehensive reference for Australian steel roof design covering cold-formed purlins (C and Z sections), bridging and sag rod systems, metal roof sheeting profiles from Lysaght and Stramit, wind loading per AS 1562.1, and practical layout guidance for portal frame buildings in Australian conditions. Includes worked examples for purlin selection, bridging layout, and roof sheeting specification.

Quick access: Australian Steel Sections Guide | Australian Steel Framing Systems | AS 1170 Wind Load Guide | AS 4100 Steel Design

Cold-Formed Purlin Systems — Australia's Default Secondary Steel

Every portal frame building in Australia uses cold-formed steel purlins spanning between the main frames to support the roof sheeting. The purlin system accounts for approximately 20-25% of the total structural steel tonnage and perhaps 40-50% of the individual steel pieces in a typical warehouse. Getting the purlin specification right — depth, gauge, lapping, and bridging — directly affects the steel cost, erection speed, and long-term serviceability of the roof.

Z-Purlins (Zed Sections) — Default Choice for Portal Frame Roofs

Z-purlins have an asymmetric cross-section with two equal-width flanges bent in opposite directions from the web. The geometry creates a non-coincident shear centre but allows adjacent purlins to nest (overlap) at the support, creating a continuous beam with significantly improved structural performance.

The standard Australian Z-purlin series, produced by Lysaght, Stramit, and Metroll:

BMT = Base Metal Thickness (the uncoated steel thickness, excluding galvanising). Design to AS 4600:2018 for cold-formed steel structures.

C-Purlins (Cee Sections) — Girts and Short Spans

C-purlins have both flanges bent to the same side of the web, creating a symmetric-looking section that is easier to connect but cannot be lapped. They are primarily used as wall girts (horizontal) in portal frame buildings and as purlins for short-span roofs (awnings, carports, verandahs) where lapping is not required.

Purlin Lapping — Why It Matters

The lapped Z-purlin is one of the simplest and most effective structural optimisations in Australian steel. By overlapping consecutive Z-purlins at the intermediate support (the portal frame rafter), the section depth at the support is effectively doubled over the lap length. This provides:

• 25-35% increased bending capacity at the support (where negative moment governs for gravity + wind uplift combinations)
• 40-60% reduced deflection compared to simple-span purlins of the same section
• Equivalent performance to a continuous beam without requiring on-site welding or special continuity connections

The standard lap length for Australian Z-purlins is 10-15% of the span at each end. For a 7.5 m span, the lap length is approximately 750-1,125 mm. The lap is secured with 2-4 M12 bolts through the web (purlins are connected through the web, not the flanges, to avoid distorting the thin lip stiffeners).

Bridging and Sag Rods — Restraint Against Lateral-Torsional Buckling

Purlins in bending buckle laterally because the thin, narrow compression flange is unbraced. Bridging provides lateral and torsional restraint to the compression flange, substantially increasing the purlin's bending capacity.

Bridging Configuration per Purlin Depth and Span

Bridging Components

A typical Australian bridging system for a Z20019 purlin roof at 7.5 m bay centres consists of:

• Bridging angles: 50x50x4 EA or 65x65x5 EA spanning between purlins, bolted to the purlin web at the bridging line. The angle resists the lateral force from the purlin compression flange.
• Sag rods: 12 mm diameter threaded rod (Grade 250) connecting bridging angles at the ridge and eave to the main frame, providing positive lateral restraint to the entire bridging truss.
• Bridging cleats: 6 mm thick x 65 mm wide plates shop-welded to the main frame rafter, providing an attachment point for the bridging angles at the eave and ridge.

For a 30 m rafter with purlins at 1.5 m centres (approximately 20 purlin lines), the bridging material adds approximately 120-180 kg of steel per rafter bay — about 2-3% of the total structural steel but essential for purlin stability.

Metal Roof Sheeting — Lysaght and Stramit Profiles

Australian metal roof sheeting falls into three categories based on profile geometry and fixing method. The choice of profile affects water penetration resistance, aesthetic appearance, and minimum roof pitch.

Corrugated Profiles (Through-Fixed)

The classic Australian roof profile, manufactured continuously since the 1920s.

Trapezoidal Profiles (Concealed-Fixed) — Standard for Portal Frame Buildings

Concealed-fixed profiles use a clip system that clips onto the purlin and engages with the sheeting rib, hidden from view. This eliminates exposed fasteners (and associated leak paths), allows thermal expansion and contraction, and provides a clean internal soffit.

Standing Seam Profiles — Architectural and Low-Pitch Roofs

Standing seam profiles use a site-rolled or factory-formed interlocking rib that is mechanically seamed after installation. The absence of exposed fasteners and the standing seam (above the water plane) allows roof pitches down to 1 degree. These profiles are common on architectural offices, schools, and community buildings where the roof is a visible design element.

Colorbond and Zincalume Finishes

All Australian roof sheeting is manufactured from steel strip coated with a zinc-aluminium alloy (Zincalume — 55% Al, 43.5% Zn, 1.5% Si) for corrosion resistance, then optionally over-painted with Colorbond (BlueScope's registered trade name for its pre-painted steel finish).

Zincalume (unpainted) provides a bright metallic silver appearance with 20-30 year expected life in C2 corrosivity environments (typical non-coastal Australia). Colorbond adds a 20-micron primer plus 20-micron colour topcoat in 22 standard colours, extending life to 30-40 years and providing the aesthetic finish expected on commercial buildings.

For coastal sites within 200 m of breaking surf (C4 corrosivity category per AS 4312), specify Colorbond Ultra (marine-grade finish) or stainless steel sheeting. Standard Zincalume is not suitable within 500 m of marine influence without regular fresh-water washing.

Purlin Selection Worked Example — 30 m Span Warehouse, Brisbane

A distribution warehouse in Brisbane (wind Region B per AS 1170.2) has portal frames at 6 m centres, 30 m span, 7-degree roof pitch. Design the roof purlin system.

Step 1 — Determine purlin spacing:

Roof sheeting: Lysaght Trimdek (0.42 BMT), maximum support spacing per Lysaght design manual = 1.8 m for 0.42 BMT under Region B wind. Select purlin spacing = 1.5 m to allow comfortable margin for the sheeting and accommodate insulation blanket sag between purlins.

Number of purlin lines per rafter = 15,000 / 1.5 = 10 spaces (11 purlins from eave to ridge).

Step 2 — Calculate purlin loads:

Dead load per purlin (tributary width 1.5 m):

• Sheeting: 0.048 kPa x 1.5 m = 0.072 kN/m
• Insulation: 0.02 kPa x 1.5 m = 0.030 kN/m
• Purlin self-weight: 0.05 kN/m (estimated)
• Total dead: 0.152 kN/m

Live load (non-trafficable roof per AS 1170.1): Q = 0.25 kPa x 1.5 m = 0.375 kN/m.

Wind uplift (Region B, terrain cat. 2, V_R = 45 m/s, Cp_e = -0.9 for high suction zone at windward edge): p = 0.5 x 1.2 x 45² x 0.9 x 1.0 / 1,000 = 1.09 kPa.

Uplift per purlin (ULS): 1.0 x 1.09 x 1.5 = 1.64 kN/m (upward). The dead load (0.152 kN/m downward) partially offsets this: net uplift = 1.64 - 0.9 x 0.152 = 1.50 kN/m (upward, ULS wind load case).

Step 3 — Select purlin section:

Gravity (downward) bending moment, 6 m span, simple span: M* = (0.9 x 0.152 + 1.5 x 0.375) x 6² / 8 = (0.137 + 0.563) x 36 / 8 = 3.15 kN·m.

Uplift (upward) bending moment: M* = 1.50 x 6² / 8 = 6.75 kN·m (governs!).

Try Z20019 (200 mm deep, 1.9 mm BMT, Grade G550). From Lysaght purlin tables for Z20019 simply supported at 6.0 m with no bridging:

Section bending capacity φM_b = 6.80 kN·m for uplift case (bottom flange in compression, unrestrained between supports). This is very close to the 6.75 kN·m demand — adequate but with minimal margin.

Add one row of bridging at mid-span: With a single bridging row reducing the unbraced length from 6.0 m to 3.0 m, the capacity increases to φM_b = 8.90 kN·m for Z20019. The utilisation drops to 6.75 / 8.90 = 0.76 — a comfortable margin.

Final specification: Z20019 at 1.5 m centres with one row of bridging at mid-span (using 50x50x4 EA bridging angle). Lap purlins over the internal portal frames with 900 mm lap length, bolted with 2 x M12 bolts per lap.

Roof Bracing for Australian Portal Frame Buildings

The roof bracing system transfers longitudinal wind loads from the end walls to the side wall bracing, and also provides stability during erection before the roof sheeting is installed.

For a 60 m long building with 6 m bays (10 bays), roof bracing is typically provided in the two end bays and one intermediate bay (at approximately one-third points of the building length). Each braced bay uses crossed tension rods (12-24 mm diameter Macalloy or Grade 250 bar) connecting portal frame apexes and eaves.

The roof bracing also provides lateral restraint to the portal frame rafter at the purlin level. Where the bracing crosses a purlin, the purlin acts as a strut transferring lateral load between bracing nodes — check the purlin for the additional axial compression from this strut action.

Design Checklist for Australian Steel Roofs

• Purlin type: Z-purlin (lapped) for portal frame roofs; C-purlin for girts and short spans.
• Purlin spacing: Matched to roof sheeting capacity (typically 1.2-1.8 m for 0.42 BMT Trimdek).
• Bridging layout: One row for spans 6-9 m, two rows for 9-12 m, three rows for 12-18 m.
• Lap length: Minimum 10% of span each end for Z-purlins. Bolted through web.
• Sheeting profile: Trimdek/Speed Deck for standard commercial; Klip-Lok for low-pitch architectural; Custom Orb for residential/light commercial.
• Sheeting gauge: 0.42 BMT standard; 0.48 BMT in high-wind regions (Regions C and D) and cyclonic areas.
• Wind suction: Check uplift case for purlins and sheeting fixings (uplift often governs over gravity).
• Corrosion: C2 (Zincalume) for non-coastal; C3 (Colorbond) for coastal 200 m-1 km from surf; C4+ (Colorbond Ultra or marine-grade stainless) within 200 m of breaking surf.

Frequently Asked Questions

What is the difference between Z-purlins and C-purlins in Australian roof design?

Z-purlins (Zed sections) are the standard for Australian portal frame roof systems because they can be lapped at the supports, creating a continuous beam that reduces deflection and bending moment by 25-35% compared to simple-span C-purlins. C-purlins (Cee sections) are used primarily for wall girts and shorter-span applications where lapping is impractical.

How should bridging be spaced in Australian portal frame roofs?

Bridging spacing depends on purlin depth and span. For Z200 purlins spanning 6-9 m, one row of bridging at mid-span is standard. For Z250 purlins at 9-12 m, two rows at third points are required. For Z300+ purlins at spans exceeding 12 m, three rows of bridging (quarter points) are typical. AS 4600 Clause 3.4.5 governs cold-formed steel bridging requirements.

What roof sheeting profiles are standard in Australian construction?

The Australian market is dominated by three families: Lysaght Custom Orb (corrugated, cover 762 mm, residential/heritage), Lysaght Trimdek and Stramit Speed Deck (concealed-fixed trapezoidal, cover 700-785 mm, standard for commercial/industrial), and Lysaght Klip-Lok/Stramit Lok-Klip (standing seam, cover 400/406 mm, architectural low-pitch roofs down to 1 degree).

How much does a typical Australian shed roof weigh?

For a typical portal frame shed, roof self-weight is approximately 0.22-0.35 kPa of plan area. This comprises roof sheeting (0.04-0.06 kPa), purlins (0.06-0.12 kPa), insulation (0.02 kPa), bracing and bridging (0.03 kPa), and services (0.05-0.10 kPa). The upper bound includes photovoltaic panels and heavier purlin sections.