Metal roofing and wall cladding do not fix directly to rafters, studs or portal frames. Between the structural frame and the sheet sits a secondary support layer: timber battens, steel top hats, or cold-formed steel purlins. Getting this layer right determines whether your roof stays on in a storm, whether your cladding sits flat, and whether your fastener pattern actually complies with the span tables that back the product warranty.
This post covers each support type, when to use which, and how support spacing connects to sheet profile, BMT and wind region.
Why a Secondary Support Layer Exists
Rafters and portal frame columns are spaced for structural loads, not for the spanning capacity of a thin steel sheet. A 0.42 BMT corrugated sheet spanning 1,800 mm between rafters in a high-wind zone will fail the span table long before the rafter does. The secondary support layer brings the sheet's actual span down to a distance the profile can carry under the design wind pressure for the site.
There is also a practical reason. Rafters and top chords are rarely perfectly coplanar. Battens or top hats let you pack and align the fixing surface so the finished sheet plane is flat, which matters for appearance on wall cladding and for weathertightness on low-pitch roofs.
Timber Roof Battens
Timber battens remain the standard secondary support for residential metal roofing on timber-framed buildings. They run horizontally across the rafters, perpendicular to the sheet length, and the roofing screws pass through the sheet into the batten.
Species, Size and Treatment
Battens are typically 75 x 38 mm or 75 x 50 mm dressed hardwood or treated pine. In bushfire-prone areas, AS 3959 requires non-combustible or appropriately rated construction depending on the BAL rating, so confirm whether timber battens remain acceptable for the BAL level on the site. For BAL-FZ, non-combustible construction is required throughout.
In coastal and high-humidity environments, H3 treated pine is the minimum. Where the batten will be in contact with COLORBOND or ZINCALUME steel, use only H3 or better treated timber and avoid direct contact where possible, since some preservative treatments are corrosive to zinc coatings. BlueScope's installation guidance specifically flags this for ZINCALUME substrates.
Fasteners into Timber Battens
Hex-head Type 17 screws are the standard fastener for metal roofing into timber. The Type 17 point cuts its own hole through the steel sheet and bites into the timber without pre-drilling. For 0.42 to 0.48 BMT steel into timber battens, a 10-gauge x 65 mm hex head with a neoprene washer is the typical specification. Class 3 coating is the minimum for inland areas; Class 4 is required within 1 km of a surf coast or in severe marine zones. The existing post on Class 3 vs Class 4 fasteners covers this in more detail.
Fasten at every batten at the sheet ends and at every second batten in the field, unless the span table or wind region requires every batten. Always check the profile's installation guide because Trimdek, Klip-Lok and corrugated each have different fastener patterns.
Batten Spacing and Span Tables
This is where many installations go wrong. Batten spacing is not a fixed number. It is the maximum span the sheet profile can carry at the design wind pressure for the wind region and terrain category of the site.
AS 1562.1 is the governing standard for metal roof and wall cladding installation. Every profile sold by manufacturers like Lysaght, Stramit and Fielders comes with span tables derived from testing under AS 1562.1. These tables give maximum support spacing for each combination of:
- Sheet profile (corrugated, Trimdek, Spandek, Klip-Lok, etc.)
- BMT (0.42, 0.48, 0.55 mm)
- Wind classification (N1 to N6 for non-cyclonic, C1 to C4 for cyclonic)
- Rafter or purlin position (end span, internal span, overhang)
A 0.42 BMT corrugated sheet might allow 1,200 mm centres in N2 but only 900 mm in N4. Stepping up to 0.48 BMT recovers some span capacity. If the required spacing is still too close for the frame layout, the answer is either a stiffer profile or additional battens.
Never assume that what worked on a previous job in a different postcode applies to the current site. Download the current span tables from the manufacturer's website and check them against the NCC wind classification for the address.
Steel Top Hats
Top hat sections are cold-formed steel sections with a hat-shaped cross-section: two flanges that fix to the substrate, two webs, and a flat crown where the cladding fixes. They serve two purposes that timber battens do not: levelling an uneven substrate and creating a drained, ventilated cavity behind the cladding.
Wall Cladding Applications
On wall cladding, top hats fix vertically to the studs or masonry substrate. The cladding then fixes horizontally across the top hat crowns. The cavity between the back of the cladding and the face of the substrate allows any water that penetrates the cladding joints to drain down and out at the base rather than sitting against the wall framing.
This cavity is also a ventilation path, which matters for NCC 2025 compliance. The 2025 NCC introduced tighter requirements around condensation management in wall assemblies. A drained and ventilated cavity behind metal cladding is one of the more straightforward ways to manage interstitial moisture risk in climate zones where condensation is a concern.
For facades on Class 2 to 9 buildings, AS 4284 covers weatherproofing testing of wall cladding systems. The cavity created by top hats is part of the drainage plane that allows a system to meet those requirements.
Levelling and Packing
On masonry or concrete substrates, top hats can be shimmed or packed to bring the cladding face to a consistent plane. This is common on commercial refits where the existing wall face is out of plumb or irregular. The alternative, direct-fixing cladding to an uneven substrate, produces a wavy finished surface that is immediately obvious on flat-pan profiles.
Top Hat Sizes and Fixing
Top hats come in various depths, commonly 25 mm, 38 mm and 50 mm. Deeper sections create a larger cavity. The material is typically G550 or G300 steel in 0.55 to 1.0 BMT, depending on the load the top hat must carry across its span between studs or frame members.
Fix top hats to steel framing with self-drilling screws, typically 10-gauge hex head TEK screws. Fix cladding to the top hat crown with the fastener specified by the cladding manufacturer, usually a smaller self-drilling screw than the roofing equivalent.
Spacing of top hats follows the same logic as batten spacing: it must not exceed the maximum support spacing from the cladding profile's span table for the relevant wind classification.
C and Z Purlins
For sheds, industrial buildings and commercial structures with portal or post-and-beam frames, the secondary support is cold-formed C or Z section purlins rather than battens. Purlins span between the portal frame rafters or columns, running parallel to the ridge on the roof or horizontally on the walls.
C Purlins vs Z Purlins
C purlins have a plain channel cross-section. They are self-supporting and used where lapping is not required or where single-span conditions apply. Z purlins have an offset flange, which allows them to be lapped at the rafter and bolted together. The lap creates a continuous beam effect over the support, which increases the spanning capacity significantly compared to a simple span.
For most shed and industrial roof applications, Z purlins in lapped configuration are the standard because they allow wider rafter spacing while still meeting the sheet span requirements. Suppliers like Stramit and Lysaght publish span tables for their Z purlin sections that account for lap length, purlin depth (150 mm, 200 mm, 250 mm are common), BMT and wind loading.
Purlin Spacing for Sheds
The same span table logic applies. The purlin spacing must not exceed the maximum support spacing for the roof sheet profile and BMT at the design wind pressure. On a shed in a N3 wind region with 0.42 BMT Trimdek, the purlin spacing might be limited to 900 mm. Moving to 0.48 BMT or a stiffer profile like Spandek may allow 1,200 mm centres, which reduces the number of purlins and the associated labour and material cost.
For cyclonic regions (C1 to C4), the span tables tighten considerably. Purlin spacing in C2 and above often requires 600 to 750 mm centres on standard profiles, which affects the entire framing layout. This needs to be resolved at the design stage, not after the portal frames are erected.
Bridging and Anti-Sag Rods
C and Z purlins in long spans require lateral restraint to prevent rotation under load. Bridging (steel rod or flat strap between purlin webs) and anti-sag rods are specified by the purlin manufacturer based on span and load. Skipping bridging on long-span purlins is a common shortcut that compromises the structural capacity the span table assumes.
Matching BMT to Support Spacing
The relationship between sheet BMT and support spacing is direct. Thicker steel spans further because it has greater bending stiffness and higher yield strength. The practical implication is that BMT selection and support spacing are not independent decisions.
If the frame layout is already fixed and the purlin or batten spacing is set, the BMT must be at least what the span table requires for that spacing and wind region. If the BMT is set by budget or availability, the support spacing must be reduced to match. Trying to run 0.42 BMT corrugated at 1,500 mm centres in N4 because the battens are already fixed is not a span table outcome you will find in any manufacturer's documentation.
For owner-builders in particular, this is worth checking before ordering sheets. Download the span table for the profile you are buying, confirm your wind classification using the NCC wind map or a site-specific report, and verify that your batten or purlin layout is within the allowable spacing for the BMT you have selected.
Putting It Together
The support layer between frame and sheet is not a detail to work out on site. It drives the framing layout, the sheet specification, the fastener selection and the compliance outcome under AS 1562.1. Timber battens suit residential timber-framed roofs. Top hats suit wall cladding applications where levelling and a drained cavity are needed. C and Z purlins suit sheds and commercial portal-frame buildings where the spans between structural members are too large for battens.
In each case, the spacing is determined by the span tables for the profile, not by what seems reasonable or what was done on the last job.
ACS supplies battens, steel top hats, C and Z purlins alongside the roofing and cladding profiles they support. If you are working through a framing layout and want to confirm that your purlin spacing, sheet profile and BMT are compatible, visit acsupplies.com.au or contact the trade desk with your wind classification and frame centres.