If you're building, you'll hear the word "purlin" on site and in your plans, usually mixed up with "batten". It's worth knowing the difference, because the members that hold your roof sheets down are what stop them leaving in a storm.
What a purlin is
A purlin is a horizontal member that runs across the roof and carries the roof sheeting. It sits on the main structure (rafters, beams, trusses or portal frames) and spans between them. The roof sheets are screwed to the purlins, and the purlins transfer the roof's weight and, just as importantly, the wind uplift down into the frame.
A batten does the same job on a smaller scale. On a typical house the trusses are spaced closely, usually 600 to 900 mm apart, so the members running across them are light battens rather than heavier purlins.
Where you'll see them on a house
Most new homes in Queensland are built like this:
- Prefabricated timber roof trusses carry the roof.
- Steel top-hat battens (a hat-shaped light steel section) run across the trusses.
- Metal roof sheets are screwed to the battens. For tiles, timber or steel tile battens are used instead.
You'll see true purlins where members have to span further between supports:
- Skillion and raked roofs, where purlins span between rafters, beams or walls.
- Patios, verandahs and carports, often steel C or Z sections spanning between beams.
- Large open-plan rooms with raked ceilings, where the roof is built from beams and purlins rather than trusses.
- Sheds and garages with steel portal frames.
The common types
- Steel C sections (Cees): cold-formed from galvanised steel strip into a C shape. Simple to fix and common on patios, carports and sheds.
- Steel Z sections (Zeds): similar, but Z-shaped so they can be lapped over supports. The lap makes them continuous over several spans, which lets them span further. Used on bigger roofs and portal frame sheds.
- Top-hat battens: light steel hat-shaped sections used over closely spaced trusses or rafters on houses.
- Timber purlins: sawn hardwood, treated pine or engineered timber such as LVL. Still common on skillion roofs and timber-framed patios. Sizes come from the span tables in AS 1684 or the manufacturer's tables.
In Australia you'll often hear the steel versions called by the Lysaght brand names "Zeds and Cees". Other suppliers make equivalent sections.
Corrosion: galvanised, not painted
Light-gauge steel purlins and battens are protected by a zinc or zinc/aluminium coating applied at the steel mill, not by paint. For example, steel purlins are commonly made to AS 1397 with a Z350 coating, and a heavier Z450 coating is available for harsher conditions.
If you're near the coast, near a pool, or the roof space is likely to stay damp, ask what coating is being supplied and check it against the manufacturer's environment guide. Some products aren't suitable within a set distance of breaking surf. Also watch for cut ends, scratches and swarf (steel filings from cutting) left on the sheets, which rust quickly.
Who designs them
The size, spacing and fixing of purlins and battens depend on:
- the span between supports
- the roof material and its weight
- the wind classification of your site (under AS 4055), which in Queensland is often the bigger load than the roof's weight
- whether the area is cyclonic
For a standard house, the truss manufacturer's design sets the batten spacing and fixings. For skillion roofs, patios and carports, the sizes come from span tables or an engineer's design, and the approved plans should show them. The roofing manufacturer also specifies how many screws go into each batten or purlin, which changes at the edges and corners of the roof where wind uplift is highest.
What to check on site
Before the roof sheets go on, have a look at:
- Sizes and spacing match the plans, truss layout or engineer's design.
- Fixings are as specified: the right number of screws or bolts at every connection, plus any tie-down brackets or cleats.
- Closer spacing at edges and corners, if the design calls for it.
- Laps on Zeds are the length shown in the design.
- Coating is right for your site, and there's no rust, damage or swarf.
- Line and level look straight. A wavy batten line shows up later as a wavy roof.
Our C7 Frame and C8 Metal Roof checklists cover these items step by step. You'll find them on our checklists page.
C7 - Frame Stage Inspection Checklist
$119.00 AUD
Last updated 28 Oct 2026 · Revision 5 · Free updates for life
Checklist Type: Major (Frame Stage)
Form Factor: Fillable, print-ready PDF (A4, 48 pages)
When to use: Frame stage: after the frame and trusses are up, before linings go in
A 146-point frame stage checklist. Check walls, openings, trusses, bracing and tie-downs against your drawings before the linings go in, while changes are still simple.
Roof trusses: read the installation guide
If your roof uses prefabricated trusses, they'll arrive with the manufacturer's installation guide. In Australia the two main truss systems are MiTek and Pryda, and each has its own guide.
The guide usually turns up in an A4 ziplock bag with a couple of carpenter's pencils, with the leads broken, of course, so you spend five minutes whittling to find any lead at all.
Humble packaging aside, the guide matters. It sets out the installation sequence, bracing, tie-downs and hardware the truss design depends on. "We've always done it this way" won't help if a defect turns up and the installation didn't follow the guide. Your builder's carpenters should follow it to the letter.
MiTek installation guide
Pryda installation guide (not the DJ, if you're wondering)
Frequently asked questions
What's the difference between a purlin and a batten?
Both carry the roof sheets. Purlins are heavier members that span further between supports. Battens are lighter members fixed over closely spaced trusses or rafters, which is what most houses use.
Can I use timber instead of steel?
Yes. Timber purlins are common on skillion roofs and patios. They must be sized from the span tables or by an engineer, and treated to suit their exposure if they're not hardwood.
Why does my patio need bigger purlins than my house roof?
Because the supports are further apart. A patio roof often spans several metres between beams, while house trusses are under a metre apart.
Further reading
Roofing Elements Simplified: A Guide to Roof ConstructionDemystify roofing elements with this beginner-friendly guide. Covering gables, Dutch gables, verge overhangs, and more, it breaks down complex roofing terms into simple language, helping you communicate effectively with architects and builders and understand the structure of your home’s roof.
Comprehensive Guide to Roof Types, Styles, and MaterialsGet familiar with various roof types, styles, and materials available in the world of construction through this guide. Understanding the advantages and charm of different roofing options can significantly influence the aesthetic and functional outcome of your project.
Metal Roof & Flashing Inspection Checklist GuideLearn how the Metal Roof + Flashing Inspection Checklist lets you inspect roof installations for quality and compliance. Avoid delays, reduce costs, and ensure your home’s roof is built to last with this checklist.
Roof Sarking Explained: Essential Protection for Your HomeLearn about roof sarking, a vital component in home construction. This guide covers what roof sarking is, its benefits like moisture protection, temperature control, and energy efficiency, and why it’s a crucial investment for the longevity and safety of your home’s roofing system.
