Most steel buildings in New Zealand use three types of coated steel, often all on the same building. Knowing which is which helps you read a quote and pick the right finish for your site.
The three finishes #
| Finish | What it is | Where it’s used | Look |
| Galvanised | Steel with a zinc coating | Frames, purlins and girts; Steelspan uses Z275, a 275 g per m² zinc coating | Dull grey |
| Zinc-aluminium coated | Steel with an aluminium and zinc alloy coating | Unpainted roof and wall cladding, especially on farm and utility buildings | Bright silver, dulling with age |
| Pre-painted | Steel with paint layers over a metallic coating | Roof and wall cladding where colour, looks or extra durability matter | Wide colour range |
Unpainted or pre-painted? #
Unpainted zinc-aluminium coated steel is usually the cheaper cladding option and is common on farm sheds and utility buildings. It’s tough, but it has limits: some environments and some contact with other materials don’t suit it, so check the manufacturer’s guidance for your site and use.
Pre-painted steel costs more, but you choose the colour, and the paint adds a layer of protection. On lifestyle blocks and commercial sites, colour is often non-negotiable. Near the coast, the higher-grade pre-painted products are often the right call.
Not all pre-painted steel is the same #
Manufacturers make several grades of pre-painted steel for different environments. Their guidance rates each for moderate, severe and very severe coastal conditions, and some aren’t recommended for the harshest sites. There are also anti-condensation sheets, which have a fleece layer bonded to the underside of the sheet to absorb condensation, and can replace a separate roof underlay in suitable buildings. These sheets aren’t recommended for continually wet interiors such as intensive animal shelters.
Thickness #
Roof and wall sheets come in different base metal thicknesses, commonly 0.40 mm and 0.55 mm. Thicker sheets are stiffer and tougher underfoot, and can span further between purlins. Check what a quote includes.
Keep metals compatible #
Some metals don’t mix. Runoff from copper or brass, for example, can damage zinc and aluminium-zinc coatings. Flashings, gutters, fixings and anything that drains onto the roof need to be compatible with the cladding.
How we choose #
We start with your site’s environment and the building’s use, then pick a finish that suits both, and explain the maintenance it needs. Our quotes typically offer two profiles: a five-rib trapezoidal profile or a corrugated profile. Both span well and are great value for money. These days most customers choose a pre-painted finish, which lets them pick the colour and gives better corrosion protection. Pre-painted steel normally comes in around 20 colours. Ranges vary between manufacturers, but we can usually source the colour you want, and pricing may vary slightly for some colours.
Pre-painted steel costs more up front than zinc-aluminium coated steel, but you’d be very unlikely to paint the building afterwards for that same difference. Once you allow for scaffolding, edge protection, water blasting, multiple coats and all the labour, painting later often costs many times more. It’s worth weighing up at the quote stage.
Talk to Steelspan #
Not sure which finish suits your site? Tell us where you’re building. Use our Easy Shed Quote or call 0800 787 726.
