What roof sheeting is
Most metal roofs on Australian homes are steel. Colorbond is pre-painted, Zincalume is coated steel without the paint, and older roofs may be plain galvanised iron, which BlueScope no longer recommends for most roofing.
Sheet makers roll that steel into profiles. Lysaght, a BlueScope business, sells its corrugated sheet as Custom Orb and its concealed-fix sheet as Klip-Lok; other makers use their own names. Those names are BlueScope and Lysaght trade marks, and Find My Roofer has no connection with either company.
The three main profiles side by side
| Corrugated | Trapezoidal (ribbed) | Concealed-fix | |
|---|---|---|---|
| How it's fixed | Screws through the crests | Screws through the sheet | Hidden clips underneath, no holes through the sheet |
| Lowest pitch | 5 degrees for Custom Orb | Varies with rib depth; ask for the maker's figure | Down to 1 degree for some Klip-Lok versions |
| Longest internal span at 0.42 mm (2022 code) | 1200 mm | 1700 mm, or 2400 mm for close-pitched trapezoidal | 2100 mm |
| Where a roofer walks | Across at least two corrugations | In the pans | In the pans, never on the ribs |
| Typical home use | Most pitched roofs and curved verandahs | Extensions, sheds and homes after a bolder rib | Low-pitched and near-flat roofs, skillions |
Thickness and coating: the two numbers on a quote
BMT (base metal thickness) is the steel without its coatings, and Lysaght's home profiles come in 0.42 mm and 0.48 mm. BlueScope says strength comes from the BMT and steel grade, while rust resistance comes from the metallic coating. Thicker sheet spans further: Custom Orb's internal span grows from 1200 mm at 0.42 mm to 1700 mm at 0.48 mm.
Those span limits in the table are from the 2022 National Construction Code. Where your state has switched to the 2025 edition, adopted from 1 May 2026, the figures may differ.
The coating is the coast question. BlueScope recommends standard Colorbond more than 200 m from breaking surf, Colorbond Ultra (a heavier AM150 coating) from 100 m, and its Superdura Stainless product closer in, measured from the high-water mark. Our Zincalume page covers the unpainted option, which is stricter near calm water.
How long steel lasts, and what keeps it going
A NSW Government heritage schedule gives steel roofs 20 to 40 years, and suggests a check every 7 years for loose sheet edges, dents from foot traffic, rust stains around fittings and mismatched metals at flashings.
Rain washes the open roof. BlueScope's October 2026 maintenance bulletin says sheltered spots rain can't reach should be washed at least six-monthly, or three-monthly near the sea or in dusty areas, and never with turps, petrol, thinners or abrasive cleaners on painted steel.
Hail dents don't usually shorten a Colorbond roof's life, BlueScope says, though big dents on a low pitch can hold water and bent overlaps can let it in.
Old grey corrugated sheets may be asbestos
If an old corrugated roof is grey and cement-like rather than metal, it may be asbestos cement, often called 'Super Six'. It was made up to 1990, and later non-asbestos sheets look the same, so only a lab test tells them apart. Don't drill, cut, water-blast or walk on it. Read our guide to Super Six roofing before getting quotes.
Questions for a roofer quoting steel
- Which profile, BMT and coating are you quoting, and why for my roof?
- Will the sheets run full length from gutter to ridge?
- What's going under the sheets: foil, a blanket, or both?
- Which sealant do you use? BlueScope says neutral cure suits Colorbond, while ones smelling of vinegar or ammonia can corrode it.
- How will you clear swarf (metal filings) off the roof each day?
- Will you register the BlueScope warranty, or give me the code to do it?
How we'd choose
We'd let the pitch narrow the field, then pick on looks. On an ordinary pitched house, corrugated does the job and most roofers can repair it; we'd only pay for concealed-fix when the pitch is low or you want no screws through the sheet. Whatever the profile, we'd want a blanket under it and a clear answer on swarf, because those two details decide whether a steel roof stays quiet and free of rust stains.
Want a roofer to take a look?
Tell us the job and your postcode. We match it with a Find My Roofer certified roofer who specialises in that kind of work, and they call you to quote. Your number goes to them alone.