Coating weight decoded — Z80, Z275, AZ150 and why the number matters
Every galvanised coil ships with a coating-weight spec: Z80, Z100, Z275, AZ150. The number is grams of zinc (or aluminium-zinc) per square metre — total, both sides added together. That's the primary lever between a five-year roof and a fifty-year one.
What the number is measuring
Z275 means 275 g/m² of zinc coating, both sides combined. That works out to roughly 137 g/m² per side — a coating about 20 microns thick. Z80 is 40 μm total; roof panels in a coastal atmosphere corrode through Z80 in 3–5 years. Z275 will last 15–20.
The GI grade ladder
- Z60–Z80 — indoor use, ducting, non-visible ceiling grids
- Z120–Z180 — general roofing, dry inland climates
- Z275 — the industry standard for outdoor structural use
- Z350–Z600 — heavy industrial, chemical exposure, marine roofing
AZ — the aluminium-zinc alternative
AZ150 means 150 g/m² of a 55% aluminium / 43.4% zinc / 1.6% silicon coating. The aluminium forms a passivating oxide layer that resists atmospheric corrosion 2–4× longer than the same weight of pure zinc. AZ150 outperforms Z275 in coastal or humid environments — same buyer, less coating weight, longer roof.
What to specify on a PO
Do not accept "GI 0.5 mm × 1250 mm". Insist on "GI 0.5 mm × 1250 mm, Z275 g/m², JIS G3302 SGCC". The coating weight is the price and the lifespan.
Every coating weight you specify has a mill cost, an atmospheric-life expectancy, and a real-world break-even in years. Underspec by one grade and you'll re-roof twice in a working lifetime. Overspec by two and you paid 30% more than you needed to.
