Metallic Coating Designations (G90, AZ50, Z275)

Steel used for roofing is protected by a thin metallic coating, and the amount of that coating is stated with a short code. The letters identify the coating alloy and the number gives the minimum coating mass, totalled over both faces of the sheet. Reading these codes is the simplest way to compare how much corrosion protection two steel products carry before any paint is applied.

Common North American codes

• G (galvanized, ASTM A653): G60 = 0.60 oz/ft2; G90 = 0.90 oz/ft2.
• AZ (55% aluminium-zinc, ASTM A792): AZ50 = 0.50 oz/ft2; AZ55 = 0.55 oz/ft2.
• GF (zinc-5% aluminium): for example GF60 and GF90; this alloy is sold under the Galfan name.
• ZM (zinc-aluminium-magnesium, ASTM A1046): ZM60 and ZM90, used where extra corrosion resistance is wanted.

Metric equivalents

One ounce per square foot equals about 305 g/m2. Metric designations therefore read G90 as Z275 and G60 as about Z180, while AZ50 becomes AZM150 and AZ55 becomes AZM165.

How the number is measured

The figure is a "triple-spot" average: three samples are taken across the strip width, at the centre line and at least 50 mm (2 in) from each edge, and the coating mass on both sides is averaged. A single spot may fall slightly below that value. Producers usually measure on-line with X-ray or isotope gauges, with chemical strip-and-weigh tests as the reference method.

Comparing coatings

Equal weights do not mean equal thickness, because zinc is denser than aluminium: AZ coatings are thicker than G coatings of the same mass. For plain zinc, life is roughly proportional to coating weight, but alloy coatings do not follow a simple ratio, so codes should only be compared within the same alloy family.