The Z-number on a galvanized steel coil tells you the zinc mass per square metre, both sides combined. Z30 gives roughly 4 µm per side, Z120 about 8.5 µm, Z275 about 19 µm.
Service life scales with coating: in a rural inland site, Z120 may last 15–25 years, while Z275 can exceed 40. Coastal, industrial, and humid environments cut that sharply, so specify heavier coatings there. Tight bends crack thick coatings, and cut edges expose steel. Match the Z-number to exposure and fabrication, not to price.
