Z120 vs Z275 Galvanized Coating — Which Zinc Weight Do You Need?

Zinc coating weight is specified in grams per square meter (g/m²) of total coating across both sides combined. So Z275 means 275 g/m² total — approximately 137.5 g/m² per side, which translates to about 19 microns thick per side (zinc density ~7.14 g/cm³).

The 'Z' prefix is the European/ISO designation. US ASTM uses different notation: G60 corresponds approximately to Z180 (60 means 0.6 oz/ft² total, ~183 g/m²). G90 ≈ Z275. G140 ≈ Z450. When importing from China to multiple regions, we typically dual-cert with both Z and G designations on the MTR.

ISO 12944 defines 6 atmospheric corrosion categories that match coating selection to environment:

- **C1 Very Low**: Heated, dry buildings. Office interiors. Almost no corrosion. - **C2 Low**: Unheated buildings, low humidity. Sheltered rural outdoor. - **C3 Medium**: Urban/industrial atmospheres with moderate SO₂. Coastal areas with low salinity. This is where most rooftop and outdoor structures sit. - **C4 High**: Industrial areas with high humidity, urban high-pollution, coastal with moderate salinity. Aggressive. - **C5 Very High**: Industrial humid + chemical exposure. Marine high-salinity (within 1 km of coast). - **CX Extreme**: Offshore areas, hot wet tropical, persistent industrial chemical exposure.

For most B2B applications: C2 (sheltered outdoor) → Z120. C3 (general outdoor) → Z275. C4 → Z350 or galvalume. C5 → Galvalume AZ150 minimum.

Z120 is appropriate for:

- **Sheltered outdoor structures**: Eaves, soffits, sheltered carports, building inner panels not directly exposed to weather. - **Indoor humid areas**: Bathroom partitions, basement framing, light industrial interiors. - **HVAC ducting** (ventilation systems): Air doesn't carry significant moisture or pollutants. - **Short-life applications** where 8-12 years is acceptable: temporary structures, modular buildings with planned obsolescence. - **Substrate that will be additionally protected**: PPGI applications where the paint provides additional barrier.

Z275 is the workhorse for outdoor durability. Choose Z275 when:

- **Roofing for buildings** in non-marine, non-industrial environments - **Agricultural buildings**: Barns, grain silos, animal facilities (high humidity from animals + occasional ammonia) - **Highway sound barriers, bridge components** (general outdoor, no marine influence) - **Light commercial buildings**: Warehouses, gas stations, fast-food chains - **Solar mounting** in non-coastal areas (you need 25+ year service life to match solar panel lifespan)

Go Z350 when you have: - Industrial atmospheres with active chemical exposure (paper mills, chemical plants) - Coastal proximity (within 5-10 km of saltwater) but not directly on coast - Critical infrastructure where premature replacement is costly

If you're considering Z350 or higher, you should also evaluate galvalume (AZ coatings — 55% Al + 43.4% Zn + 1.6% Si). Galvalume's aluminum-zinc combination provides 2-4× better corrosion resistance than equivalent-weight pure zinc:

- AZ100 ≈ better than Z275 in most environments - AZ150 ≈ better than Z350+ pure zinc - AZ150 is recommended for: marine within 5 km of coast, severe industrial, premium 25-year roofing, applications where pure GI would corrode faster than acceptable.

Note galvalume's silver spangle appearance differs from GI's traditional crystalline pattern — relevant if appearance matters.

Higher zinc coating means more zinc material consumed, hence higher cost. Approximate price additions over Z80 baseline (per ton, mid-2026 pricing):

- Z80 → Z120: +$15-25/ton - Z120 → Z180: +$25-40/ton - Z180 → Z275: +$40-60/ton - Z275 → Z350: +$30-50/ton - Switch from Z350 → AZ150 galvalume: +$50-80/ton (different process, premium product)

The lifespan extension typically far exceeds the cost premium when amortized over service life. A 25-year Z275 roof beats a 12-year Z120 roof on total cost-of-ownership (TCO) by 30-50% even with the higher upfront cost.