A buyer in Kenya received a container of galvanized roofing sheets last year. The supplier's quotation said "SGC400, Z275." Six months later, the sheets near the coast started showing white rust. We tested a sample: the coating was Z120, and the base metal was SGCC, not SGC400.
This kind of thing happens more often than you'd think. The specs look right on paper, but the material doesn't match. So let me walk you through what these grades and coatings actually mean, and how to verify what you're getting.
SGC400: What the Numbers Mean
SGC400 is a Japanese steel grade (JIS G 3302) for hot-dip galvanized steel. The "SG" means Steel Galvanized. The "C400" means the base metal has a minimum yield strength of 400 MPa.
| Grade | Standard | Yield Strength (min) | Tensile (min) |
|---|---|---|---|
| SGCC | JIS G 3302 | 205 MPa | 270 MPa |
| SGC400 | JIS G 3302 | 400 MPa | 490 MPa |
| SGC490 | JIS G 3302 | 490 MPa | 540 MPa |
The difference between SGCC and SGC400 is significant. SGC400 is nearly twice as strong. For load-bearing roofs that need to support snow loads, workers walking on them, or solar panel installations, SGC400 is the right choice. For decorative wall panels where strength isn't critical, SGCC is fine and cheaper.
If you're buying for the US market, SGC400 is approximately equivalent to ASTM A653 Grade 340. For Europe, it maps to EN 10346 S350GD.
Z275: Coating Weight, Not Thickness
This is the single most misunderstood spec in galvanized steel. Z275 means the total zinc coating on both sides of the sheet weighs 275 grams per square meter. That translates to about 19.3 micrometers per side.
| Coating | Zinc (g/m2) | Per-Side Thickness | Outdoor Life |
|---|---|---|---|
| Z80 | 80 | 5.6 um | 5-8 years |
| Z120 | 120 | 8.4 um | 8-12 years |
| Z180 | 180 | 12.6 um | 12-16 years |
| Z275 | 275 | 19.3 um | 20-25 years |
| Z350 | 350 | 24.5 um | 25-35 years |
Those service life numbers assume standard atmospheric conditions. In coastal areas with salt spray, cut the life by 30-40%. In dry rural areas, it can be longer.
For roofing, Z275 is the industry standard. Anything less is a compromise. For coastal or industrial environments, I push buyers to Z350.
International Standards Cross-Reference
| Standard | Region | SGC400 Equivalent | Coating Notation |
|---|---|---|---|
| JIS G 3302 | Japan | SGC400 | Z + weight |
| ASTM A653 | USA | Grade 340 | G + oz/ft2 (G90 = Z275) |
| EN 10346 | Europe | S350GD | Z + weight |
| AS 1397 | Australia | G450 | Z + weight |
| GB/T 2518 | China | DC51D+Z (approx.) | Z + weight |
How We Verify Coating Quality
After the Kenya incident, we made quality verification mandatory for all ZTRFM roofing sheet orders. Here's our checklist:
1. Coating thickness gauge. We use a magnetic induction gauge. Z275 should read 19 +/- 3 um per side. Takes 10 seconds, costs $200 for the gauge, saves you thousands.
2. Mill test certificate (MTC). The supplier provides this document showing heat number, grade, coating weight, and mechanical properties. Cross-check the heat number against the physical material marking.
3. Visual inspection. Look for: uniform spangle pattern (the crystal pattern on the zinc surface), no bare spots, no white rust (zinc oxide -- means the sheets got wet during storage), no black spots (means the zinc didn't bond properly).
4. Bend test. Bend a sample 180 degrees around a mandrel with diameter = 2x material thickness. If the zinc flakes or cracks, the coating adhesion is poor. This is a pass/fail test -- no gray area.
5. Third-party inspection. For orders over $10,000, we recommend SGS or BV inspection before shipment. Costs $300-$500. Worth every cent.
Common Quality Issues
Issue: Coating weight below spec. This is the most common problem. Supplier quotes Z275, delivers Z180 or Z120. Prevention: Always test with a thickness gauge on arrival. Keep the MTC for comparison.
Issue: White rust on delivery. White powdery spots mean the sheets got wet during transit. Minor white rust can be brushed off and doesn't affect long-term performance. Severe white rust (covering more than 10% of the surface) is grounds for rejection.
Issue: Uneven coating. Some areas have thick zinc, others thin. This indicates poor process control at the galvanizing line. The bend test will catch this -- zinc will flake at the thin spots.
Issue: Wrong base metal grade. Supplier ships SGCC instead of SGC400. The sheets look identical. Only a tensile test or MTC will catch this. If strength matters for your application, always request the MTC.
