Choosing the Optimal Al-Zn Coating for Durability and Lifetime ROI
Understand how coating thickness (GSM) directly impacts corrosion protection, service life, and cost, ensuring you choose the right Al-Zn specification for your specific environment—from inland sheds to coastal structures.
1. Understanding Your Roof’s Armor: What is Al-Zn (Galvalume)?
When choosing pre-painted metal roofing sheets, the coating is the most critical factor for how long the roof will last. Both AZ 70 and AZ 150 refer to a protective **Al-Zn (Galvalume)** metallic layer. The number represents the
total amount of coating in grams per square metre (g/m²) across both sides of the sheet.
The basic rule is simple: A higher AZ number means a thicker metallic coating. This translates directly to much stronger protection against rust, especially in bad weather, near the coast, or in industrial areas.
1.1 The Meaning of “AZ” and the Numbers (GSM)
| Term | What It Means | Typical Mix | How It Protects |
|---|---|---|---|
| AZ | **Al**uminium and **Z**inc | ~55% Al, 43.5% Zn, ~1.5% Si | It protects in two ways: the Aluminium creates a tough barrier, and the Zinc sacrifices itself (galvanic action) to stop the steel from rusting. |
| GSM | Grams per Square Metre (g/m²) | Total weight of the coating (Al-Zn alloy) on both sides of the base steel. | It shows the coating thickness. A thicker coat takes much longer for rust to break through. |
2. The Simple Choice: AZ 70 (Good Value) vs. AZ 150 (Maximum Protection)
The best coating depends on your location, how long you need the roof to last, and your budget. Here is a quick summary of when to choose each standard.
| Coating Standard | Recommended Location | Typical Use | Expected Lifespan |
|---|---|---|---|
| AZ 70 | Inland, away from the coast, low pollution | Projects focused on budget, standard warehouses (usually >25-30 km from the sea). | Medium Term |
| AZ 150 | Near the Coast, High Rainfall, Industrial Areas | Sites near the sea (<10 km), industrial pollution, projects requiring low maintenance over many years. | Long Term (20+ years) |
3. Matching the Coating to Your Location (Coastal vs. Inland)
You must consider the entire roof system, including the thickness of the steel (BMT) and the type of paint layer (RMP/SMP/SDP).
| Location / Use | Distance from Coast | Corrosion Risk | Coating | Suggested Paint Type | Steel Thickness (BMT) |
|---|---|---|---|---|---|
| Standard inland warehouse | >25-30 km | Low | AZ 70 | RMP (Basic) or SMP | 0.45-0.60 mm |
| Semi-industrial town | 10-25 km | Medium | AZ 70 or **AZ 150** (for a longer life) | RMP/SMP | 0.50-0.60 mm |
| Coastal city / Salt Air | <10 km | High | AZ 150 (Essential) | SMP | 0.50-0.60 mm |
| Chemical/Industrial exposure | Any | High | AZ 150 + Excellent Installation | SDP (Best Durability) | 0.55-0.60 mm |
| Premium Building / Long Life | Any | Medium–High | **AZ 150** (or AZ 200) | SDP | As needed for structure |
Paint Type Descriptions:
The most basic and budget-friendly paint. Provides standard colour and protection.
Better than RMP, with improved colour retention and stronger resistance to UV light.
The best choice for durability in harsh climates (lots of sun or chemical exposure).
4. Examples from the Market: Popular Indian Brands and Availability
When purchasing metal roofing, here are some common brand names in the Indian market associated with the different AZ coating standards:
AZ 70 GSM Range
- AMNS
- Indradhanush by JSW
- Colorshine
- Jindal Sabrang, and others
AZ 150 GSM Range
- TATA BlueScope Prisma
- JSW Coloron+
- Apollo, and others
Emerging Standard: Some companies are introducing **AZ 200 GSM**. However, availability may be limited in retail and is typically reserved for specialized, extremely long-life or coastal projects.
5. Protecting Your Investment: Cost and Quality Factors
While AZ 70 is cheaper initially (initial price), AZ 150 often saves money over the full lifespan of the building (lifecycle cost) in tough environments.
5.1 The Long-Term Value of AZ 150
- Much lower chance of rust starting early at cut edges and where screws are placed.
- Fewer expensive repairs and repaints needed over the building’s life.
- Helps maintain the building’s value for selling or leasing.
5.2 It’s More Than Just the Coating Thickness
A strong roof needs quality in every part. Only upgrading the coating without checking these factors can still lead to failure.
| Factor | Why It Matters |
|---|---|
| Steel Thickness (BMT) | Thinner sheets bend easily, which can crack the paint layer at stress points. **Avoid underspecifying BMT** just to upgrade the coating. |
| Fasteners & Screws |
Screws must be designed to work with the metal sheet. Using the wrong kind of metal screw can cause a fast, local type of rust called galvanic corrosion. |
| Design & Drainage | Good design prevents water from sitting on the roof. Proper sealing and drainage are vital, especially around edges and overlaps. |
| Installation Quality | Correct screw tightness and removing all metal filings (swarf) after cutting are critical. Poor installation creates weak points for future rust. |
| Maintenance | In harsh areas, periodic washing helps remove corrosive debris and pollutants that stick to the roof. |
6. Final Summary
Choosing between AZ 70 and AZ 150 is a decision based on your location, not just the upfront price. For mild inland areas, AZ 70 is economical and works well. However, for coastal areas, high rain, or pollution, AZ 150 is the correct choice. Always make sure the coating is balanced with the right steel thickness (BMT), the correct paint type, and high-quality installation.