ASTM A653 Galvanized Steel (GI): Grade Designations, Coating Weights & Supplier Checklist

ASTM A653 (GI) — Designations, Coatings & What to Ask Suppliers

Engineering Breakdown for Roll Forming & Structural Applications

ASTM A653 is the primary North American specification governing hot-dip galvanized steel sheet in coil form.

It defines:

• Mechanical grade (G250, G350, etc.)

• Coating weight (G60, G90, etc.)

• Coating type

• Surface condition

• Chemical and mechanical tolerances

For roll forming manufacturers, misunderstanding A653 designations can result in:

• Unexpected springback

• Edge cracking

• Corrosion failure

• Warranty disputes

• Structural non-compliance

This guide explains how to properly interpret ASTM A653 and what must be specified when purchasing galvanized coil.

1️⃣ What Is ASTM A653?

ASTM A653 is the standard for:

“Steel Sheet, Zinc-Coated (Galvanized) or Zinc-Iron Alloy-Coated (Galvannealed) by the Hot-Dip Process”

It applies to:

• Structural coil

• Commercial coil

• Forming coil

• Roofing coil

• Stud & track material

• Purlins and deck

The standard controls both substrate properties and metallic coating weight.

2️⃣ Mechanical Grade Designations

ASTM A653 uses two main classification systems:

2.1 Structural Steel Grades

Grade	Minimum Yield Strength
Grade 33	230 MPa
Grade 37	255 MPa
Grade 40	275 MPa
Grade 50	345 MPa
Grade 80	550 MPa

These are commonly referenced in ksi (33 ksi, 50 ksi, etc.).

Grade 50 (approx. 345 MPa) is equivalent to G350 in many global markets.

2.2 Forming & Commercial Grades

• CS (Commercial Steel)

• FS (Forming Steel)

• DDS (Deep Drawing Steel)

• EDDS (Extra Deep Drawing Steel)

These grades prioritize ductility rather than strength.

Important:
Structural grades are required for purlins and load-bearing sections.

3️⃣ Coating Weight Designations

Coating weight is expressed in oz/ft² total both sides.

Designation	Total Coating Weight	Approx. g/m²
G30	0.30 oz/ft²	~90 g/m²
G60	0.60 oz/ft²	~180 g/m²
G90	0.90 oz/ft²	~275 g/m²
G115	1.15 oz/ft²	~350 g/m²

G90 is the most common roofing and structural coating in North America.

4️⃣ What G90 Actually Means

G90 = 0.90 ounces of zinc per square foot (total both sides).

This does NOT mean:

• 0.90 per side

• 90 microns thickness

It is total coating mass distributed across both surfaces.

4.1 Why Coating Weight Matters

Higher coating weight:

• Increases corrosion life

• Improves cut-edge protection

• Supports longer warranty

But:

• Slightly increases cost

• May affect formability slightly

• Increases coating thickness at bends

5️⃣ Galvanized vs Galvannealed (A653)

ASTM A653 covers both:

5.1 Galvanized (GI)

• Pure zinc coating

• Better corrosion resistance

• Used in roofing and structural framing

5.2 Galvannealed (GA)

• Zinc-iron alloy surface

• Better paint adhesion

• Used in automotive panels

Galvannealed is rarely used for roofing roll forming.

6️⃣ Forming Behavior in Roll Forming

ASTM A653 material forming behavior depends on:

• Grade (strength)

• Coating thickness

• Slit quality

• Base metal thickness

6.1 Springback

Higher grade (Grade 50 / 80):

• Increased springback

• Greater overbend required

• More passes recommended

6.2 Edge Cracking

High tensile structural grades combined with:

• Slit edge burr

• Tight bend radius

• Heavy gauge

Can cause flange splitting.

6.3 Coating Cracking at Bends

Zinc coating may crack on tight radii.

However, sacrificial protection continues unless:

• Bend radius extremely tight

• Coating excessively thick

• Slit edge severely damaged

7️⃣ Thickness Tolerance

ASTM A653 defines:

• Thickness tolerance

• Width tolerance

• Camber limits

Important for roll forming dimensional consistency.

Tolerance variation affects:

• Profile symmetry

• Oil canning

• Punch alignment

8️⃣ Surface Finish Designations

A653 allows for:

• Regular spangle

• Minimized spangle

• Zero spangle

• Skin-passed surface

Surface finish affects:

• Paint adhesion

• Aesthetic appearance

• Oil canning visibility

9️⃣ Corrosion Performance

Typical service life estimates (G90):

Environment	Approx. Life
Rural	20+ years
Urban	15–20 years
Coastal	5–15 years (depending on salt)

Higher coating weights extend lifespan.

🔟 What to Ask Suppliers (Critical Checklist)

When purchasing ASTM A653 coil, always request:

1. Grade (e.g., Grade 50, 33, 80)

2. Coating designation (G60, G90, etc.)

3. Base metal thickness

4. Mill test certificate (MTC)

5. Yield and tensile results

6. Coating weight verification

7. Slit edge or mill edge condition

8. Surface condition (skin passed?)

9. Country of origin

10. Compliance confirmation with ASTM A653 latest revision

1️⃣1️⃣ Buyer Strategy (30%)

When G60 Is Acceptable

• Interior applications

• Stud & track

• Low corrosion exposure

When G90 Is Required

• Roofing

• Exterior structural

• Commercial buildings

When Higher Than G90 Is Needed

• Coastal exposure

• Long design life

• Industrial zones

Common Buyer Mistakes

1. Ordering Grade 33 instead of Grade 50 for purlins

2. Confusing coating weight with thickness

3. Ignoring slit edge quality

4. Not verifying mill certificates

5. Using G60 in coastal roofing

6. Assuming ASTM A653 equals Galvalume (it does not)

1️⃣2️⃣ Machine Design Implications

Switching from:

• Grade 33 → Grade 50

• Grade 50 → Grade 80

Increases:

• Springback

• Tooling stress

• Shaft load

• Gearbox torque

High-strength A653 material requires structural roll forming platforms.

6 Frequently Asked Questions

1. What does G90 mean in ASTM A653?

It means 0.90 ounces of zinc per square foot total coating weight (both sides combined).

2. Is Grade 50 the same as G350?

Yes, approximately 345 MPa yield strength equivalent.

3. Is ASTM A653 the same as Galvalume?

No. A653 covers galvanized steel, not aluminum-zinc coated steel.

4. Can G60 be used for roofing?

It can, but G90 is typically recommended for exterior roofing durability.

5. Does coating weight affect roll forming?

Yes. Thicker coatings may crack at tight bends and slightly change forming behavior.

6. Should burr height be controlled in A653 coil?

Yes, especially for structural grades to prevent edge cracking.

Final Engineering Summary

ASTM A653 governs both mechanical strength and zinc coating mass.

Correct specification must define:

• Structural grade

• Coating designation

• Thickness

• Edge condition

• Surface type

Misunderstanding these variables leads to:

• Structural failure

• Early corrosion

• Production instability

• Warranty disputes

Proper supplier communication and specification control protect both forming performance and long-term durability.