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

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

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:

Grade (e.g., Grade 50, 33, 80)
Coating designation (G60, G90, etc.)
Base metal thickness
Mill test certificate (MTC)
Yield and tensile results
Coating weight verification
Slit edge or mill edge condition
Surface condition (skin passed?)
Country of origin
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

Ordering Grade 33 instead of Grade 50 for purlins
Confusing coating weight with thickness
Ignoring slit edge quality
Not verifying mill certificates
Using G60 in coastal roofing
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.