Coating Adds Thickness — Why “Nominal” Causes Steel Coil Disputes

One of the most common disputes in steel coil purchasing begins with this sentence:

“We ordered 0.60 mm. This measures 0.63 mm.”

Or worse:

“This measures 0.57 mm. It’s under thickness.”

The problem often is not the steel.

The problem is misunderstanding what “0.60 mm” refers to:

• Base metal thickness (BMT)?

• Total coated thickness (TCT)?

• Nominal thickness?

• Minimum guaranteed thickness?

Coatings such as zinc and paint add measurable thickness.
If buyers and suppliers are not aligned on definitions, disputes are inevitable.

This guide explains:

• How coating adds thickness

• What nominal really means

• Where disputes come from

• How to specify correctly

• How this affects roll forming and structural performance

1. What Is Base Metal Thickness (BMT)?

Base Metal Thickness (BMT) refers to the steel thickness before coating is applied.

This is the true structural thickness.

If you specify:

0.60 mm BMT

You are referring to the steel core only.

Zinc or paint layers are not included.

BMT is what determines:

• Load capacity

• Section modulus

• Forming force

• Springback

2. What Is Total Coated Thickness (TCT)?

Total Coated Thickness includes:

Base steel

• Zinc coating

• Paint (if prepainted)

Example:

• Base metal thickness: 0.60 mm
• Zinc coating may add ~0.01–0.02 mm
• Paint system may add ~0.02–0.04 mm

Total coated thickness may measure:

0.63–0.66 mm

If buyer measures with micrometer across full thickness, they measure TCT — not BMT.

This causes confusion.

3. What Does “Nominal Thickness” Mean?

Nominal thickness is the target thickness.

It is not a guarantee of exact measurement.

Example:

Nominal: 0.60 mm
Tolerance: ±0.03 mm

Acceptable BMT range:

0.57–0.63 mm

If coating adds thickness, micrometer reading may exceed nominal value.

Nominal does not mean exact.

4. Zinc Coating Thickness Contribution

Zinc coating mass is specified in grams per square meter (g/m²).

Higher coating mass generally increases coating thickness.

For example:

Z275 coating may add roughly 0.02 mm combined both sides.

However, coating thickness is not perfectly uniform.

Zinc forms alloy layers and surface texture.

Measuring with standard micrometer measures steel + coating.

Special tools are required to isolate BMT.

5. Paint Adds Additional Thickness

Prepainted coil includes:

• Primer layer
• Top coat
• Back coat

Combined, paint layers can add:

0.02–0.05 mm

In thin gauge roofing (0.40–0.60 mm), this is significant.

If someone expects:

0.60 mm total

But receives:

0.60 mm BMT + paint

Measurement appears higher.

6. The Most Common Dispute Scenario

Buyer orders:

0.60 mm galvanized coil

Supplier supplies:

0.60 mm BMT, Z275

Buyer measures:

0.63 mm with micrometer

Buyer claims:

“Over thickness.”

Supplier responds:

“Base metal is 0.60 mm within tolerance.”

Both may be technically correct — but specification was unclear.

7. Structural Risk of Confusion

If structural design requires:

Minimum 0.60 mm steel

And supplier delivers:

0.57 mm BMT within tolerance

But coating raises total thickness to 0.60 mm

Micrometer may show 0.60 mm total, but structural steel is only 0.57 mm.

This reduces structural capacity.

Design must reference BMT — not total thickness.

8. Roll Forming Impact

Forming force depends on:

Base metal thickness

Not coating thickness.

However, coating thickness affects:

• Surface friction

• Tool clearance

• Punching burr formation

Incorrect assumptions about thickness alter:

• Roll gap settings

• Shear blade clearance

• Punch clearance

Production instability may follow.

9. Micrometer Measurement Trap

Standard micrometer measures:

Total thickness.

To measure BMT only, you need:

Coating thickness gauge (magnetic or eddy current method)

Without proper equipment, field measurements may be misleading.

This fuels disputes.

10. International Market Differences

In some markets:

Thickness quoted refers to BMT.

In others:

Thickness quoted refers to total thickness.

Roofing markets vary by region.

Never assume universal interpretation.

Clarify in contract.

11. Minimum vs Nominal Language

Two different approaches:

Nominal thickness: 0.60 mm ± tolerance
Minimum thickness: not less than 0.60 mm

Minimum specification costs more because:

Mill must control thickness tighter
Average thickness must increase

If you need guaranteed minimum thickness, specify it clearly.

12. How to Write Proper Specification

Professional example:

• Base metal thickness: 0.60 mm
• Tolerance: ±0.03 mm
• Coating: Z275
• Surface: Minimized spangle
• Total coated thickness for reference only

This eliminates ambiguity.

13. Why Coating Thickness Is Not Structural Thickness

Zinc coating:

Provides corrosion resistance.

It does not significantly contribute to:

Structural load capacity
Section modulus

Design engineers calculate based on BMT.

Using total thickness in structural calculation is incorrect.

14. Common Buyer Mistakes

1. Not specifying BMT

2. Measuring only total thickness

3. Confusing coating mass with coating thickness

4. Assuming nominal means minimum

5. Not including tolerance in contract

Clarity prevents disputes.

15. FAQ Section

Does coating increase thickness?

Yes.

Is 0.60 mm galvanized equal to 0.60 mm base metal?

Not necessarily.

Should structural calculations use total thickness?

No, use base metal thickness.

Can micrometer measure base metal only?

No, unless coating is removed or special gauge used.

What causes thickness disputes?

Unclear specification of BMT vs total thickness.

Does paint add measurable thickness?

Yes.

Should RFQ specify BMT?

Always.

Is nominal equal to exact?

No.

Can coating thickness vary?

Yes, within coating mass tolerance.

What is safest contract wording?

Specify base metal thickness and coating separately.

16. Conclusion

Coating always adds thickness.

“Nominal thickness” without clarification leads to confusion.

To avoid disputes:

Specify:

• Base metal thickness

• Tolerance

• Coating mass

• Whether minimum thickness is required

Structural performance depends on steel thickness — not coating thickness.

Clear documentation protects both buyer and supplier.