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
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
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Not specifying BMT
Measuring only total thickness
Confusing coating mass with coating thickness
Assuming nominal means minimum
Not including tolerance in contract
Clarity prevents disputes.
Yes.
Not necessarily.
No, use base metal thickness.
No, unless coating is removed or special gauge used.
Unclear specification of BMT vs total thickness.
Yes.
Always.
No.
Yes, within coating mass tolerance.
Specify base metal thickness and coating separately.
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.
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