What Steel Grade Should You Use? (G250 vs G350 vs G550 Guide)

Learn how to choose the right steel grade for roofing, purlins and structural deck. Compare G250, G350 and G550 strength and forming impact.

What Steel Grade Should You Use?

Complete Engineering Guide for Roll Forming & Structural Profiles

1️⃣ What Is Steel Grade?

Steel grade defines:

• Yield strength (MPa)

• Tensile strength

• Ductility

• Forming behavior

In cold-formed roll forming applications, grade is typically defined by minimum yield strength.

Common grades:

• G250 (250 MPa)

• G350 (350 MPa)

• G450

• G550 (550 MPa)

Higher number = stronger steel.

2️⃣ Yield Strength vs Tensile Strength

Yield strength = point where permanent deformation begins.

Tensile strength = maximum stress before fracture.

In roll forming and structural design, yield strength is the critical value.

3️⃣ Why Steel Grade Matters

Steel grade affects:

✔ Structural capacity
✔ Deflection
✔ Span performance
✔ Springback
✔ Minimum bend radius
✔ Machine load
✔ Tool wear

Higher strength improves structural performance — but makes forming more demanding.

4️⃣ G250 vs G350 vs G550 (Practical Differences)

5️⃣ For Roofing Profiles

Most roofing panels use:

G300–G550

Why G550 is common in roofing:

✔ Allows thinner material
✔ Maintains structural rigidity
✔ Reduces weight
✔ Improves span capability

However:

High tensile increases springback and forming stress.

For architectural standing seam:

Grade selection affects seam tension.

6️⃣ For Purlins

Common grades:

G350–G550

Higher grade allows:

• Reduced thickness

• Increased span

• Lower weight

However:

High tensile increases punching force and forming load.

Structural engineering must evaluate grade and thickness together.

7️⃣ For Structural Deck

Common grades:

G350
G450

Higher grade increases bending capacity without increasing thickness.

However:

Composite behavior and embossing performance must also be considered.

8️⃣ Forming Impact of Higher Grade

As grade increases:

✔ Springback increases
✔ Forming force increases
✔ Roll pressure increases
✔ Tool wear increases
✔ Risk of edge cracking increases

Machine must be sized correctly.

9️⃣ Minimum Bend Radius & Grade

Higher strength steel requires:

Larger minimum bend radius.

Too tight radius in G550 can cause:

• Cracking

• Coating fracture

• Micro-fracture at edges

Profile geometry must adapt to grade.

🔟 Steel Grade & Developed Width

Steel grade influences:

• Effective forming radius

• Springback compensation

• K-factor assumptions

Higher grade may slightly increase developed width due to radius adjustment.

Small per bend — significant across many bends.

1️⃣1️⃣ When Higher Grade Is Beneficial

Use higher grade when:

✔ Weight reduction important
✔ Long span required
✔ Structural efficiency needed
✔ Wind load high
✔ Section depth limited

Higher strength = more load capacity per thickness.

1️⃣2️⃣ When Lower Grade Is Better

Lower grade is preferable when:

✔ Tight bend radius required
✔ Complex seam geometry
✔ Aesthetic finish critical
✔ Machine is lower capacity
✔ Cost sensitivity present

Mild steel forms more easily.

1️⃣3️⃣ Cost Considerations

Higher grade steel:

✔ May cost slightly more per ton
✔ Allows thinner material
✔ Reduces shipping weight

But:

✔ Increases forming complexity
✔ Increases tooling stress

True cost must consider production impact.

1️⃣4️⃣ Regional Variations

Different countries use different grade standards:

Australia: G250, G350, G550
USA: 33 ksi, 50 ksi, 80 ksi
Europe: S250GD, S350GD, etc.

Always confirm grade equivalence.

1️⃣5️⃣ Machine Selection & Grade

When buying a roll forming machine:

Always specify:

✔ Maximum yield strength
✔ Thickness range
✔ Production speed

A machine designed for G250 may not perform well with G550.

1️⃣6️⃣ Common Buyer Mistakes

❌ Choosing grade without structural calculation
❌ Not matching grade to bend radius
❌ Ignoring springback impact
❌ Not checking machine torque capacity
❌ Assuming higher grade always better

Grade must match application.

1️⃣7️⃣ Practical Selection Guide

Residential Roofing:

G300–G550 (depending on thickness)

Standing Seam:

Often G300–G550 (evaluate forming carefully)

Purlins:

G350–G550

Structural Deck:

G350–G450 typically

Always confirm with structural engineer.

1️⃣8️⃣ Engineering Summary

Steel grade determines:

✔ Structural strength
✔ Span capability
✔ Forming behavior
✔ Springback
✔ Tooling load
✔ Machine requirements

Higher grade improves strength but increases forming complexity.

Correct grade selection balances:

Load + span + geometry + thickness + machine capacity.

FAQ Section

Is G550 better than G350?

Structurally yes, but harder to form.

Does higher grade change coil width?

Slightly, through radius and springback influence.

Should roofing always use G550?

Common, but depends on thickness and forming complexity.

Does grade affect punching?

Yes. Higher grade increases punching force.

Can my machine run higher grade?

Only if designed for required torque and stiffness.

Is higher grade more expensive?

Often slightly per ton, but may reduce thickness requirement.