How to Specify a Z Purlin (Complete Structural & Machine Guide)

Roof purlins in steel buildings

Complete Engineering & Procurement Guide

Z purlins are primarily used for:

Roof purlins in steel buildings
Industrial sheds
Warehouses
Agricultural buildings
Long-span roof systems

The defining characteristic of Z purlins:

They can lap over each other at supports, creating structural continuity.

This makes them more efficient than C purlins for roof applications.

1️⃣ What Defines a Z Purlin?

A Z purlin is defined by:

✔ Web depth
✔ Flange width
✔ Lip size
✔ Thickness
✔ Steel grade
✔ Overlap length
✔ Punch pattern
✔ Length

Common naming format:

Z200 x 70 x 20 x 2.0

Meaning:

200 mm web
70 mm flange
20 mm lip
2.0 mm thickness

All dimensions must be specified.

2️⃣ Standard Section Sizes

Common web depths:

120 mm
150 mm
200 mm
250 mm
300 mm
350 mm

Common flange widths:

50 mm
60 mm
70 mm
75 mm

Common lip sizes:

15 mm
20 mm
25 mm

Changing flange or lip changes:

Section modulus
Buckling resistance
Load capacity

Never specify only web depth.

3️⃣ Thickness Range

Common thicknesses:

1.5 mm
1.6 mm
1.8 mm
2.0 mm
2.5 mm
3.0 mm

Heavy industrial may go higher.

Machine must be designed for:

Maximum thickness at maximum grade.

4️⃣ Material Grade

Common grades:

G350
G450
G550

G550 is widely used for Z purlins because:

Higher yield strength improves structural performance.

However, G550 increases:

Forming force
Springback
Punch tonnage requirement

Grade must be defined before tooling design.

5️⃣ Coating Specification

Common coatings:

Z275
Z350
Z450

High corrosion environments may require heavier coating.

Indoor applications may use:

Uncoated steel.

Coating affects:

Tool wear
Punch wear
Corrosion life

Always specify coating mass.

6️⃣ Typical Coil Width

Coil width =

Web + 2 flanges + 2 lips + bend allowance.

Example:

Z200 x 70 x 20:

200 + (70 × 2) + (20 × 2) = 380 mm
Add bend allowance → approx. 400–420 mm

Exact developed width must include:

Bend radius
Thickness compensation
Springback correction

Never estimate coil width.

7️⃣ Overlap Design (Critical Difference vs C Purlin)

Z purlins overlap at supports.

Specify:

✔ Overlap length (commonly 10% of span or structural engineer defined)
✔ Bolt hole alignment in overlap zone
✔ Reinforcement requirements

Overlap improves load distribution.

Incorrect overlap design reduces structural continuity.

8️⃣ Punch Pattern Specification

Z purlins often require:

✔ Cleat holes
✔ Bolt holes
✔ Slotted holes
✔ Service holes

Define:

Hole diameter
Hole spacing
Hole edge distance
Tolerance

Punching layout must align in overlapping sections.

Incorrect punch alignment causes installation failure.

9️⃣ Length Specification

Common lengths:

6 m
9 m
12 m
Custom

Overlap requires precise hole alignment between pieces.

Length tolerance typically:

±2 mm

Longer sections require:

Proper support handling.

🔟 Structural Load Requirements

Before selecting Z size, define:

✔ Span length
✔ Wind load
✔ Snow load
✔ Roof pitch
✔ Deflection limit

Z purlins are selected based on structural calculation.

Never guess section size.

1️⃣1️⃣ Machine Engineering Requirements

Typical Z purlin line:

16–22 forming stands
80–100 mm shafts
30–55 kW motor
Servo punching system
Hydraulic cut-off

Because Z geometry includes offset flanges:

Roll design must control twist carefully.

Thicker G550 requires strong frame design.

1️⃣2️⃣ Production Speed

Typical speeds:

15–25 m/min

Heavy thickness reduces speed.

Punching cycle may limit line speed.

1️⃣3️⃣ Tolerance Requirements

Typical tolerances:

Web depth ±1–2 mm
Flange width ±1 mm
Twist control critical
Length ±2 mm

Twist is especially important for Z purlins due to asymmetric shape.

Poor twist control causes installation difficulty.

1️⃣4️⃣ Z vs C Structural Comparison

Feature	Z Purlin	C Purlin
Overlap	Yes	No
Span Efficiency	Higher	Lower
Structural Continuity	Yes	Limited
Common Use	Roof	Wall / Short spans

Always confirm whether project requires Z or C.

1️⃣5️⃣ Export Market Considerations

Australia:
High-strength G550 common.

Africa:
G550 widely used.

Middle East:
G350–G550 depending on design.

Europe:
Must align with EN structural standards.

Always confirm local structural code requirements.

1️⃣6️⃣ Common Specification Mistakes

❌ Not defining overlap length
❌ Not specifying thickness
❌ Not defining steel grade
❌ Ignoring punch alignment
❌ Underestimating coil width
❌ Not controlling twist

These mistakes are costly and structural.

1️⃣7️⃣ Developed Width Reminder

Developed width includes:

✔ Web
✔ Flanges
✔ Lips
✔ Bend allowance
✔ Thickness compensation
✔ Springback correction

Incorrect developed width leads to:

Wrong coil purchase
Section dimension errors
Machine overload

Calculation must be precise.

1️⃣8️⃣ Final Z Purlin Specification Checklist

Before tooling or machine approval:

✔ Confirm web depth
✔ Confirm flange width
✔ Confirm lip size
✔ Confirm thickness range
✔ Confirm steel grade
✔ Confirm coating
✔ Calculate developed width
✔ Confirm coil availability
✔ Define punch layout
✔ Define overlap length
✔ Confirm structural load requirement
✔ Confirm production speed target

Only then proceed to production.

FAQ Section

Why use Z instead of C?

Z purlins overlap and provide structural continuity.

Is G550 common?

Yes — especially in roof structures.

How thick can Z purlins be?

Commonly up to 3 mm on light-gauge lines.

Does overlap increase strength?

Yes — if designed correctly.

Is twist a common issue?

Yes — asymmetric geometry makes twist control critical.

Can one machine run multiple Z sizes?

Yes — if adjustable tooling system is designed properly.