C Purlin vs Z Purlin: How to Choose the Right Section

Choosing between a C purlin and a Z purlin is mostly a question of how the roof members connect over the rafters. The c purlin vs z purlin decision follows the span, the roof load, the wind uplift and the way the roof will be erected, and the final section should be confirmed by the structural engineer.

C purlin vs Z purlin: how the two sections differ

Both are cold-formed from thin steel strip, and both carry the roof sheeting between the main frames. The difference is in the shape and in how they are connected.

A C purlin has a symmetrical channel shape with the lips turned inward. A Z purlin has the flanges turned in opposite directions, which lets one purlin nest inside the next at a lap. Both sections are described in general terms in this overview of how purlins are used in roof construction.

Simple span or continuous lap

The practical difference shows up at the supports.

• C purlins are commonly used as simple-span members, one purlin per bay. They are easy to detail and to check, and they suit shorter bays and lighter roofs. They are also common as wall girts.
• Z purlins are often lapped over the rafters so that the roof behaves as a continuous run. Continuity reduces the bending in the middle of the span, so a lapped Z purlin can reach a longer bay with the same section.

A lapped Z purlin needs correct lap length, bolt count and hole position, because the lap is what makes the member continuous. A mistake at the lap defeats the reason for choosing the Z section.

Wind uplift, bracing and fixings

On many roofs, uplift controls the purlin design more than downward load. The purlin, its cleat and the cladding fixings must all resist suction, and the corner and edge zones need more fixings than the middle of the roof. The wider picture is covered in our guide to wind uplift and roof bracing in steel buildings.

Purlins also help to brace the top flange of the rafter. Where the roof relies on that restraint, the purlin line and the bracing must be designed together.

What to confirm before you order

• The bay spacing and the purlin spacing along the slope
• Whether the design assumes simple spans or laps, and the lap length
• The roof load, including any solar panels or equipment hung from the roof
• The wind uplift loads in the corner, edge and field zones
• The sag rods or bracing that hold the purlins in line
• The coating, which should match the site exposure

Coating and fabrication

Purlin coatings are chosen for the exposure and the service life. At Truong Thinh, C and Z purlins are hot-dip galvanized, and the reasons to specify galvanizing are explained in our guide to hot-dip galvanized steel structures. If you compare steel purlin manufacturers before settling the c purlin vs z purlin choice, ask which section sizes and thicknesses they roll, how the holes are punched, and how bundles are labeled so that each purlin reaches the right bay. Our page on C and Z purlin supply lists what we can quote.

If you are specifying roof purlins, share the building span, bay spacing, roof slope and site wind speed, and discuss your structural design requirements with our engineering team.

Frequently Asked Questions