The two most common types of purlins are C purlins and Z purlins. C purlins are shaped like the letter "C" and are typically used for supporting walls and floors, while Z purlins are shaped like the letter "Z" and are often used for roofing and walls, offering greater strength.

1. C Purlins:
   Shape: Resemble the letter "C", with two flanges and a web. 
   Usage: Commonly used for supporting walls and floors, and also for roofing. 
   Characteristics: Generally lighter than Z purlins, making them suitable for applications where load-bearing capacity is not the primary concern. 
   Installation: Often used in single spans or unlapped continuous spans in multi-bay buildings.

2. Z Purlins:
Shape: Resemble the letter "Z", with flanges that are not the same size. 
Usage: Primarily used for roofing and walls, often in heavy-duty applications. 
Characteristics: Offer greater strength and load-bearing capacity compared to C purlins. 
Installation: Allow for lapping during construction, which can increase rigidity and stability.