Purlins are used for two types of building systems: overlapped and sleeved connections. The purlin overlap system is mainly used for long spans in large buildings, as it gives additional strength and stiffness over the rafter area. In the sleeved connection, Z sections are used for buildings which are two or more bays long, and are made using continuous purlin sleeves.
Purlins are created in a variety of ways:
An end extension, where purlin or side rails are extended in length to suit desired overhang. Purlins are normally produced with standard end holes in every purlin, then overhand fixing holes are added to suit individual requirements.
Continuous extension used to achieve desired overhand where overhang is greater than half of standard sleeve length.
Continuous extension where standard sleeve is cut to suit desired overhang.