The roof beams spanning between the trusses are known as purlins.
A horizontal member in the roof.
Roof sheets are connected with purlins. Purlins are connected with panel points of the truss. There is no direct connection between roof sheathing and truss top chord. Uniformly distributed roof load is carried by purlins. This UDL is converted into point loads acting at panel points of truss.
The load acting at purlins is inclined that is why these beams subjected to bi-axial bending along with torsion.
Reference : Steel structures by Zahid Ahmad siddiqi and Muhammad ashraf