The design of purlins interacting with roofing systems offers enhanced load resistance over nominal sections. Typically, the roofing systems are of two types: through fastened systems (TF systems) and standing seam metal roofing systems (SSMR systems). The structural behavior of purlins interacting with TF systems is well discussed in literature and the analytical and experimental design methods are available in codes of practice. For SSMR systems, due to the presence of hidden clips, the generic design tool is not available, and often, the codes of practice neglect the beneficial effect. The analytical methods for purlin design provided in the Eurocode is conservative as reported in the literature. The conservatives of the method are approximating the cross-section and relative slenderness. In the design of purlins interacting with SSSMR systems, the effect as provided by the TF systems is not well recognized due to the presence of hidden clips and connection methodologies. The codes of practice account the restraint in the form of 'R' factor through the experimental procedure. The objectives of the study are 1) Develop a new analytical model for predicting the stress distribution and buckling loads. The torsion strength parameter Bimoment is introduced in the analytical method, and the restraint effect is accounted for. 2) To compute the relative slenderness of the purlins interacting with roofing systems accounting for the entire cross-section in the analysis where the Euro code assumes the part of the cross-section. 3) To propose the 'R' factors for enhanced purlin design. To demonstrate the influence of the restraints to the purlins supporting standing seam roofing systems through experiments. To develop the FEM models and parametric study of the various standing seam roof systems with the test variables like clip height and width.