The complex nature of cold-formed roof purlin-sheeting system has always been attracting research attention. Roof sheets, once been installed, will increase the loading capacity of purlin by providing both lateral and rotational restraints. While the lateral restraint is usually considered to be fully effective, the flexible rotational restraint always plays a significant role on the buckling behaviour of the attached purlin. Existing prediction of rotational restraint provided in EC3 is mainly based on experimental data for zed and channel sections. A series of rotational restraint tests on sigma purlin sections with attached sheets have been conducted, to which the results have suggested a lack of accuracy in EC3 after comparison. The obtained values of rotational stiffness are then used as an input for numerical simulations of full-length sigma purlin-sheeting experiments. Results have indicates that the rotational stiffness makes a significant impact on the ultimate strength of the structure and therefore there is a need to improve the current calculation model in EC3 for more economical design purposes.