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Until nowadays, surface roughness effects were ignored in the analysis, due to the difficulty to generate a rough surface model and also to simplify the model in order to reduce calculation time. However, many engineering fields, such as MEMS, seek to improve the behaviour of the system at the surface level or the interface between surfaces. Thus, with the advance of numerical capabilities, the topography of the surface can be included in finite element simulations. This paper presents two methods for generating rough surfaces, one using the real shape with an original reverse engineering method and the other one by using a parametric design language to generate a normally distributed rough surface. As an application to demonstrate the power of these methods, we choose to predict by simulation the electrical contact resistance and the real contact area between rough surfaces as a function of the contact force. This application is a major concern in RF MEMS ohmic switches and shows an original approach to extract a guideline in choosing a design, materials and process flow to minimize the contact resistance. The agreement between the numerical model and an analytical model is very good and validates this novel numeric approach.