Surface roughness plays a very important role in particle deposition and coagulation, especially in the presence of repulsion forces. A simple model of rough surfaces is proposed in this work for evaluating double-layer interactions. The model allows estimation of the tangential forces between two surfaces by taking into account the interaction of their asperities. Analytical and numerical methods are examined for the solution of the mathematical problem. Their accuracy, stability, and computational efficiency are investigated over a large range of model parameters. The influence of these parameters on the repulsion energy is studied for the case of constant surface potential and asperities smaller than double-layer thickness. The results of the simple model can be generalized, at least qualitatively, for arbitrary surface roughness.