Powder particles are projected from a thermal spray gun towards substrates to generate protective coatings. A clear understanding of the dynamic impingement when droplets make contact with substrates is critical for controlling and optimizing the thermal spray process. A droplet impingement model is developed to simulate the transient flow dynamics during impact, spreading and solidification. The volume of fluid surface tracking technique is employed within a fixed Eulerian structured mesh. The numerical model is validated with experimental data from tin droplet measurements. The results prove that thermal contact resistance is the key element in characterizing the substrate surface roughness for impingement modelling. It is found that spreading, solidification and air entrapment are closely related to surface roughness.