The study evaluates the Rain on Grid (RoG) hydraulic model’s sensitivity to Digital Elevation Model (DEM) resolution when simulating an extreme flood in Slovenia. The RoG model is validated against a high-resolution benchmark, showing strong agreement with a Kling-Gupta Efficiency of 0.913 and Pearson correlation of 0.964 for a 1 m DEM. Differences are observed in peak shapes and concentration times, attributed to rainfall propagation in RoG grids. DEM resolution significantly impacts performance, with the largest decrease between 1 m and 5 m resolutions. Coarser DEMs yielded higher depths, indicating slope decreases and terrain smoothing. The study concludes that high-resolution DEMs (<1m) are needed for adequate RoG performance, while commercially available coarser DEMs (30m) degraded accuracy and should be avoided using this method. Differences from semi-empirical concentration time models are also discussed, and an emphasis is given also on the impacts on water velocity and numerical stability.