Building elements allow you to model a polygon-based structure in the 2D computational grid. At a minimum, providing both the elevation and density of the building footprints allows the 2D simulation to better account for the obstruction volumes, and dramatically improves the simulated velocities and direction of flow. Adjusting the underlying grid cells for the roughness (Manning’s n) or impermeability associated with the building itself is much less important in a minor inundation scenario.