In this paper, an efficient approach to model the diffuse scattering in the ultra wide band (UWB) indoor channel is presented. It extends 3D ray tracing by associating a scattering cluster with each interaction point of the ray. The scattering clusters were defined by examining the simulated responses of each object while taking into account its surfaces roughness or small details. The comparisons with measurements performed in a residential area show that spatio-temporal channel characteristics like delay and azimuth and elevation angular spreads were better predicted when scattering was added into simulations.