The understanding of tomograms can be a difficult task when the observed scene is complex. The Synthetic Aperture RADAR (SAR) ranging acquisition induces geometrical distortions depending on the local slope of the back-scattering surface. Over dense urban areas, this particular geometry produces tilted facades instead of straight buildings. Moreover the high dynamic in SAR images of urban areas may lead to 3-D representation with very bright voxels eclipsing other echoes. In this paper we present a graph-cut approach adapted to any tomographic reconstruction technique to segment the urban surface in a 3-D tomographic reconstruction. Results on real data from a stack of 40 TerraSAR-X images are presented.