This paper proposes the idea of emulating fat-trees in overlays for multi-source multicast applications. Fat-trees are like real trees in that their branches become thicker the closer one gets to the root, thus overcoming the “root bottleneck” of regular trees. We introduce FatNemo, a novel overlay multi-source multicast protocol based on this idea. FatNemo organizes its members into a tree of clusters with cluster sizes increasing closer to the root. It uses bandwidth capacity to decide the highest layer in which a peer can participate, and relies on co-leaders to share the forwarding responsibility and to increase the tree’s resilience to path and node failures.