In this paper, we solve the problem of relay robot placement in multi-robot missions to establish or enhance communication between a static operator and a number of remote units in an environment with known obstacles. We study the hardness of two different relay placement problems: 1) a chain formation of multiple relay robots to transmit information from an operator to a single unit; and 2) a spanning tree of relays connecting multiple remote units to the operator. We first build a communication map data structure from a layered graph that contains the positions of the relays as the unit moves. This structure is computed once and reused throughout the mission, significantly reducing plan re-computation time when compared to the best-known solution in the literature. Second, we create a min-arborescence tree that forms a connected component among the operator, relays, and units, and that has an optimal communication cost. Finally, we validate our ideas through software simulations, hardware experiments, and a comparison of our approach to state-of-the-art methods.