For a robot to perform a grasping and manipulation task, it has to determine possible robot placements in the workspace, from which target objects or environmental elements relevant to the given task are reachable. This work presents a novel approach for finding placements for the mobile base of a humanoid robot in an unknown environment with multiple support planes. We propose a novel type of reachability map - the Oriented Surface Reachability Map - that takes inclined surfaces in the environment into account and has the same complexity as reachability maps designed for flat surfaces. The resulting robot placements are not limited to SE(2) but can be applied to arbitrarily oriented planes in 3D space. The proposed method was evaluated in simulation and on the humanoid robot ARMAR-6 in real-world grasping experiments. The results show that a placement can be found for over 80% of the poses that are reachable in complicated, simulated environments, with only a small runtime overhead.