Tree routing (TR) is a well-known algorithm for static environment surveillance in ZigBee networks because there are no routing table or route discovery requirements. However, TR only uses parent-child link to relay packets. Parent-child relaying requires more hops and energy expenditure when both source and destination nodes are close but belonging to different tree branches. In addition, nodes close to the coordinator may relay more packets and lead to more congestion and higher energy consumption. This is termed the single point of failure (SPOF) problem. In this paper, we propose the Relieving SPOF Tree Routing (R-SPOFTR) algorithm to provide shortest routing and congestion avoidance. In the R-SPOFTR algorithm, the source can deliver packets to the destination directly from its neighbor nodes with the same tree depth without disturbing upstream nodes. The simulation results show that the R-SPOFTR algorithm is capable of reducing average hop count, shortening end-to-end delay, increasing throughput and prolonging lifetime of tree nodes.