Wireless sensor networks are envisioned to support an increasing number of applications having various quality of service (QoS) requirements. A possibility to provide a large variety of QoS is to deal with multi-stack WSN where several routing and MAC protocols coexist in the same network. However, routing loops can occur when several routing protocols are used alternatively. Such loops can yield to large delays and high packet loss, and should therefore be avoided. In this paper, we propose a three-step solution to solve the loop problem. The first step describes a sufficient condition to determine when two arbitrary routing protocols can be used without producing loops. The second step states that loops can be avoided if some nodes refrain temporarily from sending a packet. The third step proposes a mechanism that guarantees that no loops are produced for any pair of routing protocols. Our solution is proved through theoretical analysis, and its performance is evaluated through extensive simulations. It requires a limited energy overhead and limited computation capabilities for the network devices. Index Terms—Routing protocols, multipurpose wireless sensor networks, routing loops.