The purpose of the invention is to prevent the blockages that occur in the general drainage traps of buildings. The invention consists in connecting a conduit by positioning it in the inlet part above the trap water float level (M) and in the outlet par below a line (N), thereby forming a new auxiliary trap. The invention is suitable for use in traps in buildings which are connected to the common sewerage system. The invention provides a technical solution that prevents the obstruction of the trap by means of an alarm which is installed in the conduit wherein the water is diverted due to gravity when the conduit is obstructed. Said alarm signals any obstruction in the conduit so that it can be cleared without waiting for the consequences of such a situation to occur. Other versions of the invention consist in installing said conduit in the upper or lateral part of the trap, on the connection points indicated. More specifically, the two-way trap provides an inlet pipe (22) and an outlet pipe (24). A general trap section (26) is attached to and disposed between the inlet pipe (22) and outlet pipe (24) such that when filled with water to an equilibrium water level (30) equal to the lowest point of the outlet pipe (24), air does not freely communicate between the inlet pipe (22) and the outlet pipe (24). An auxiliary trap section (28) is attached to either the inlet pipe (22) or the general trap section (26) at a point above the equilibrium water level (30) at a first end and attached to the trap section at a point below the equilibrium water level (30) at a second end, such that if the general trap section (26) became blocked water would flow through the auxiliary trap section (28) yet air would not freely communicate between the outlet pipe (24) and the pipe (22).