By definition, a kitchen island is a stand-alone cabinet not connected to a wall and accessible on all sides. Adding a sink to the kitchen island increases the functionality of the unit but adds to the difficulty of the project. The drain of the sink must connect to a rooftop vent. This vent pipe is commonly encased in a wall, which may be difficult to reach from an island sink. The project requires extensive work with 1 1/2 PVC pipe and fittings but can be accomplished by most do-it-yourselfers.
Connect a T-fitting to the drain trap of the sink. This can be connected directly to the outlet of the trap, or a short section of straight PVC pipe can be installed between the trap and the T-fitting. The openings of the T-fitting are oriented straight up and down.
Connect the down end of the T-fitting to the plumbing drain pipe below the floor. This will serve as the waste water drain for the sink.
Cut and fit a piece of PVC pipe to the top opening of the T-fitting that extends above the bottom surface of the sink. This pipe begins the loop that ultimately connects to the roof vent.
Add an elbow, a short segment of PVC pipe and another elbow to the pipe extending from the top of the T-fitting. Add another piece of PVC pipe to the second elbow. This piece will extend to just below the floor level of the kitchen.
Add a T-fitting to the bottom of this pipe end below floor level. The pipe will attach to the top opening of the T-fitting, with openings pointed down and horizontally.
Connect the bottom opening of the T-fitting to the main waste water drain pipe. This connection will likely be just a few inches from the main connection from the sink made in Step 2. This pipe allows any waste water that inadvertently enters the vent pipe to drain away rather than accumulate in the vent pipe.
Connect a pipe to the horizontal opening of the T-fitting. This pipe will run to the nearest wall space, where a vent pipe is then extended up through the wall space, attic and roof to create the vent for the island sink.
Cut and test-fit the pieces together before gluing the PVC pipe connections. Measure each pipe, as the lengths of the components will vary depending on the dimensions of the island cabinet, size of the sink and the location of the waste water pipe. Once all components test-fit properly. use PVC pipe glue at each junction and finish the project.
Plan the plumbing for the sink so all components are accessible for repairs if needed. Also confirm that the pipes won't interfere with any drawers in the cabinet.