How to Do Plumbing for an Island in a Kitchen

Measure the cutout needed for the prep sink in the island counter top. Use a circular saw with a diamond-tip blade, if the countertop is of granite or stonelike material. Cut the four sides for the sink, trimming off corners at an angle. Lift out the countertop material with a suction handle. Do some final grinding with the saw blade to smooth all of the edges and cut in sharp corners for the sink space.

Lower the sink into the prepared space. Drop a plumb bob through the drain hole to find the exact place to cut through the bottom of the cabinet. Mark the spot where you will cut the drain hole using a 2-inch hole saw. Cut this hole to open into basement space under the island. Cut carefully to avoid damaging existing copper lines and drain lines in the basement or crawlspace.

Cut off the water supply or shut-off valves before tapping into hot and cold water lines. Use a small pipe cutter to make cuts on the existing copper water lines, cutting out a small section to allow room for a T-joint on each line. Sand the pipe’s exterior to make it smooth for applying soldering flux. Apply the T-joint with the leg of the "T" pointing up to join the new pipe. Solder the joints and prepare to fit the drain pipe for the island sink.

Cut into the drain pipe about 1 foot away from the existing drain in the basement space. Use a Y-assembly to hold the new sink drain to the drain in place. Brush the PVC drain lines with PVC primer and PVC cement. Slip on a rubber sleeve over the joint and tighten mission clamps over the assembly. Attach a copper strap to the adjacent floor joists to hold the new drain in place.

Drill water lines under the island cabinet using a 2-inch hole saw. Connect water lines in the sink to the basement water lines. Cut a drain space through a hole in the cabinet bottom as well. Use PVC cement to glue the drain pipe assembly together after you fit them in a "dry run" to check everything. Mark all of the drain pipes in their proper positions, using a marking pen to denote where they each should align. Don't glue them yet. Take them apart and then glue them together in the correct places, lining up your markings, to fit the complete, new drain system.

Fit copper water pipes together for the island by using pre-soldered fittings. Add heat from the blow torch to secure them. Cut off the water lines to slip on shut-off valves.

Attach all fittings to the sink before you lower it into the island. Use plumber’s putty to seal the basket to the sink. Install the faucet and drop water lines through the sink hole. Tighten water lines with a plastic socket wrench and attach the sprayer hose. Use silicone sealant around the edge of the sink and tighten clips to secure the sink in place. Slip on the P-trap and connect the drain system. Snug up all the water lines and turn on the water.