I find sinks that are not vented property all the time. When you talk to the home owner they will be the first to tell you that the sink gargles and gulps and drains fast then slow. Remember for water to leave your sink it needs to draw in air. If the vent is not installed correctly it will cause drainage issues. Island sinks are a special problem because there is no place for the drain vent. No one wants a vent pipe coming up through their granite counter-top. To solve this problem, make a loop
vent in the island. Create the beginning of the drain and loop vent by test-fitting a drain trap, waste T, two 45º elbows, and a 90º elbow, 
linking them with 2" lengths of pipe. Measure the width of the loop between the center-points of the fittings. The top of the loop should be as high as possible under the sink. Both legs of the loop attach to a drainage line under the floor, but one leg has a vent tee installed immediately under the floor and above the drainage line. Attach your horizontal vent line to this tee and run it to the nearest vent. You need your horizontal vent pipe to rise 1/4 inch per foot from the vent tee to the vertical vent, and any horizontal drain pipe to fall 1/4 inch per foot. Consult your local plumbing code for specific municipal regulations.www.homeinspectionservices.org
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