Q: In a kitchen I'm building for a client, the dishwasher drain line would need to pass through a drawer cabinet to connect to the sink drain. The waste line for the sink passes under the floor directly below the dishwasher. Is there any problem with running the dishwasher drain directly into the waste line?
A: Mike Casey, a licensed plumbing contractor and co-author of responds: Whenever possible, it is always best practice to connect the dishwasher drain hose to the kitchen sink drain. Most drawer cabinets have a 3-inch to 4-inch space below the drawers, or space at the back of the cabinet behind the drawers, where the hose can be located.
Nothing in the code specifically prohibits you from connecting a dishwasher drain directly into a waste pipe below the floor, but doing so creates many potential problems. First, the drain line should always have a high loop (at least 20 inches above the finished floor) to prevent waste water from backing into the dishwasher, so you would need to loop the drain hose over the dishwasher and back down to the waste line. Second, the drain must connect to a proper fitting with a trap that prevents sewer gases from backing into the dishwasher. This trap arrangement already exists at the kitchen sink, but it would need to be duplicated if you made a separate connection directly into the waste line. Finally, when a drain connects to a waste line below a point that is 20 inches from the kitchen floor, an air-gap device—which must be installed above the rim of the sink—is required for the drain.
So again, while there is nothing in the code that says you can't run the drain directly into the waste line, it is almost always easier and certainly less complicated to connect the dishwasher drain hose to the drain at the kitchen sink.