a word of warning, in 1999 I built an addition with a bathroom, kitchen and laundry room. They are all vented appropriately and have worked fine for years but lately the washing machine drain is causing "glugging" in the kitchen sink. I opened the wall behind the washer to confirm a vent, check. I went on the roof to confirm the vent was clear, check. If I had it to do again I would separate the washing machine and make it 2" instead of 1.5". I'm still trying to figure this thing out.