I couldn't get behind the washing machine/dryer stack but, there was no visible vent stack from the roof. This was an addition which had a different roof than the main home, no stack visible. On the main home, where the kitchen is located, the sewer stacks vent up on the outside of the home on the opposite side of the home from the kitchen. Old school cast iron type stacks. At first I thought that the pipe may be for the dishwasher(air gap), but again, I didn't see a vent stack through the roof or running up the side of the home.