How do I tie my new 2" washer drain into existing 1 1/2" bathroom sink drain under bathrooms? Can I "Y" joint into the sink drain and replace the last section of 1 1/2" with new 2"? Or do I have to increase to 3" from "Y" to the stack? My girlfriend and I just bought a 1968 single story home with a crawlspace and are going to remodel the kitchen, laundry room and bathrooms. We are moving the laundry room from the kitchen to the garage. I have rough framed in a new 7' by 9' room in garage and raised the floor approx 6" to match floor with rest of house. All ready to start putting in the drain system. Washer will be sitting on an exterior wall so I will vent straight up to roof. I plan to run a new 2" drain through the crawlspace across the house and tie into existing drains under the bathrooms, approx 25 ft away so I need about 8 inches of fall. There is a 1 1/2" drain from bathroom sink that is in perfect spot. I'm pretty sure I can "Y" into it easy enough and run new 2" from "Y" to stack, but will 2" be big enough, or do I need to increase it to 3" or whatever? I've already opened the city permit and waiting for approval so it will be inspected, which is a good thing. But I want to do it right so the inspector doesn't have any problems with it. Thanks for any advice you can give me.