I live in a two story co-op and occasionaly when my upstairs neighbor runs his washing macine soap and water back up into my sink. No one knows what to do and a few tried to fix what they thought was the problem but nothing worked. Any ideas of what the problem is or anything on how to fix the problem?

You share a common drain pipe with your neighbor upstairs. The drain blockage is either at your level or below. Usually it is where common drain pipe turns from vertical to horizontal and further down the slope. Have it cleaned. Milo

Milos take was bang on. Now how do we fix it? As Milo stated you have a partial blockage downstream from your unit. This blockage can be snaked from one of two places,
1. You can snake from the kitchen roof vent, ( Put out enough snake to reach the base and 16 more feet) **OR**
2. If you have a cleanout tee in the basement you can snake from there. Good luck, Tom

Thanks Tom and Milo I'm going to try that and keep you posted on what happens.

So this backup never shows itself in your tub or shower? You could have more than one drain running in common with your neighbor but it would be unusual. Do you own or rent?

No it does not come up through anything but my sink and I do own it.

OK you own it but can you tell me if the co-op is like a condo home owners association? Is there a management company and monthly/yearly fee?