I live on the first floor of a four story 80 year old Brooklyn apartment building with (most of) the original 2 inch cast iron drain and vent pipes. Twice in the last three months and once two years prior an obstruction (or gunk buildup) has stopped up the kitchen drain line, somewhere between the second floor and the basement. This seems to happen when residents above the second floor use their dishwashers. The soapy water hits the obstruction or gunk-narrowed section of pipe and reverses flow back up the drain line and then out of the second floor sink and dishwasher. Twice now the water has filled the second floor sink and dishwasher, overflowing onto her floor and down to my ceiling causing significant damage. My first conclusion was that the obstruction must be between my floor and the floor above it, since waste water has not come up into my sink. However, it turns out that my kitchen, for whatever reason, is not connected to the kitchen drain line but the vent line. This means that the obstruction or built-up section(s) of pipe can be anywhere from the basement where it meets the sewage line to the second floor. A plumber noticed two 45 degree elbows connected to the drain pipe right above the basement ceiling. He thinks that this is a likely area of congestion. However, he can not be certain and says that even if he replaces this section of pipe the area prone to these obstructions could still be above it. It would be hard to tell visually and the pipe is too small for a camera. Since this has happened twice over the period of a few months we must find a solution. What should we do? Replacing the entire drain pipe from the second floor down would seem to be the most thorough course of action but it would entail major expense and inconvenience. Periodic snaking of the lines may lead to further damage. Bio-Clean in a multi-unit apartment building sounds tricky. Replacing only some of the pipe sounds like a gamble. What do you recommend? All help is greatly appreciated.