Hopefully someone in this forum can help answer this question. I live in a three story condo, on the second floor. Twice since I have lived here, my kitchen sink has filled up with dirty dishwater, apparently from my neighbor upstairs. This last time, the backup occurred when I was out, and it spilled out of my sink and all over my kitchen floor, and leaked through the floor to the first floor as well! This is no ordinary tap water; it was filled with food particles and soap. This flooding occurs when my upstairs neighbor drains her sink (after washing dishes) and runs the garbage disposal. I also have a garbage disposal in my unit, but my sink has been empty and disposal clear of food each time this has occured.

Why does the water come into my sink, and then fail to drain? Is it because the pressure of her disposal? All I can think is that maybe her disposal "forces" the water up my sink, and then all the food garbage in the water subsequently blocks my drain, causing the water to back up and flood my kitchen. The condo association plumber said that nothing is wrong with the pipes in the building, and suggested I get a "backflow preventer valve". Is this adequate to prevent this from happening again?

Thanks in advance for any help you can provide!!