My drain in the basement backs up frequently while doing the laundry, flushing toilet, tub drain, etc. I have had the drains snaked, and even purchased my own, but the problems keep coming up. I know I have a some type of back flow prevention because when the city’s lines were backed up to my house I did not get water in my house, but none would go out either. When I snake a clog it is usually cleared before the snake gets outside of my house, leading me to believe the problem is UNDER my basement floor. The city has ran a video on my outside lines from my house to the main sewer line, for which I am waiting to view. Is it possible that my backflow valve has a defect preventing my drains to drain outside of the house, causing it not to open, but stay in my house and come out of my trap? I plan on having someone run a video cam from inside of my home to the outside, but right now anyones thoughts are appreciated. I realize the video will tell me what I will most likely need, I am just trying to gather thoughts if the video does not give any results.