80 year old/gut renovated 6 years ago, 7 family house in Gowanus Brooklyn. Extreme back flow problems in the basement whenever there are extreme rains. I understand our proximity to the Gowanus Canal is a major part of the problem, however our neighbors building does not have any back flow problems. The roof drain is not connected to the house trap. We have gone through 5 plumbers and none have been able or seemingly willing to help. There is a check valve in the main sewer line, but it is obviously not working or is not capable of handling the volume of water coming out. I have a bathroom in a finished basement that back flowed for 20mins yesterday. There is a check valve in the main house trap, but it is either not sufficient or it doesn't work. Is a backflow preventer more reliable?