When a sewer backs up, the "low man on the totem pole" will flood first, then as the water level in his basement rises, the next lower house will start to fill up. It is strictly a matter of elevation. I once had a customer whose finished lower level started to flood about Midnight. After some diagnosis, I decided it was a city problem and called the emergency number. We finally determined that a manhole which was buried under the blacktop was the part of the solution because there was about a square mile of sewer backed up. My customer was one of the few with a basement in the area, and also the lowest house. IF the city had not removed the stoppage, my customer would have been the recipient of ALL the water used for showers and sinks in the area the next morning. After chopping out the blacktop and beating on the manhole cover to loosen it, a "million" cockroaches came scurrying out of the manhole and we jumped up on our trucks to let them go by.