I recently had my sewer back up for the first time in the 19 years that I have lived in this house. It backed up through the drain in the basement floor (there are no sinks, toilets or any other plumbing fixtures in the basement). The clog cleared itself after a toilet flush - my plumber said that it must have been a temporary blockage, and to keep and eye on it and hope that it doesn't happen again.
My question is this - how did the sewage get up through the drain? Is there some kind of backflow valve under the floor in the pipe that goes from the house to the street sewer?