Sewer backups develop when something clogs the lines that drain water from your home. This can cause overflowing toilets or make water back up into your bathtub, dishwasher, or sink. Backed up sewer lines can flood your home with waste, creating a health hazard. Tree roots, broken pipes, grease clogs, and poor drain designs can all cause a sewer backup. If just one drain is slow, you might be able to solve the problem with a plunger or drain cleaner. More serious backups require a plumber.