Blockages

• All the waste water drainage systems in a home eventually drain into the same system, so a blockage could exist at many different points. In this case, it is likely at the convergence of the washing machine and sink waste water lines. Sometimes tree roots cause blocks, but often hair, grease, food and other household debris stick in the drain.
  
  

Vents and Pipes

• If the drains are clear, have a professional plumber or drain rooter use a camera and examine pipes for breaks. Also, each sink needs a vent to allow air in for easy drainage. If there are not pipe blockages and no broken pipes, check rooftop vents for bird or rodent nests.
  
  

Solutions

• Although a homeowner may be able to use a plumber's snake to clear a blockage, it is difficult for an amateur to clear all connected lines. To prevent more backups, call a professional and have them use a heavy duty rooter on all waste lines. Use a drain camera to be sure there is no blockage anywhere, including between the home and sewer.