If you have cleaned out likely clog sites inside the house and in individual drains but still have water backing up in your house, your main drain–the drain that leads from your house into the city’s sewer system–may be clogged. If the main drain has been clogged by tree roots, there is little that you can do yourself to remove them without risking damaging the main drain pipe–you need to call a professional plumber. But if the main drain is simply clogged by debris, you might be able to remove it with the aid of a rented or purchased plumber’s power snake.
Step One
Locate the main drain clean out ports. These are short, stubby tubes that branch off the main drain pipes at a "Y" angle, fitting with a steel or plastic cap that has a square fitting on top, and may be located in the basement or crawl space. Some homes might have these access ports in the garage, pantry or outside the house along the foundation. Just follow exposed drain pipes until you find them.