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.
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.
Have a waste bucket handy to catch any overflow that comes out of the main drain clean out. Spread a tarp, old towels or plastic sheeting around the main drain as well to help contain any overflow.
Loosen the clean out cap with a pipe wrench. Backed-up water will probably flow out of it as you open it. Continue loosening slowly to control the flow of water. Once the water has stopped, you can remove the cap.
Insert the auger head of a plumber's power snake through the main drain clean out and into the main drain pipe. Feed it into the drain pipe as far as you can until you encounter resistance--this signals a clog.
Follow the manufacturer's instructions for operating the power snake. You should feel the vibration as it cuts through the clog.
Continue feeding the auger in to the limit of its cable length, cutting through clogs as you go. Then pull the entire length of cable back and feed it into the main pipe going the opposite direction.
Cut through clogs as you encounter them.
Replace the cap on the drain clean out. Make sure that it is tightly sealed and secured.
Flush a toilet several times to check and make sure that there is no further blockage in the system.