Overflowing toilets and bathtubs in your house indicate a problem, obviously, with the house's plumbing system. A single clog in one of the plumbing fixture's drains will not cause multiple fixtures to overflow at once. If toilets and bathtubs are backing up, filling or overflowing at the same time, the problem is a serious one. Contact your city first to determine if any problems with the sewer system are causing the overflowing problem.
All of the drains in your house eventually connect together. The drains in one portion of your house, such as a bathroom, will all combine into a larger drain pipe. These larger drain pipes all connect to an even larger drain pipe, the sewer main, that feeds into the sewer system or septic tank for the house. Clogs or in the drain pipes, or even in the larger sewer drain pipe, will lead to water going up the drain pipes and coming out of the drains in your house.
Wastewater that flows the wrong way through the drain pipes normally comes up through bathtub and toilet drains, but not sink drains. Bathtub and toilet drains sit closer to the ground than sink drains, since sinks sit higher off the ground. Because toilets and bathtubs have lower drains, the water that gets pushed up the drain pipe as backflow will exit into the tub and toilets first unless something blocks the effluvium from doing so.
Vent Pipe Obstructions
Every larger drain pipe in your house connects to a vent pipe. Vent pipes run vertically in your house, their openings coming through the house's roof. The vent pipes allow sewer gases trapped in the drain pipes to escape above the house's roof, keeping the gases from finding a way into the house. These vent pipes replace any air washed down the drain pipes with the waste water. Without this air balance, the wastewater in the drain pipes will rise, possibly even coming out of the drain pipes. Use a garden hose with a high-pressure attachment to dislodge any obstructions out of the vent pipes. You need to be on the house's roof to clear the obstructions.
Drain Pipe Obstructions
If clearing obstructions from the vent pipes does not alleviate the problem, a larger clog in the drain pipes likely is the source of the problem. Check the cleanouts in the front of the house, which sit along the sewer drain line that connects to the city's sewer system. Sewage water coming out around the cleanouts indicates a backup in the sewer drain line or a problem in the city's sewer system. Call a plumber to eliminate these tougher clogs.