The toilet backing up into the tub usually happens when the water has no place to go due to a sewer drain clog. The water will then come back into the house in one way or another. This problem is quite common and damaging which makes it a plumbing emergency. When a sewer drain clog has occurred, one should not use any of the plumbing in the home until the stoppage is cleared.
The toilet backing up into the tub is a sign of a sewer drain clog below the point where the tub drain connects. Overflowing from any drain in the household, including sink drains, are all signs of sewer drain clogs. Strange reactions while using plumbing fixtures is also a sign of sewer drain clogging. Waste build up in the drain is caused by careless disposal of waste in toilets and sinks. This problem is considered an emergency and cannot be left unfixed.
1. Use duct tape to cover the tub and sink trip levers. If there are any other places affected by the overflowing, their trip levers should also be covered by duct tapes. This is done to cover the overflow openings and makes the plumbing pipes airtight.
2. Pour water into the toilet or flush it to make water come out of the bathtub drain. Then all the drains in the bathtub and sinks should be closed by use of the drain plugs.
3. Then using the plunger preferably the one with the rubber flange pulled out to get a better seal pump the toilet bowl vigorously a few times.
4. Check the tape you applied and stick it back down if it came off or fortify it with another layer. Then try plunging again.
5. If the plunging has no effect, use a toilet auger to clear the blockage. The auger is inserted through the toilet and pushed until it encounters the blockage. Crank the handle of the auger through the clog repeatedly until the clog is cleared and the blocked water flows through the drain and the problem is fixed.
6. Remove the auger and the tapes and flush the toilet to confirm that the problem is solved.
If the drain remains clogged, you will need to consult a plumber. The professional plumber can try more aggressive methods such as power jetting.