Toilet Backing Up into a Sink: Causes & Solutions


If your toilet gets clogged, it may back up into your bathroom sink. This is because toilets and bathroom sinks usually share a waste line. Sink drains may be connected to the toilet pipe that links your toilet to a soil stack. If there is a long distance between the toilet and stack, your sink drain can also act as a toilet vent. Here is the main cause of this problem and an effective solution.

Blocked Waste Line

The toilet can back up into a sink if the waste line is blocked. When toilet waste water moves into the obstructed pipe, it pushes some air through your sink vent and fills up the pipe, making it enter the sink drain. Sink drains tend to be thinner than toilet drains and this allows water to rise up higher compared to a large toilet pipe.


The most effective solution for this problem is to plunge the drain, but make sure you close off your bathroom sink before you begin plunging. Close your sink stopper and close off any overflow holes in the sink rim. This will allow you to plunge more effectively. When you do this, you reduce avenues for air and water to escape and they will put more pressure on the obstruction, making it easy to remove it. You can also get someone else to use a plunger on the sink at the same time, putting more pressure on the line.

While closing the sink drain is an effective strategy, it may not always work. When this happens, you can use a toilet snake or auger to remove the obstruction. Feed the auger into the toilet until you can feel some resistance and then crank its handle to remove the obstacle. Water should start draining from your toilet if the obstacle was successfully removed. When the water drains, you can run your bathroom faucet to clear the sink.

A clogged toilet can be very frustrating and if all these do-it-yourself methods fail to open the clog, you will need to call a professional plumber. Continuing to force the snake down the drain can cause damage to your pipes. A plumber can use other, heavy-duty methods of cleaning your pipes.