When water comes up in the shower after you flush the toilet, or both fixtures drain slowly at once, a blockage is usually sitting downstream of where the two drains join. That is more serious than a simple hair clog in the shower. This article explains what is likely happening, what to do right now, and when to stop and call a plumber.
What to do right now
- Stop using water. Do not flush toilets or run sinks, showers, dishwashers, or washing machines. More water has nowhere to go and can overflow.
- Keep people and pets away from the affected area. Backed-up water from drains can carry bacteria.
- Do not use chemical drain cleaners. If the blockage is deep or in the main line, caustic products tend to sit in the pipe, may splash back, and can create a hazard for the plumber who works on it afterward.
- Avoid using electrical devices in or near standing water. If water reaches outlets, appliances, or a basement electrical panel, stay out and call an electrician or your utility. Electrical safety comes before cleanup.
- Call a plumber, especially if more than one fixture is affected.
Why the toilet and shower back up together
Fixtures in a home drain into pipes that eventually join and run to the sewer or septic system. When a blockage sits where those pipes combine, or in the main line, wastewater has no path forward. Water follows the easiest route, which is often back up through the lowest, emptiest drain. In many homes that is a shower or tub, because its drain opening sits low and the trap is often not full of water.
So when you flush a toilet and water appears in the shower, the water is being pushed back from a shared line that is blocked or restricted.
Common causes
- Main sewer line blockage. Grease, paper products, wipes, or other debris can build up and block the line leaving your house.
- Tree roots. Roots can enter small cracks in older pipes and grow into blockages over time.
- Items that should not be flushed. “Flushable” wipes, rags, feminine products, and paper towels do not break down like toilet paper.
- A clog in a shared branch line. The pipe where the bathroom drains join can clog before reaching the main line.
- Damaged or aging pipes. Cracked, collapsed, or offset sections can trap debris.
- A blocked plumbing vent. Vents let air into the system so water can flow. A blocked vent can cause slow draining and gurgling. Any work on a roof vent is a fall risk, so leave that to a professional.
- Pipe scale or buildup. Mineral buildup narrows pipes over many years.
Warning signs of a serious drain blockage
- Water rising in a shower or tub when you flush or run another fixture
- Gurgling sounds from drains
- Several drains running slowly at once
- Sewage smells
- Standing water around floor drains or the lowest fixtures in the home
- Toilets that bubble when a washing machine drains
A single slow shower drain is usually local. Several symptoms at once suggest a shared problem. If it is only the shower, our guide on how to unclog a shower drain covers local fixes.
Will the water eventually go down?
Sometimes slow drains recover on their own if the blockage is partial or shifts, but a toilet that is clogged and standing high will not reliably go down, and you should not keep flushing to find out. Treat repeated backups as a sign of a blockage that needs clearing rather than waiting.
Is it okay to pour boiling water down the drain?
It is best not to. Boiling water can soften or damage PVC pipes and joints and can crack porcelain. It also rarely helps with a deep, solid blockage. Hot tap water is a safer choice for light buildup near a drain. If you suspect the main line, hot water will not solve it.
Cleanup and sanitation
Water from a sewer backup is considered contaminated. If it has reached floors or belongings:
- Wear rubber gloves, boots, and eye protection.
- Ventilate the area.
- Remove standing water carefully and avoid contact with skin.
- Porous materials such as carpet, drywall (see is drywall porous), and insulation that were soaked may need professional cleaning or removal.
- Wash hands thoroughly afterward.
For large backups, consider a professional water damage restoration service and check your homeowners insurance policy for coverage details.
What a plumber will do
A plumber will typically locate the nearest cleanout, which is a capped access point on the main drain, and use a drain machine to clear the blockage. They may use a camera to inspect the line and find roots, breaks, or other damage. For heavy buildup, hydro jetting uses high-pressure water. Severe damage may require pipe repair or replacement. The cost depends heavily on your location and the problem, so ask for a written estimate.
Septic systems
If your home is on a septic system, a backup can also mean a full tank or a failing drain field. Do not keep running water. Contact a septic professional.
Preventing backups
- Flush only toilet paper and human waste.
- Keep grease and oils out of drains.
- Use drain strainers and clean them regularly.
- Be careful where trees and large shrubs are planted near the sewer line.
- Consider a periodic camera inspection if your home has older clay or cast-iron pipes, or a history of backups.
For other troubleshooting, see our guide to common shower problems.