Short answer
If one fixture backs up only when you use that fixture, the clog is in its own drain line. If water comes up in a different fixture, usually a tub, shower or the lowest drain, the blockage is further down in a shared branch or the main sewer line. Stop running water, skip chemical drain cleaners, and call a plumber for anything beyond a single-fixture clog.
Read the pattern
Where the water comes up, and when, narrows things down quickly.
Kitchen sink backs up
- Only when the sink or disposal runs: a clog in the sink drain or the kitchen branch line. Grease and food waste are the usual cause.
- When the dishwasher drains: the dishwasher shares the sink drain. A clog in the sink trap or disposal backs up both. Check that the disposal isn't jammed; see garbage disposal not working.
- Both sides of a double sink fill: the clog is below where the two sides join.
Bathtub or shower backs up
- When you shower: hair and soap buildup in the tub drain.
- When you flush a toilet or run a sink: water is being forced up the lowest opening on a shared line. This points to a branch or main line blockage. See sewer line problems.
Toilet backs up
- One toilet, others fine: a clog in that toilet or right after it. Plunging usually helps.
- Several toilets, or toilets and tubs together: main line.
Washing machine standpipe overflows
- The laundry drain can't handle the fast discharge. Lint buildup in the standpipe or branch is common. If other drains also gurgle when the washer drains, suspect the main line.
Floor drain or garage drain
- Floor drains are often low points connected to the main line. Water coming up there usually means a blockage further downstream.
What to do right now
- Stop using water in the house. Turn off the washing machine and dishwasher, and ask everyone to hold off on flushing.
- Soak up and contain water with towels. Put a bucket under a leaking trap.
- Try a plunger if it's a single fixture: a cup plunger for sinks and tubs (block the overflow opening with a wet rag), a flange plunger for toilets.
- Check the cleanout in the yard if multiple drains are affected. A capped pipe near the house, usually 3 to 4 inches across. If water is standing in it, the blockage is between there and the street. Don't unscrew a cap that's under pressure.
- Call a plumber if plunging fails or more than one fixture is involved.
Common causes of backups
- Grease from kitchen sinks, which hardens as it cools
- Hair and soap scum in tubs and showers
- Wipes, paper towels and hygiene products flushed down toilets
- Food waste overloading the disposal and kitchen line
- Lint from laundry
- Tree roots in the sewer line, causing repeat backups every few months
- Damaged or sagging pipe where waste collects
- Heavy rain, occasionally, if groundwater is getting into a damaged line
What not to do
- Don't keep flushing a toilet that's rising. Take the lid off the tank and lift the flapper back down, or close the toilet's supply valve, to stop it overflowing.
- Don't use chemical drain openers on a backup. They're harsh on pipes, rarely work on solid clogs, and are hazardous to handle.
- Don't rent a large drain machine for the main line unless you know what you're doing. A stuck cable becomes a second problem.
- Don't ignore repeated backups. Clearing the line treats the symptom. If it keeps happening, the cause needs finding.
DIY or call a plumber?
Reasonable DIY:
- Plunging a single sink, tub or toilet
- Cleaning hair out of a tub or shower drain
- Removing and cleaning a sink P-trap (with a bucket under it)
- Clearing a jammed disposal with its hex key
Call a plumber when:
- More than one fixture is affected
- Water comes up somewhere other than the fixture you're using
- There's sewage in the house
- Plunging doesn't work, or the clog returns within weeks
- You've had to clear the same line more than once this year
What to tell the plumber
- Which drains back up, and what triggers it (flushing, shower, washer)
- Whether you know where the cleanout is
- Any history of root problems or previous clogs
- Your home's age
A plumber will typically clear the line and, for repeat or main line problems, recommend a camera inspection to see what's causing it. See drain cleaning for how different clogs are cleared, and sewer line repair if damage is found.
Preventing backups
- Scrape plates into the trash. Let grease cool and throw it away.
- Use hair catchers in showers and tubs.
- Flush only toilet paper.
- Run plenty of cold water with the disposal and don't overload it.
- Clean the laundry lint trap and consider a lint filter on the washer discharge hose.
- If roots are a known issue, ask about scheduled maintenance cleaning.
Frequently asked questions
Why does my shower back up when I flush the toilet?
The toilet and shower share a drain line further down. When that line is blocked, flushed water takes the easiest way out, which is often the shower or tub drain. This usually means a branch or main line clog.
Why does my kitchen sink back up when the dishwasher runs?
The dishwasher drains through the sink plumbing, often via the disposal. A clog in the sink trap, disposal or kitchen drain line blocks both. Clearing the disposal and trap often fixes it.
Is a drain backup an emergency?
Sewage coming into the house, or a backup you can't stop, needs prompt attention. A single slow sink can usually wait for a scheduled visit. Either way, stop using water until it's cleared.
Can heavy rain cause drains to back up?
It can if groundwater enters a damaged sewer line or if the public sewer is overloaded. If backups line up with storms, mention it to your plumber and consider a camera inspection.