Why Does My Drain Keep Clogging? How to Find the Real Cause

Graphic: summary of this guide, made by our team from the cited sources.

Step 1: one drain or several?

This is the most useful check, and it takes two minutes. Run water in the problem drain, then in the fixtures near it and on the lowest floor.

What you see	Where the clog likely is	What usually fixes it
Only one sink, tub or shower is slow	In that fixture's trap or the short pipe right after it	Cleaning the stopper and trap, or a hand snake
Both sides of a double kitchen sink are slow	In the shared drain below the sink or in the wall	Snaking the branch line
A bathroom group (toilet, tub, sink) is slow or gurgles together	In the branch drain serving that bathroom	Snaking from a cleanout, often a plumber's job
The lowest drains (basement floor drain, first-floor tub) back up, or water comes up when a toilet is flushed or the washer drains	In the main building drain or sewer line	A plumber with a drain machine and, ideally, a camera

If it is a single fixture, keep reading. If it is the last row, stop using water as much as you can and call a plumber. A backup from the main line can push sewage into the house, which the Boston Water and Sewer Commission warns carries bacteria and other harmful germs.

Why the same drain clogs again

Hair that was only partly removed

In bathroom drains, hair wraps around the stopper linkage and the inside of the trap. Pulling out the visible clump opens the drain for a few days, but the strands left behind catch soap and the next round of hair. The fix is to take the stopper out and clean the trap, not just the top of the drain.

Grease and food in kitchen drains

Grease goes down warm and liquid, then cools and sticks to the pipe walls. The Boston Water and Sewer Commission says grease should never be poured down sinks, toilets or garbage disposals; pour it into a covered container and put it in the trash. Each new layer narrows the pipe a little more, which is why kitchen clogs tend to come back in the same spot.

Wipes and other things that do not break down

"Flushable" wipes are a common cause of repeat toilet and main line clogs. The Boston Water and Sewer Commission says many wipes labeled flushable do not break down in pipes and lists bathroom wipes, baby wipes, disinfecting wipes and towelettes as items for the trash.

Something stuck in the line

A toy, a bottle cap or a chunk of debris can sit in a trap or fitting and catch everything that comes after it. A clog that returns quickly after every snaking, always at the same depth, often points to an object or a damaged section of pipe.

Roots, sags and broken pipe outside

In older sewer lines, roots can enter through joints, and a sagging section can hold water and solids. Boston's water and sewer authority lists a blocked or collapsed private sewer lateral among the causes of sewer backups (BWSC). These clogs come back no matter how often the line is snaked, until the pipe is repaired. A camera inspection is the way to know.

A worked check for a single slow bathroom sink

Here is a check you can run on a sink that keeps slowing down. You need a bucket, a towel, pliers, a flashlight and an old toothbrush.

1. Pull the stopper. Most pop-up stoppers lift out, or come free when you loosen the nut on the pivot rod behind the drain pipe under the sink. Clean the hair off it.
2. Put a bucket under the trap (the U-shaped pipe) and unscrew the two slip nuts by hand or with pliers.
3. Clean the trap and look into the pipe going into the wall with a flashlight. Hair and sludge here are common.
4. Reassemble hand-tight plus a little more, then run water and check for drips.
5. Test the flow. Fill the basin and pull the stopper. If it drains quickly, the clog was local. If it is still slow, the clog is in the wall, and a hand snake or a plumber is the next step.

While you are there, check whether the faucet is weak too. A slow drain and a weak faucet are separate problems; a weak flow often comes from a clogged aerator screen, which Pfister says you can unscrew and rinse.

What about drain cleaners?

Chemical drain openers are a common first move, and a common reason clogs keep coming back: they can burn a small channel through soft buildup without removing the hair or grease around it. They are also dangerous. Poison Control says many liquid drain cleaners contain concentrated acids or alkalis that can cause serious burns, and that splash-back is a risk when too much is poured into a fully blocked drain (Poison Control). If you use one:

• Follow the label exactly and wear goggles and gloves, as Poison Control advises.
• Never mix it with another drain product or cleaner.
• Tell anyone who later plunges or snakes the drain that chemicals are in it.

For the home remedy, Poison Control says baking soda and vinegar is unlikely to be strong enough to clear tough grease clogs (Poison Control).

How to stop the cycle

• Bathroom: use a hair catcher in tubs and showers, and clean the sink stopper every month or two.
• Kitchen: wipe greasy pans with a paper towel before washing, and put grease in a container for the trash.
• Toilets: flush only toilet paper and human waste; wipes go in the trash.
• Main line: if you have had a main line backup, ask the plumber for a camera inspection so you know whether roots or a damaged pipe are involved.

When to call a plumber

• More than one fixture is slow or backing up.
• Water comes up in a tub or floor drain when you use another fixture.
• The same drain clogs again within weeks of being cleared.
• You have used a chemical drain cleaner and the drain is still full of standing water.