Why does my kitchen sink keep clogging?

Kitchen clogs start with fats, oils and grease, and the sink is where they enter. The City of Phoenix Water Services Department says "The biggest problem is coagulated grease and cooking oil" in the city's sewer lines, and the same thing happens in your own pipe first. EPA's report on sewer overflows found that grease from homes, restaurants and industry is the most common cause of reported blockages, at 47 percent of them, ahead of grit and debris at 27 percent and roots at 22 percent.

Hot water and dish soap do not fix it. They only carry the grease a few feet down the line, where it cools and settles again, usually at the first bend or the first long horizontal run. Phoenix's advice for grease you cannot pour out is to let it cool in the cookware, then scrape it into the trash. Wipe greasy pans with a paper towel before washing. A coffee can or jar works for a large amount of bacon fat or frying oil, and it goes in the trash when full. Our page on what not to put down the drain lists the rest, and what FOG is explains why the term matters.

Food is the second layer. Coffee grounds, rice, pasta and potato peels swell or pack together in the trap and stick to the grease film. A strainer basket in the drain catches most of it. If you have a double sink, notice which bowl drains slowly. Both bowls usually meet at one trap, so a slow bowl and a fast bowl point at the tailpiece or the strainer of the slow one, while two slow bowls point at the trap or beyond.

A disposal breaks food into small pieces, but small pieces still have to travel down the pipe. InSinkErator, a disposal manufacturer, states "Never put fats, oils, or grease into a garbage disposal." The company also says to run a steady stream of cool water before switching the unit on, feed food in gradually, and let water run for a few seconds after grinding so the drain pipe flushes.

Those habits matter because a disposal turns bulky scraps into a paste that settles in the trap and the horizontal pipe behind it. Fibrous food such as celery, corn husks and onion skins tends to stringy up the paste, and starchy food such as pasta, rice and potato peels turns it gluey. Neither one rinses away. Feeding a little at a time, with water running, keeps the paste thin enough to move.

Here is a quick way to tell where the blockage is. Run the disposal with water. If it spins freely but the sink still fills, the clog is downstream of the unit, in the tailpiece, trap or trap arm. If the disposal hums or will not spin, the problem is the unit itself, which is a different fix. Our page on a slow-draining garbage disposal walks through the downstream case, and a smelly disposal covers the buildup that comes before the clog.

Your dishwasher usually empties into the same place as the sink, either the disposal or the sink tailpiece. That means a partial clog below that point slows both, and each dishwasher cycle pushes food residue into the trap. If you notice the sink gurgles or water rises in the basin while the dishwasher runs, the shared line is already narrow.

On a new disposal, the dishwasher inlet has a knockout plug. InSinkErator's installation page says "If using a dishwasher, you will need to tap out the dishwasher knockout plug." A plug left in place blocks the dishwasher's water, so a sink that was fine before a disposal swap and a dishwasher that will not drain afterward is worth a look at that inlet. The hose may also run through an air gap, the small fitting on the counter or sink deck, which InSinkErator's installation page mentions. That fitting can clog with debris too. See what a dishwasher air gap is for how to check it, and a dishwasher that will not drain if the machine is the only thing standing in water.

Scrape plates into the trash before loading. The dishwasher rinses whatever is left into the same pipe your sink uses, and greasy pots that run through a hot cycle send warm grease down the line.

The P-trap is the U-shaped bend under the sink, and the trap arm is the pipe that runs from the trap to the wall. Water slows there, so grease and paste build up first at the trap and along the arm. A snake pushed through the disposal or the tailpiece can punch a small hole in that buildup, and the drain seems fixed until the hole closes. That can explain why a cleared drain backs up again.

Here is how to do a proper clean.

1. 1Put a bucket under the trap and unscrew the two slip nuts by hand.
2. 2Empty the trap into the bucket and scrape the inside with a brush.
3. 3Reach into the trap arm as far as you can and pull out the residue.
4. 4Reassemble with the washers seated flat, then run water and check for drips.

Follow the trap maker's tightening instructions rather than applying a universal extra turn. Overtightening cracks plastic nuts and washers, and a crooked washer leaks. If you have a disposal, its discharge tube connects to the trap the same way, so check that joint too. For the full sequence and other fixes, see how to fix a slow-draining sink and what a P-trap is. Skip chemical drain cleaner, which can splash and does not remove a grease lining.

If the trap is clean, water still backs up, and the arm looks clear as far as you can reach, the blockage is beyond your hand. Older homes with galvanized or cast iron drain pipe can have rough, scaled interiors that catch buildup sooner, which cast iron drain pipe lifespan covers.

Sometimes the kitchen sink is the first place a shared clog shows up. A clog can sit beyond the trap arm, out of reach of a hand tool. In Phoenix, tree and shrub roots matter as well. The city says the second biggest problem in its sewer lines is roots from trees and invasive shrubs such as oleanders, which find their way through pipe joints and form a root ball. Roots more often affect the line to the street than the pipe under your sink, but a kitchen that clogs along with other fixtures is worth a camera look. Signs the problem is past the sink:

• Water backs up in another fixture, such as a shower or tub, when the sink or dishwasher drains.
• Gurgling from other drains.
• The clog returns within days of a clean trap.

Main line versus branch clog explains how to tell them apart. A licensed plumber matters here because the fix needs equipment and a look inside the pipe. A cable or camera reaches what a hand snake cannot, and it shows whether the lining is grease, a belly in the pipe or roots. Jetting, which scours the pipe wall with high-pressure water, removes a grease lining that a cable only bores through. HQ Plumbing & Air offers 24/7 emergency service, gives upfront pricing and free estimates, and is licensed and insured.

Prevent the next one: scrape fats into the trash, use a strainer, run cool water with the disposal, and clean the trap when the sink starts to slow. That's the plan.