Bathroom Sink Draining Slowly? Fix It in 4 Steps (and Replace the Pop-Up)

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

Why bathroom sinks slow down

• The pop-up stopper catches hair. The stopper and the rod that lifts it sit inside the drain, right in the water's path. Hair wraps around them and soap glues it in place.
• Soap curd. University of Nebraska-Lincoln Extension's hard water guide says soap used in hard water combines with the minerals to form a sticky soap curd, and that soap curds deposit on all water and plumbing fixtures. In a sink, that film builds up on the stopper, the tailpiece and the trap.
• A stopper that barely opens. Kohler's pop-up drain installation guide has you adjust the stopper so it lifts about 3/8 inch (1 cm) when opened. If yours only rises a little, the water has a smaller gap to get through.
• Buildup in the trap. The curved P-trap holds water to block sewer gas, and it is also where heavier debris settles.

Step 1: Pull and clean the pop-up stopper

Close the stopper and try to lift it out. Some types come straight up, and some turn a quarter turn first.

If it will not budge, look under the sink at the back of the drain pipe. You will see a horizontal rod going into the pipe, held by a nut, and joined by a clip and a slotted strap to the vertical lift rod that comes down from the faucet. In Kohler's design, the short end of that horizontal rod goes into the lift rod hole and under the stopper rod, and the rod nut is hand tightened. Unscrew the nut, pull the rod back an inch, and the stopper lifts free.

Clean the stopper and its rod with an old toothbrush and hot water. Wipe inside the drain opening as far as you can reach. Put it back, push the rod in under the stopper again, and hand tighten the rod nut. Then check the lift: open the stopper and see if it rises about 3/8 inch. Kohler says to adjust it by turning the threaded stopper rod in or out.

Step 2: Plunge it the right way

With the stopper clean and back in, run some water. Still slow? Plunge. Korky's guide on how to plunge a sink gives the details that make this work on a bathroom sink:

1. Use a cup plunger, the kind made for flat drains (not a flanged toilet plunger).
2. Plug the sink's overflow hole. A wet rag pressed into it works. Without this, the pressure escapes out of the overflow instead of pushing on the clog.
3. Hold the lift rod down so the stopper does not block the drain while you plunge.
4. Seal the plunger over the drain and push and pull with steady strokes for about 10 to 15 seconds.
5. When it clears, run hot water for a few minutes to flush away loosened debris.

Step 3: Clean the P-trap

If plunging does not fix it, the clog is probably in the trap. The University of California IPM program, on its moth and drain fly page, says that for sinks with a J trap underneath, the trap should be removed and cleared of organic debris, and that you should run water afterward to make sure it flows freely.

1. Put a bucket under the trap and have a towel ready.
2. Loosen the two slip nuts that hold the curved section. Plastic ones usually turn by hand.
3. Pull the trap off, empty it, and scrub it out. Check the washers inside the nuts.
4. Look into the short pipe going into the wall. If it is packed with gunk, clean it with a brush.
5. Reassemble, hand tighten, run water, and check for drips. Snug a nut a little more if one weeps.

The kitchen version of this job, with photos of each part, is in our guide to cleaning a smelly P-trap under the kitchen sink.

Step 4: Snake the pipe in the wall

With the trap off, feed a small hand auger (drain snake) into the pipe that goes into the wall. Turn the handle as you push, and pull back whatever it catches. Put the trap back on and test. This clears clogs past the trap without forcing a cable through the stopper hardware.

What about drain cleaner?

It is not the first step. Most slow bathroom sinks are a hair and soap problem at the stopper, which no liquid removes as well as your hands do. If you do use one, Poison Control's article on what is in drain cleaner says to wear goggles and rubber gloves and to pour carefully and close to the drain. It warns that drain cleaner can splash back if poured from too high or if too much is poured into a completely blocked drain. Do not plunge or take the trap apart after a chemical cleaner has gone in.

Replacing a worn pop-up drain

Replace the assembly if the stopper will not stay up, the drain leaks at the flange, or the parts are corroded. You need an adjustable wrench and plumber's putty. These are the steps from Kohler's installation guide for its K-7114 and K-7126 pop-up drains; check the instructions for your own model.

1. Disconnect the P-trap and the lift rod linkage, then unscrew the old drain body nut under the sink and push the old flange up and out. Clean off old putty.
2. Unscrew the new flange from the drain body. Apply a ring of plumber's putty to the underside of the flange.
3. From under the sink, push the drain body up into the drain hole. From above, hand tighten the flange onto it.
4. Thread the nut up the body until the flange sits snug. Do not let the drain rotate as you tighten. Make sure the lift rod hole faces the back of the sink, then tighten the nut with the wrench and wipe off excess putty.
5. If the tailpiece was removed, Kohler says to double check that the plastic washer is inside the flange body, or the drain will leak.
6. Connect the P-trap to the tailpiece. Press the seal into the lift rod hole, put the stopper in, and slide the short end of the rod into the hole and under the stopper rod. Hand tighten the rod nut.
7. Adjust the stopper to lift about 3/8 inch when open.
8. Fit the clip and the link onto the rod with the thumbscrew facing the back. Push the lift rod down through the faucet and into the link, and tighten the thumbscrew so the lift rod knob sits 1/2 inch (1.3 cm) above the lift rod hole.
9. Run water and check for leaks.

Step 1 is our addition, since Kohler's guide covers a new install.

When the problem is bigger than the sink

If the tub, shower or toilet is slow at the same time, the clog is further down the line. The City of Minneapolis page on sewer problems in the home says slow drainage in toilets, showers and sinks is typically caused by clogs in the property's own plumbing, and that a blockage in the city sewer is rare. Call a plumber when several drains are slow, when water comes up in the tub as the sink drains, or when the sink is still slow after the four steps above.

A slow shower in the same bathroom has its own list of causes, covered in shower drain clogged with no hair. If this sink keeps clogging again a few weeks after you clean it, read why a drain keeps clogging.