Quick Answer:

If the drain slows only after you wash hands or dishes, soap scum or a gel-like mix of soap + fats/minerals is likely building up just inside the drain or trap. Start with simple checks — observe whether the water comes as a steady stream or foam (check aeration), flush with hot water, and remove the stopper to clean the visible surfaces. If those steps don’t clear it, inspect the P-trap for a soft, gel-like clog before moving to stronger actions.

Why This Happens

Most household soaps and detergents can combine with grease, skin oils, and dissolved minerals to form a sticky residue. That residue clings to the drain walls and the stopper, narrowing the opening over time. When the stream from the faucet is bubbly instead of a solid flow, the drain’s aeration is reduced and the soap can settle more easily. If the trap is partially full of a viscous, gel-like substance it will slow drainage until the trap fills and passes the liquid slowly.

Step-by-Step What to Do

1. Check aeration (water vs foam)

• Turn the tap on at normal speed and watch the flow. A clear, steady stream means good aeration; a bubbly, foamy flow suggests soap-rich water and poor aeration.
• If the flow is mostly foam, run the hot water for a minute to see if foam clears and flow improves.

2. Flush with hot water

• Run the hottest water available from the tap for 2–5 minutes. Hot water can soften and carry away fresh soap buildup. For kitchen sinks, pouring a kettle of near-boiling water (careful with PVC pipes — use shorter pours and let cool between) helps dissolve grease-bound soap.

3. Remove and clean the stopper and drain walls

• Pop or unscrew the sink stopper and wipe it clean. Use an old toothbrush or small bottle brush to scrub the drain opening and the first few inches of the visible drain wall.
• Remove any hair, sticky residue, or visible scum. Rinse the stopper and reinsert it to test the flow.

4. Check the trap for gel-like buildup

• Place a bucket under the P-trap, loosen the slip nuts and remove the trap. Inspect for a soft, jelly-like substance that smells like soap and grease. Clean the trap with hot water and a brush.
• If you’re not comfortable removing the trap, stop here and consider calling a pro rather than forcing it.

5. Run a small dish soap + hot water test to confirm

• Pour a tablespoon of liquid dish soap down the drain, then follow with a steady stream of hot water for 1–2 minutes. If the drain slows more or foams heavily, you’ve reproduced the condition and confirmed soap is the likely cause.
• After cleaning the trap and stopper, repeat the test to confirm the problem is resolved.

What Not to Do

• Do not mix cleaners (bleach + acids);
• don’t rely on harsh chemicals that can damage old plumbing.
• Avoid shoving sharp objects or wire deep into the drain — you can scratch pipes or push clogs further down.

When to Call a Professional

• If the drain remains slow after cleaning the stopper and trap, or multiple drains slow at once, call a plumber.
• If you detect persistent sewer smell, visible pipe damage, or backups into other fixtures, get professional help.
• For recurring soap-related slowdowns despite routine cleaning, a plumber can inspect venting and deeper parts of the line.
• Related situations you may want to check next: Drain Overflows When Washing Machine Drains and Drain Backs Up Only When Dishwasher Runs.

Safety Notes

• Wear gloves and eye protection when removing traps or scrubbing drains.
• Use caution with hot or boiling water to avoid scalding; pour slowly and in stages for plastic piping.
• Never mix chemical cleaners and ventilate the area if you use any chemical product.

Common Homeowner Questions

• Why does the drain slow only after using soap?
  Soap mixes with grease and minerals to form sticky residue that narrows the drain or clogs the trap.
• Will vinegar and baking soda remove soap scum?
  They can help loosen mild soap scum but won’t remove heavy, gel-like buildup in the trap.
• How often should I clean the trap and stopper?
  Every few months for busy kitchens, or whenever you notice slower drainage or buildup.