Quick Answer:
When the cabinet gets wet only while the sink is used, the leak is usually at a connection that only carries water under flow — a loose slip joint, a worn washer, or a fail of a hose or drain connection. Start by turning off the water, drying fittings, and visually inspecting the drain and supply connections. If tightening slip joints or replacing worn washers doesn’t stop it, call a plumber.
Why This Happens
Most leaks that appear only during use come from places that only see pressure or flow when the faucet is on. Common causes:
- Loose slip joints under the drain — they may weep when water moves through the trap.
- Worn or missing washers in supply or drain fittings that hold when idle but fail under flow.
- Cracked or split hoses and seals that leak under pressure or when the sink bowl is full.
If you notice water pooling right after using the sink, also watch for continuing drips from the tailpiece or P-trap — a symptom similar to “drips after you turn it off” but occurring during use. A trap that shifts or compresses under flow can act like a valve and release water only when the sink runs.
Step-by-Step What to Do
1. Turn off the water and clear the area
Shut off the sink’s cold and hot supply valves under the cabinet. If there are no individual shutoffs, turn off the main house water. Remove items from the cabinet so you can work and dry fittings with a towel.
2. Dry fittings and perform a visual check
Wipe every pipe, connection nut, and flexible hose dry. Run the sink briefly and watch carefully to see where water first appears. Drying fittings beforehand makes fresh leaks easier to spot.
3. Inspect slip joints and tighten by hand
Look at the slip joint nuts on the tailpiece and P-trap. Many leaks come from these loose nuts. Gently hand-tighten each slip joint nut a half-turn — do not over-tighten. If the nut keeps turning or the joint seals poorly, remove, inspect the washer and seating, then reassemble.
If the trap keeps moving or you find the trap is loose after tightening, this is the common “trap keeps loosening” problem that can need a replacement nut or a different trap design; check the alignment and seating before testing under flow. If you see cracked threads or bent parts, replace them.
4. Replace worn washers
Remove the slip joint nut and the washer inside the joint. If the washer is flattened, cracked, brittle, or missing, replace it with a new one of the same size and material. Reassemble and hand-tighten the nut, then test.
Also check small rubber or neoprene washers on supply line connections; replace any that look compromised.
5. Test under normal use
- With everything reassembled, turn supplies back on slowly.
- Run the sink for several minutes and watch for leaks under the cabinet and at each connection you worked on.
- If the leak only appears when the sink is filled or used heavily, recheck the tailpiece and overflow path for clogs or misalignment.
What Not to Do
- Do not wrap cracked pipes with tape. Temporary tape fixes hide the real problem and can fail, causing more damage.
- Avoid over-tightening plastic nuts with a wrench — that can crack fittings.
- Don’t ignore a persistent leak; repeated wetting of cabinet materials leads to rot and mold.
When to Call a Professional
Call a plumber if:
- You cannot find the leak source after drying and watching connections.
- Parts are cracked, stripped, or corroded and simple replacement isn’t obvious.
- Leaks continue after replacing washers and tightening slip joints, or if water damage is already present in the cabinet.
A plumber can also advise if a trap or tailpiece must be replaced with a different size or a rigid replacement to stop repeated movement.
Safety Notes
- Always shut off the water supply before removing fittings.
- Wear gloves and eye protection when working under the sink — standing water can hide sharp edges and dirty build-up.
- If you smell gas or find electrical components wet, stop and call a professional immediately.
Common Homeowner Questions
- Why is the cabinet only wet after I use the sink?
Because the leak is likely at a joint or part that only carries water or pressure while the sink runs. - Can I just tighten everything to stop the leak?
Tightening helps if nuts are loose, but worn washers or cracked parts usually need replacement. - How long can I wait to fix it?
Fix it as soon as possible — ongoing wetting damages cabinet materials and encourages mold.