If you’re smelling something musty under your sink, or you noticed the cabinet floor feels soft when you press on it, water has probably been leaking for longer than you’d think. Most under-sink leaks are slow and hidden, which is exactly why the damage is already done by the time homeowners notice it.
Why Under-Sink Leaks Are So Easy to Miss
The space under a kitchen or bathroom sink is dark, crowded, and rarely opened. A drip from a supply line fitting or a slow seep at a drain slip joint might only produce a tablespoon of water per day. That’s enough to saturate particleboard and start mold colonies over weeks or months without ever pooling visibly on the cabinet floor.
Particleboard (the material in most stock cabinets) swells and softens fast when it gets wet. Solid wood fares better, but it will still rot and warp with prolonged moisture. By the time you see black staining or feel that spongy floor, you’re typically looking at damage that started weeks or months ago, not days.
The Most Common Sources, in Order of Likelihood
Supply line connections. The braided supply lines that feed your faucet connect to shutoff valves below and to the faucet body above. The fittings at both ends can work loose over time, especially with older plastic-bodied shutoffs. Water here comes out slowly and often runs down the outside of the line before dropping off, so it looks like condensation at first.
Drain slip joints and P-trap connections. Under most sinks, the drain runs through a P-trap held together with hand-tightened plastic slip nuts. These don’t corrode, but they do loosen. If the slip joint under the drain basket isn’t sealed properly, every time you run water you’re dripping a little into the cabinet. Look for a white mineral ring or soft wood directly below the P-trap.
Garbage disposal. Disposals leak in a few specific spots: the sink flange at the top (where it mounts to the sink), the discharge port where the drain pipe exits the unit, and the dishwasher inlet if the knockout plug wasn’t properly removed during installation. A flange leak usually shows up as water right under the disposal body. A discharge port leak tends to run along the drain pipe and drip further back.
Dishwasher drain hose. If you have a dishwasher, the drain hose typically routes through or under the sink cabinet before connecting to the disposal or drain. Hose clamps can loosen. Check the connection point and trace the hose for any cracks or worn spots.
Water filter or under-sink filtration systems. Saddle valves and quick-connect fittings on under-sink filters are a common slow-leak source. The fittings work fine for years then start weeping at the connection point.
Signs to Look For
Soft or spongy cabinet floor is the clearest indicator. Press firmly on the wood, starting at the corners and working toward the center. If it gives or makes a crunching sound, the particleboard core is saturated and probably breaking down.
Black or dark gray spots on wood surfaces are likely mold. Spores can begin germinating within 24 to 48 hours in wet conditions, but a visible colony big enough to see usually means sustained moisture over weeks.
White mineral deposits on pipes or on the cabinet wall indicate water has been evaporating and leaving minerals behind, meaning the leak is slow and ongoing.
A persistent musty smell even after you’ve cleaned the cabinet is almost always mold growing somewhere in the structure, not just on the surface.
Paint or finish bubbling on cabinet walls adjacent to the sink means moisture has gotten behind the surface material.
How a Plumber Diagnoses It
A licensed plumber will typically start by drying everything out and running water through each source independently. They’ll fill the sink and watch the drain connections while it drains. They’ll run the disposal separately, check the dishwasher cycle if one is connected, and feel each supply line fitting under pressure. They may use a moisture meter on the cabinet floor and walls to map how far the saturation has spread, which matters a lot for any cabinet repair or replacement that needs to follow.
If mold is found in the structure of the cabinet or in the wall behind it, the remediation scope changes, and a plumber may bring in a separate contractor for that work.
What You Can Do Right Now
Turn off the shutoff valves under the sink and dry out the cabinet completely with towels and a fan. This stops ongoing damage while you assess the situation. If the shutoffs themselves are the source of the leak (common with older gate valves), turn off the main house supply instead.
Photograph everything before you clean it: the staining, the soft spots, the exact location of discoloration. This helps whoever works on it understand where the leak was coming from and gives you documentation if a cabinet replacement ends up being part of the repair.
Removing stored cleaning products and other items from under the sink is worth doing anyway. It lets air circulate and makes it much easier to spot future leaks early.
If the cabinet floor is soft, don’t put items back until the source is repaired and the wood has dried or been replaced. Wet particleboard won’t recover to structural integrity just from drying out.
When to Call a Licensed Plumber
If you’ve found visible structural damage, mold, or a source you can’t identify, stop and call a licensed plumber. Diagnosing the source and shutting off the water are the right homeowner moves. The actual repair, whether it’s a fitting, a slip joint, a disposal flange, or anything behind the cabinet wall, belongs to someone with the license and liability insurance to back it up.
California plumbers must hold a C-36 specialty contractor license. Verify any contractor at cslb.ca.gov before they start work. It’s free and takes under a minute.
The plumbing fix is usually the cheap part. Cabinet replacement and mold remediation are what run the bill up. Catching it early and getting a plumber in fast is the whole game.