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Water damage under the kitchen sink: what to check and what's salvageable
Water damage under a kitchen sink almost always comes from one of four sources, supply-line connections, the P-trap drain assembly, the garbage disposal, or the dishwasher line that runs through the cabinet. The swollen particleboard cabinet floor you're looking at won't shrink back when it dries, but the more important question is what's behind it: the wall cavity, the toe-kick space, and the flooring beyond the cabinet, where moisture hides after the cabinet itself looks handled.
The under-sink cabinet is where slow leaks go to hide. It's dark, it's full of cleaning supplies, and nobody looks at the back corners for months at a time. By the time most homeowners notice, a swollen cabinet floor, peeling laminate, a musty smell when the door opens, the leak has usually been running for a while. This guide covers finding the source, why cabinet floors are usually replaced rather than repaired, and the hidden spread that turns a cabinet problem into a kitchen problem.
Finding the source: four suspects
Empty the cabinet, dry everything with a towel, and lay down paper towels or newspaper, they'll show you exactly where the next drops land. Then check each suspect in order.
- 1
Supply lines and shutoff valves
The hot and cold lines and their angle-stop valves are pressurized around the clock, so even a pinhole drip runs 24 hours a day. Feel each connection and valve stem with a dry fingertip. These small braided or plastic lines are among the most common water-loss sources in a home.
- 2
The P-trap and drain connections
Drain leaks only run when water is flowing, so fill the sink, then pull the stopper and watch every slip joint while it drains. A drain leak that only appears under full flow is easy to miss with a quick look.
- 3
The garbage disposal
Disposals leak from the sink-flange seal on top, the dishwasher inlet or drain connections on the side, and, when the internal seals fail, from the bottom of the unit itself. A disposal dripping from the bottom housing is typically replaced, not repaired.
- 4
The dishwasher supply and drain lines
Both usually route through the sink cabinet. Run a dishwasher cycle and watch the connections, a dishwasher line leak shows up under the sink even though the dishwasher looks fine.
Why swollen cabinet floors get replaced, not dried
Most cabinet boxes are particleboard or MDF with a melamine or laminate skin. When particleboard absorbs water, the wood fibers expand and the resin bonds holding them break, that swelling is permanent, which is why a cabinet floor that puffed up stays puffed up after it dries. The material also loses structural strength once it's been saturated.
The practical fix is usually a cabinet-floor replacement: the swollen bottom panel is cut out and a new panel fitted, without replacing the whole cabinet box. If water wicked up the cabinet's side panels or the face frame, or the box has gone soft at the corners, replacement of the unit is on the table, and with older kitchens, matching a single cabinet can be the hard part.
The spread you can't see from inside the cabinet
The cabinet interior is the visible part. Water that pooled on the cabinet floor also finds the gaps: down through the base into the toe-kick cavity under the cabinet, backward into the wall cavity behind it, and sideways under the adjacent flooring. None of those spaces are visible without removing the toe-kick or cutting an access opening, and all of them are dark, enclosed, and slow to dry, ideal mold conditions if the leak ran for weeks.
This is the judgment call that separates a DIY wipe-down from a professional dry-out: if the leak was slow and long-running, or the cabinet floor was wet enough to swell, assume the spaces around the cabinet took on water too. A restoration pro checks the wall and toe-kick with a moisture meter in minutes, without demolition, and in a slab-on-grade Arizona kitchen, also checks whether moisture is sitting between the flooring and the concrete.
The musty-smell test
If the cabinet smells musty when you open the door, moisture has been present long enough for microbial growth to start somewhere, and it's often not on the surfaces you can see. A smell with no visible mold usually means the wall cavity or toe-kick space behind the cabinet is the problem.
Insurance: the leak vs. the damage
Homeowners policies generally distinguish the failed part from the resulting damage. A sudden supply-line or disposal failure that soaks the cabinet is typically a covered loss for the resulting damage, though the failed line or disposal itself usually isn't. A slow drip that ran for months is where coverage gets contested, because carriers can classify long-term seepage as a maintenance issue rather than a sudden loss. If you're filing, photograph everything before repairs and keep the failed part.
Common questions
- Can a swollen cabinet bottom be repaired instead of replaced?
- Not really, particleboard swelling is permanent because the fiber structure breaks when it saturates. The standard fix is replacing the bottom panel of the cabinet box, which is far cheaper than a new cabinet. If the sides or face frame swelled too, replacing the unit becomes the realistic option.
- Does insurance cover water damage under the kitchen sink?
- Usually yes for sudden failures, a burst supply line, a failed disposal seal, covering the resulting damage to cabinets, walls, and flooring, though not the failed part itself. Slow leaks that ran for months are often disputed as maintenance. Document the source and the damage with photos before anything is repaired or discarded.
- How do professionals dry inside and behind a cabinet?
- Without tearing the kitchen out: they remove the toe-kick to open the cavity under the cabinet, sometimes drill small vent holes in the cabinet base or wall (hidden areas), and direct air from air movers into those spaces while dehumidifiers pull the moisture out. Moisture meters confirm the wall, base, and slab are actually dry before anything is closed up.
- Should I shut off the water if I find a leak under the sink?
- Shut the angle-stop valves under the sink first, turn them clockwise. If a valve itself is the leak or won't close, shut the main water supply to the house. Then dry the cabinet out and diagnose before turning anything back on.
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