Water Damage Under the Sink:
What Swollen, Stained Cabinet Floors Tell Russ
Every sink cabinet keeps a record. A rippled, stained floor under the trap means water got there at some point. The question is whether it still does.
At 47 of Russ’s recent inspections, a sink cabinet showed damage from water. Twelve reports called it damage from old leaks. Nine called it minor moisture damage, and seven more noted moisture damage inside the sink cabinet. A Portland report gave the usual cause: long-term, slow leaks at the sink drain, supply line connections, or the garbage disposal.
The damage is easy to see. What’s harder is telling whether it’s done.
What the cabinet floor shows
Most cabinet floors are particleboard or thin plywood with a laminate or paint finish. When water sits on them, the board swells, the edges puff up, the finish peels and dark stains spread from the back corner. Russ’s report wording for the common case: the under-sink cabinet had damage from old leaks, indicated by water stains, swelling, or deterioration of the cabinet materials.
Sellers sometimes cover it. A Salem report found an OSB sheet laid over the original cabinet base, stained and with apparent fungal growth, a sign of a history of plumbing leaks. A shelf liner or a new board on top doesn’t make the leak history go away.
Old leak or active leak
Russ runs the faucet hot and cold, fills the basin and lets it drain, and runs the disposal and dishwasher if present. Then he looks at every joint under the sink with a light, and checks the cabinet floor with a moisture meter. The moisture meter readings post explains what those numbers mean and what they don’t.
Dry readings over swollen wood usually point at an old leak that’s been fixed. Russ’s reports still recommend monitoring the connections, because the trap or supply line that leaked once is the usual one to leak again.
The parts that leak
Trap slip nuts loosen. Braided supply lines age, and the supply lines post covers why plastic-nut lines get replaced. Faucet bases leak down the back of the sink, out of sight. The dishwasher drain hose and the disposal outlet add two more joints in a kitchen. A dishwasher hose with no high loop can push dirty water back into the cabinet; the dishwasher high loop post covers that.
Under the cabinet
Water that got the cabinet floor wet sometimes got the subfloor wet too. Russ looks at the framing and subfloor below kitchens and bathrooms from the crawlspace or basement when he can reach it, and checks the ceiling under upstairs vanities. Stains or soft wood down there are a different conversation than a cabinet floor.
Real Inspection Photos
Sink Cabinet Findings From Russ's Inspections
Cabinet water damage from Russ's recent reports.
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Moisture damage under a sink, with the cabinet floor swelling around the drain. -
OSB laid over the original cabinet base, stained from past leaks. -
Stains and minor damage from old leaks. -
Water rings under the disposal and drain lines. -
Cabinet damage from old leaks under a kitchen sink.
What Russ checks
He runs the fixtures, looks at the visible plumbing and the cabinet interior, takes moisture readings, and uses thermal imaging, which can show cool patterns consistent with moisture but doesn’t confirm a leak on its own. He doesn’t move stored items, remove cabinet floors or open walls. Active leaks go to a licensed plumber. Damaged cabinets get a recommendation for repair or replacement by a qualified contractor.
- Empty the cabinet and look at the floor with a flashlight
- Press on swollen spots: soft or just puffy?
- Run water and watch the trap, supply lines and faucet base
- Run the disposal and dishwasher and watch their connections
- Look at the floor framing below the sink from the crawlspace
Sink cabinet water damage came up at 47 of Russ’s recent inspections. Most of it was from old leaks that were already fixed, but the same joints tend to leak again. Run the water, check the joints, and look at the floor below the cabinet.
The guide to what to look at in the kitchen before you make an offer covers the rest of the room. To set up an inspection, you can book online.
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