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The sink cabinet stays dry until the faucet runs
When water appears under a bathroom sink only while the faucet is running, how do you narrow down whether it’s coming from a pressurized supply connection or draining down from the faucet or basin? Would it matter whether it happens with hot water, cold water, or both?
I’d like to know what can be observed safely without loosening any fittings. If the source is clearly at a supply connection, is closing that fixture’s shutoff valve the sensible immediate step, or should the response depend on how much it’s leaking?
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Dry the visible pipes, shutoffs, and cabinet floor, then briefly run cold water while watching where moisture first appears; repeat with hot water. If the leak occurs with only one temperature, that may help identify which supply side is involved, but it doesn’t establish the source by itself. Water first appearing at the drain, faucet base, or basin suggests a drain or fixture path. If a supply connection is clearly leaking, close its shutoff if it turns easily. If water keeps coming or the valve won’t close, stop using the faucet and shut off the home’s main water supply if you can do so safely; get plumbing help.
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