Check under every sink
Twice a year · typically February and August
Why it matters
The plumbing you can see fails loudly — a burst pipe announces itself. The plumbing under sinks fails silently: a supply-line fitting weeping one drop an hour, a trap slip-joint loosened by years of storing and shoving cleaning bottles, a shutoff valve with a slow crust of mineral growing on it. None of it makes a sound, all of it is staining wood the whole time, and the cabinet floor hides the evidence until it's structural.
Twice a year, every sink, ninety seconds each. That's the whole program.
What skipping it costs
$400–$6,000
Under-sink leaks are drip-per-hour slow, and cabinet floors are particleboard — they swell, then rot, then grow mold, quietly, behind the cleaning supplies. What a paper towel catches in February as a damp fitting becomes, by the following year, a replaced vanity, a patched floor, and sometimes the ceiling below: $2,000–$6,000 for a leak that never once made a sound.
Two ways to get it done
Do it yourself
You'll need: Flashlight, Dry paper towel.
- Empty the cabinet (this is half the value — leaks hide behind the dish soap stockpile).
- Flashlight on the cabinet floor first: stains, swelling, warping, or that telltale white mineral crust on fittings. Sniff — musty means water, somewhere.
- Run the water, hot and cold, then run the dry-paper-towel test: wrap the towel around each supply-line connection, each shutoff valve, and each trap joint. Any dampness prints on the towel instantly.
- Fill the basin, then pull the stopper and watch the trap under full drain flow — some joints only weep under volume.
- Wiggle check: supply lines shouldn't be tight as guitar strings or resting on anything sharp. Original rubber washing-machine-style lines or plastic lines? Braided stainless replacements are $8–$15 each — the single cheapest flood prevention there is.
- Find damage but no active leak? Dry everything, lay a fresh paper towel flat on the cabinet floor, and check it in a week. The towel is your leak detector.
Hire it out
$100–$200National fair range — local labor moves this number.
There's no service call for looking under a sink — but when your semiannual check finds something (a weeping valve, a corroded trap, a drip you can't chase to its source), a plumber's visit runs $100–$200 and should fix ALL of it in one trip: batch every fixture's issues into that visit and you've turned the trip charge into a bargain.
While they're there, have them test every shutoff valve under every sink. A shutoff that won't shut is a valve you'll discover during an emergency — and swapping stiff ones for quarter-turn valves is cheap insurance done in bulk.
Ask the tech
- Are these supply lines braided stainless or the original plastic/rubber, and how old are the shutoff valves?
- Is this trap slip-joint or glued, and is the slope right?
- Do the shutoffs under here actually close? Test one while you're in there.
Red flags you're being upsold
- A faucet-replacement pitch for a fixable drip — cartridges and washers are $5–$30 parts, and 'they don't make parts for this anymore' is rarely true for anything under 25 years old.
- Quotes over $250 to replace a P-trap. It's a $15 part and twenty minutes.
- Leak-detection-system hard sells ($500+) triggered by one damp fitting. Fix the fitting first; smart sensors are a fine upgrade, not an emergency.
The call
DIY, obviously — it's a flashlight and a paper towel. The skill isn't plumbing; it's remembering to look.