Under a standard kitchen sink you will find two supply lines with angle stops, one drain assembly with a P-trap, and often a garbage disposal, a dishwasher hose, and a filter line. Water comes in through the small braided hoses at the back and leaves through the larger curved drain pipe at the bottom.
Once you can name each part, a drip stops being a mystery. You can tell whether a leak needs a quarter-turn of a nut, a new washer, or a call. This guide walks the cabinet from back to front, then gives you a table that matches each wet spot to its likely source. For a dripping faucet above the counter, see our faucet repair service.
Start at the back wall: supply lines and angle stops
Two small valves sit on the wall or cabinet floor, one for hot and one for cold. These are the angle stops, also called shutoff valves, and each has a handle or oval knob. Turning one clockwise closes the water to that side of the faucet.
From each stop, a braided stainless or plastic supply line, usually three-eighths inch, climbs to the faucet shanks. These lines carry water under pressure all day, so they are the likeliest parts to fail suddenly. A bulging braid, rust at the nut, or a crusty white or green crystal at a fitting means it is time to replace the line.
Test your stops once a year. If a handle will not turn, do not force it, because a stop that snaps leaves you with no way to isolate the sink. Shut off the main at the house instead and have the valve replaced.
A labeled walk-through, back to front
Open the cabinet, empty it, and shine a flashlight along this path. Each item below is a part you can point to.
- Find the two angle stops at the wall and note which handle is hot (usually left) and which is cold (usually right).
- Follow each braided supply line up to the faucet, where a threaded connection or a quick-connect fitting joins it to the faucet body.
- Look straight under the sink basin for the tailpiece, a straight pipe that drops from the strainer or the disposal outlet.
- Trace the tailpiece down to the P-trap, the U-shaped bend that holds a water seal to block sewer gas.
- Follow the trap arm into the wall or floor stub-out, where the sink joins the main drain line.
- Locate any ribbed dishwasher hose, usually clamped to a small nipple on the disposal or on the tailpiece, and note how it loops upward before dropping.
- Look for a thin plastic tube feeding a separate filter faucet or cartridge, which is the filter line.
Leaks by location: a quick finder
Dry the cabinet floor, lay a paper towel under each joint, and run the tap for a minute. The towel that gets wet first points to the culprit.
| Wet spot | Likely source | What to try |
|---|---|---|
| Drips from the top of a supply line | Loose or worn compression nut or gasket | Snug a quarter turn, or replace the line |
| Water only while the faucet runs | Faucet body, cartridge, or supply connection | Check shanks, then inspect the faucet |
| Wet trap joint | Loose slip nut or flattened washer | Hand tighten, replace the washer |
| Puddle under the disposal | Flange seal, housing crack, or dishwasher hose | Check the three spots in that order |
| Wet only after the dishwasher drains | Hose clamp or air gap | Re-clamp, check the high loop |
| Dripping around the angle stop stem | Packing nut worn | Snug the packing nut, or replace the stop |
The P-trap, tailpiece, and why it holds water on purpose
The trap is not a defect. It holds an inch or two of standing water so sewer gas cannot rise into the room. If the trap dries out from a long absence, a faint sewer smell can appear until you run the tap.
Traps are held by slip nuts with a flat washer on each end. Over time the washer flattens or the plastic pipe warps, and a slow weep develops. A trap that sags, or one that has been jammed against the cabinet back, rarely seals well. Hand-tighten the nuts, then add a quarter turn only if needed, because overtightening cracks plastic threads.
Garbage disposal, dishwasher hose, and air gap
The disposal sits between the sink strainer and the trap, so its outlet connects to the trap arm with a short discharge tube. If the dishwasher drains through the disposal, a ribbed hose runs from the dishwasher to a small nipple on the disposal housing.
That hose should rise to the underside of the counter before dropping to the nipple. This high loop keeps dirty water from flowing back into the dishwasher. Some sinks use a separate air gap, a small cylinder near the faucet, instead of a loop. If your dishwasher smells or drains slowly, the loop, clamp, or nipple plug is the first place to look. Our drain cleaning service handles stubborn kitchen lines that back up even after the trap is clear.
Ten-minute check you can do tonight
Do this once a year, or any time the cabinet smells musty. It takes a flashlight and a dry towel.
- Press a dry paper towel against each supply nut and the base of each angle stop.
- Squeeze the braided supply lines gently and look for bulges, rust, or kinks.
- Turn each angle stop closed, then open, to confirm it moves freely.
- Run water for sixty seconds and watch the trap and tailpiece for beads of water.
- Look at the cabinet floor for swelling, stains, or soft spots in the particleboard.
- Check that the dishwasher hose loops up, not straight down, and that its clamps are snug.
- If you have a filter line, check the cartridge housing for sweating or a slow drip.
What affects the cost of fixing leaks under a kitchen sink
A drip under the sink may be a ten-minute fix or a sign of something larger. These are the factors that decide which.
Which part is failing
A supply line or washer is a small repair, while a corroded angle stop or cracked disposal housing takes more parts and time.
Cabinet and access
Cramped cabinets, tight corners, and stacked pipes make work slower.
How long the leak has run
A slow leak may have soaked the cabinet base or the floor beneath it, which adds repair work.
Condition of the surrounding pipe
Old, brittle fittings often crack when disturbed, so the job grows.
Shutoff valve condition
A stop that will not close can force a main shutoff and a valve replacement before anything else.
Homeowners insurance usually covers sudden and accidental damage, but typically not the failed part itself or gradual leaks and wear. Check your policy if a leak has damaged cabinets or flooring.
When to stop and call a plumber
Fix a loose nut or a flattened washer yourself. Stop if an angle stop will not close, if corrosion has eaten through a fitting, or if the leak appears while the faucet is off and you cannot find the source.
Also call if the cabinet floor is wet and the water bill has been creeping up. That points to a pressurized supply leak, which wastes water around the clock. Our crew serves homes in Fern Park and Winter Park as well as Casselberry, and a same-day appointment is possible when available.
Casselberry notes: what is in the water in your supply lines
The City of Casselberry Utilities Department draws groundwater from Floridan Aquifer wells at its North Plant, South Plant, and Howell Park facilities. It aerates the water to cut sulfur odor, disinfects with chlorine, and adds orthophosphate for corrosion control. The practical tie-back is that a rotten-egg smell from only one tap often points to a stale trap or the hot side, not the supply.
The South Plant also uses granular activated carbon filtration. If you add an under-sink filter, follow the cartridge maker's change schedule, because a spent cartridge stops doing its job. For an annual water quality report, ask the City of Casselberry Utilities Department, and test your tap if you want to know more about what reaches your sink.
Sources and standards used: City of Casselberry 2025 Consumer Confidence Report, City of Casselberry Utilities Department information
Frequently asked questions
It is the P-trap. The bend holds a small plug of water that blocks sewer gas from rising through the drain.
Yes, if the angle stops close fully. Shut off both stops, open the faucet to relieve pressure, disconnect the old lines, and install new braided ones with hand-tight nuts plus a quarter turn. If the stops will not close, shut off the main water valve and call a plumber.
The dishwasher hose, its clamp at the disposal nipple, or the high loop is usually the cause. A hose clamp that has loosened over time lets water seep out only when the drain pump runs.
Find and stop the leak first, then dry the cabinet fully with a fan. Swollen particleboard rarely recovers, so plan to replace the base panel once the source is fixed.
Run only the cold tap and sniff the water in a glass away from the drain. If the glass smells clean, the odor is coming from the drain side, often a dried trap or a dirty disposal. If the glass smells of sulfur, ask the City of Casselberry Utilities Department and check the hot side separately.
Still not sure which answer fits? Describe it to a licensed plumber.
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