How to Repair a Leaking Pipe Under the Sink: JB Rooter and Plumbing Inc Tips

A leak under the sink never announces itself on a convenient day. You hear a faint drip or catch that sour smell of damp wood, open the cabinet, and there it is: a small puddle, a sheen on the P-trap, maybe a swollen particleboard floor. As plumbers who’ve fixed thousands of these across California kitchens and bathrooms, we can tell you most under-sink leaks fall into predictable patterns. The good news, many are fixable with a bit of patience, the right supplies, and a steady hand. The better news, knowing when to stop and call a licensed plumber in California saves far more money and stress than pushing a bad repair too far.

This guide shares field-tested methods for diagnosing and repairing the common culprits under a sink, plus the little judgment calls that separate a lasting fix from a temporary patch. Whether you’re looking up how to repair a leaking pipe for the first time or refining your routine, treat the steps below as a checklist and a set of decision points.

Why small leaks matter more than they look

A teaspoon a minute sounds harmless until you do the math. That’s close to two gallons every three hours, enough to soak cabinet floors, delaminate plywood, and feed mold behind a toe kick. We’ve pulled more than one sink base where a “small drip” turned into a $3,000 cabinet replacement and flooring repair. Water always wins if you give it time. Catching leaks early protects the cabinet, keeps soft metals from corroding, and prevents mineral deposits from roughening sealing surfaces.

Start with a careful inspection, not a wrench

Slow down before you start tightening things. Over-tightening is a classic DIY misstep that cracks plastic slip nuts or warps gaskets. Dry the area with paper towels, then run the faucet and watch with a flashlight. If the cabinet is dark, set your phone to video and aim for hard-to-see angles. Look for a bead forming, not just an old water stain. A leak at the trap has a different look than a leak from a pressurized supply line.

Focus your eyes on five zones: the faucet connections above, the shutoff valves and supply lines, the trap assembly, the tailpiece and sink strainer or basket, and any dishwasher or disposal connections. If there’s a garbage disposal, check the body seam and side port hose. If there’s a water filter or soap dispenser, inspect those hoses too. Many “mystery” leaks come from add-ons.

Know what type of leak you’re dealing with

Two broad families of leaks hide under a sink, and they behave differently.

Drain-side leaks are gravity-fed. They drip when you run water in the sink or dishwasher drain cycle. They show up around the P-trap, slip joints, tailpiece, basket strainer, or disposal gasket.

Supply-side leaks are under pressure. They may drip constantly or pulse with the faucet handle. They show up at shutoff valves, braided supply lines, compression fittings, or angle stops. A pinhole in a braided line can spray a mist that looks like condensation.

A third, less obvious category is splash or overflow. Water runs behind the sink lip, down the back, and collects in the cabinet. That’s not a pipe leak at all, and the fix involves caulking the sink rim or adjusting the faucet aerator to reduce overspray.

Common drain leak repairs and the logic behind them

Slip-joint fittings are designed to seal with a nut and a beveled washer, not brute force. A properly aligned washer will hold with firm, hand-tight turns, plus a gentle tweak with pliers. If you torque down to solve misalignment, you crush the washer and create a new leak.

P-trap leaks at the U-bend or connections usually trace back to a dried or deformed washer, mineral buildup on the mating surface, or a trap knocked out of alignment by storing a heavy cleaning bottle beneath. Remove the slip nut carefully, keep track of the washer orientation, and clean both pipe ends until smooth. Reassemble while keeping the trap square to the tailpiece and wall arm. If the trap or wall arm is pitted, replace it.

Tailpiece leaks just below the sink can be sneaky. If the tailpiece nut is cracked or the washer upside down, you’ll see a drip form after a few seconds of flow. The beveled side of the washer faces the nut. Hand snug, then a quarter turn.

Basket strainer and sink drain body leaks often appear as moisture at the bottom of the cabinet, not dripping from a fitting. The tell is a damp ring around the bottom of the sink where the basket strainer meets the steel or composite. If you see green corrosion or old plumber’s putty cracking around the top of the basket, plan a reseal. For stainless sinks we still prefer oil-based plumber’s putty, rolled into a 3/8 inch rope. For stone or composite sinks, use a compatible silicone to avoid staining. Don’t overtighten the basket nut to the point the sink bows.

Garbage disposal leaks have three common sources: the sink mount flange, the dishwasher inlet hose, or the discharge to the P-trap. A leak from the bottom seam of the disposal body usually means internal failure. That’s replacement territory, not repair. If you swap a disposal, refresh the sink flange seal. Many callbacks come from reusing a flattened flange gasket.

Common supply leak repairs and what sets them apart

Supply leaks do more damage faster because they are under pressure. A braided stainless hose with a pinhole can spray the back of the cabinet and soak drywall, then the baseboard, then the floor. When you see a misting spray or hear hissing, shut off the angle stops immediately. If the stop won’t shut off fully, use the main house shutoff.

Angle stops themselves fail with age. Compression angle stops last a long time but the packing nut under the handle can loosen. A gentle eighth-turn snug on the packing nut often stops seepage. If the valve body weeps or the chromed tubing is pitted, replace the valve and consider upgrading to quarter-turn ball valves for reliability.

Toilet-style flexible hoses sometimes get reused under sinks during a remodel, which is not ideal. Use faucet-rated braided connectors of correct length. Too short and they strain the fittings, too long and they loop with a kink that fails later. Every experienced plumber for pipe replacement has a bin of correctly sized hoses for this reason.

If you have copper supply risers that connect with compression fittings, inspect the ferrule and nut. Once a ferrule bites onto copper, it’s married to that spot. Over-tightening will not fix a scratched sealing surface. If in doubt, cut back to clean copper and use a new ferrule and nut. On PEX or push-to-connect fittings, ensure the pipe is cut square and deburred. A slanted cut will leak even if it seems tight.

The step-by-step that avoids do-overs

Below is a single checklist we give new techs when training. It keeps you from chasing the wrong drip and makes sure you leave everything better than you found it.

Dry everything, then run the faucet while watching a single joint at a time. Confirm drain versus supply. For drain leaks, disassemble the suspect joint, clean, inspect washers, replace if hardened, reassemble in alignment, and snug, don’t force. For supply leaks, shut off water, relieve pressure, replace suspect hoses or angle stops, and verify with tissue paper for the faintest seep. Once fixed, run hot and cold for two minutes, fill the sink to stress the basket seal, then release and check again. Wipe cabinet dry, place a paper towel under the assembly, and recheck after an hour to catch slow seeps.

Tools and materials that make the difference

A bowl and towels handle cleanup, but a few inexpensive specialty items turn guesswork into control. Channel-lock pliers sized for slip nuts, a basin wrench for tight faucet connections, a small adjustable wrench, and a good flashlight are the basics. Keep a pack of 1.25 and 1.5 inch slip-joint washers, both beveled and flat. Have Teflon tape for threaded connections, but avoid wrapping compression threads where the seal is on the ferrule, not the threads. Plumber’s putty, a tube of 100 percent silicone sealant for stone sinks, and spare braided faucet connectors round out a handy kit.

If the sink uses ABS or PVC tubular drain parts, have a few spare sections. The time lost making a brittle old piece limp along costs more than replacing it.

When a quick patch is enough, and when it’s a trap

Not every scenario calls for a full rebuild. A loose slip nut that drips once each time the dishwasher drains can be cured with a washer and correct alignment. A weeping packing nut on an angle stop often needs a quarter turn. But we’ve also seen temporary pipe repair tape wrapped over a split trap, which turned into a full cabinet replacement after one forgotten overnight cycle.

Ask two questions: Is the component brittle, pitted, or corroded through? Is the leak coming from a sealing surface rather than a replaceable washer? If either answer is yes, replacement is the smart move. For example, a rusted steel tailpiece, a cracked plastic trap, or a disposal leaking from its lower housing all want new parts, not patches.

Edge cases that fool even seasoned techs

A slow-draining sink can push water out at slips that otherwise seal fine. If you fix a leak at the trap but still smell musty air, the real issue may be a partial clog building pressure in the line. The symptom shows up when the sink is full, then released, and you see a quick burst at the slip joints. Clearing the line with an auger or a plumber for drain cleaning may be the real fix. A quick test is to remove the trap, send a small jet of water into the wall arm, and see if it backs up.

Another confusion is condensation. In summer, cold water lines sweat in humid rooms. You dry everything, run cold water, and think you have a leak as droplets form. Wrap a paper towel around the line and watch. If the towel gets uniformly damp, and you see no bead at a joint, it’s condensation. Insulate the pipe or reduce humidity.

We also see leaks from hand soap dispensers with cracked bottles. The customer finds a puddle and assumes the trap leaks, but the trap is dry. Always check attachments.

Replacing a basket strainer like a pro

People tend to dread this job because of access. It’s manageable with a bit of technique. Remove the P-trap and tailpiece to clear space. Loosen the strainer locknut from below. If the old putty is rock-hard, lightly tap the strainer lip from above with a wood block to break the seal. Clean the sink opening flawlessly. If this surface has scratches or pitting, smoothing with fine steel wool helps.

Roll a rope of plumber’s putty and ring it around the opening. Set the new strainer and press evenly until putty squeezes out all around. From below, install the rubber gasket, friction ring, and locknut in the order the manufacturer specifies. Tighten evenly. Wipe excess putty from above. After reconnecting the drain, fill the sink, let it sit 3 to 5 minutes, then check underneath. If you see a weep, tighten the locknut a fraction more. Old sinks flex, so aim for snug and sealed, not gorilla-tight.

Compression vs. threaded vs. push-to-connect under the sink

Kitchen and bath supply connections come in a mix. Understanding where the seal happens prevents misapplied sealant. Compression fittings seal on the ferrule biting into the pipe, so Teflon tape on threads does nothing. Threaded IPS connections do seal on threads, so tape or pipe dope helps. Push-to-connect fittings seal on an internal O-ring; a clean, square pipe end is everything. If you see a slow weep at a push-to-connect, the pipe edge may be nicked. Cut back and reinsert.

If you’re upgrading old chrome-plated copper risers, braided stainless supply lines with brass ends are a safe bet. Match the faucet and stop sizes, usually 3/8 inch compression at the faucet and 3/8 or 1/2 inch at the stop.

What to do before things escalate to emergency plumbing help

Shutoff valves are a homeowner’s best friend. Know where your sink angle stops are and make sure they actually move. If a valve hasn’t budged in ten years, it might freeze the day you need it most. Work it gently now. If it leaks around the stem while turning, snug the packing nut slightly.

If a supply line bursts or you see a spray you can’t stop, go for the main shutoff. In most California homes that means a valve at the front hose bib area, the garage, or a ground box near the curb. If the valve is a stubborn gate style, it may take several full turns. Replacing that main valve with a quarter-turn ball valve is a good preventive upgrade, something a trusted plumber for home repairs can schedule during regular hours so you don’t meet us at 2 a.m.

How to avoid repeat leaks after a remodel

We handle a lot of calls after a bathroom or kitchen refresh where a new sink, faucet, or garbage disposal went in but the old trap assembly stayed. Mixing old and new components with mismatched diameters or odd geometries invites leaks. If you upgrade fixtures, rebuild the trap assembly to match the new layout. For deep farmhouse sinks, you often need a longer tailpiece and a trap repositioning to keep slope and alignment.

If you’re planning bigger changes, bring in a plumbing company in my area that does plumbing services for bathroom remodel projects regularly. Small decisions like the height of a wall stub-out or the choice between ABS and PVC adapters can save hours later.

What a pro checks that most DIY fixes skip

Any experienced plumber for pipe replacement runs a hand around every joint after the first test, then again after the cabinet sits closed for a bit. We also check for backflow air gaps if there’s a dishwasher, and we look for looped or sagging drain hoses that can siphon water back. We feel the base of angle stops for warmth that suggests a slow seep evaporating as it drips, and we verify aerator flow because high pressure can aggravate weak seals.

On older homes we look for galvanic corrosion where dissimilar metals meet, such as brass nuts on steel tailpieces. A simple change to matching materials extends the life of the repair.

When to stop and call a licensed plumber in California

There’s pride in tackling a leak, and most of the time you can. Stop and bring in help if you see any of these: the shutoff valves do not work or won’t fully close, the leak is from a corrosion pit or a cracked casting, the wall arm moves freely in the wall suggesting a broken trap adapter, or there’s visible mold growth and damaged wood beyond the cabinet floor. Also call if a disposal leaks from the body, if you’re uncomfortable re-sealing a stone sink with the right silicone, or if you have mixed materials that need a certified plumber for sewer repair evaluation because the drain doesn’t just leak, it backs up.

If a clog is part of the picture and you’ve already tried a plunger and a zip snake, a plumber for drain cleaning can clear the line without damaging the trap or pushing a clog deeper. And if the leak is tied to your water heater lines or recirculation piping, a plumbing expert for water heater repair should evaluate pressure, expansion, and thermal stress.

Preventive maintenance that costs less than a single cabinet repair

A ten-minute check twice a year pays off. Open the cabinet, run hot and cold, fill and dump the sink, and check every joint by touch. Replace any supply line older than 8 to 10 years, sooner if you see corrosion at the ferrules. Consider replacing multi-turn angle stops with quarter-turn valves during a faucet upgrade. Keep the area under the sink tidy so nothing bumps the trap. Adhesive water leak sensors are inexpensive and buy you early warning.

If you ever move into a new place and wonder how to find a local plumber who fixes water leaks and stands behind their work, ask neighbors which top rated plumbing company near me they’ve actually used. Consistent service stories matter more than a flashy ad. When you narrow it down to the nearest plumbing contractor, make sure they’re licensed and insured. In California, verify the license online in minutes.

What JB Rooter and Plumbing Inc does differently on under-sink leaks

Our team treats under-sink work as a mini system check. We don’t just swap a washer. We check alignment, replace brittle trap parts rather than over-tightening them, inspect supply lines with a mirror, and test under both static and flow conditions. If you need a plumber to install water heater in the same visit or have a reliable plumber for toilet repair on deck for another bathroom, we coordinate so your day isn’t a parade of appointments. We’ve helped homeowners who searched for an affordable plumber near me and got stuck with stopgap fixes. Our approach favors durable materials and correct geometry, which costs a little more on parts and less on professional plumbing industry service callbacks.

If you’re choosing between local plumbing repair specialists, ask how they test their repairs, what materials they stock on the truck, and whether they warranty under-sink work. A trusted plumber for home repairs should answer clearly and put it in writing.

A careful walk-through of a real job

A recent service call in a 1970s ranch house illustrates the decisions involved. The homeowner reported a musty smell and occasional puddles under the kitchen sink. The cabinet floor was swollen at the back, which suggested a long-term slow problem rather than a sudden burst.

We dried everything and ran the faucet. No drip showed at the trap, but after filling the sink and releasing, a quick bead formed at the trap arm slip joint. That pointed to a pressure pulse from a partial clog. We disassembled the tubular, found gritty mineral buildup at the washer seat, and the washer was flattened. Replaced the washers, cleaned the seat, and reassembled with correct alignment. With the trap off, we ran water into the wall arm and saw a minor slow rise, confirming partial blockage. A light auger cleared it.

While testing, we noticed the cold supply braided line had a rust halo at the ferrule. Under a paper towel, it showed a faint wick. We recommended replacing both supply lines and the multi-turn angle stops with quarter-turn valves. After installing, we tested again, filled the sink for five minutes, released, ran the dishwasher for a drain cycle, then rechecked. Dry. We placed a leak sensor as a courtesy. That homeowner had been searching for the best plumber near me after two DIY attempts. The real fix was a combination of alignment, fresh washers, a cleared line, and new valves.

If the leak turns out to be a clog problem

Under-sink leaks tied to clogs crop up more often now that many kitchens handle heavy use. Grease, fibrous foods, and coffee grounds are the classic trio. If your symptom is a leak only when the sink is full and released, and you also notice slow drainage, fix the clog first. A hand auger is safer than chemical drain cleaners, which can soften old gaskets and create future leaks. If the clog is beyond reach, call a plumber for drain cleaning who can run a proper cable and camera if needed. Clearing the line reduces pressure spikes that force water past marginal seals.

If you need help on the spot and can’t get a same-day slot, many companies offer emergency plumbing help. Use it when water is running and you cannot stop it, not for a slow drip that you can catch in a bowl overnight. That triage saves you the emergency fee and gets you a calmer, more thorough repair the next morning.

A quick word on other fixtures while you’re under there

Once you’re in the cabinet, scan for other issues. Dishwasher drain hose should loop up high to prevent backflow. Disposal cords should be intact with no tape repairs. If you see corrosion around a copper joint on the hot side, note it and plan a replacement. If your water heater is nearby and you see corrosion on its flex lines or a slow drip at the T&P discharge, schedule a plumbing expert for water heater repair before it becomes urgent.

For bathrooms, under-sink leaks often pair with a worn pop-up drain assembly. If you’re already resealing a basket plumbing industry experts in the kitchen, consider resealing a lavatory drain with the right putty or silicone. Small wins add up.

Choosing the right help, if you want it

If you decide to bring in a pro, it helps to be specific. Tell them you have a drain-side leak at the P-trap after releasing a full sink, or a constant supply-side drip at the cold angle stop. Mention any previous attempts, the age of the disposal, and whether the shutoffs work. This saves time and ensures the tech brings the right parts. When you call a top rated plumbing company near me, ask whether they stock various trap sizes, basket strainers, and angle stop types on their trucks. A well-stocked truck is a sign of a prepared team.

If you’re local, JB Rooter and Plumbing Inc can help diagnose and repair, from simple under-sink leaks to larger issues like certified plumber for sewer repair needs or planning plumbing services for bathroom remodel projects. Our scheduling team can combine tasks if you also need someone to fix clogged kitchen sink troubles in another room or coordinate with an experienced plumber for pipe replacement if we find brittle galvanized nearby.

Final checks that protect the cabinet

After any repair, leave the cabinet open for an hour. A slow seep can take time to show. Lay down a dry paper towel under the assembly. If you come back and see a dime-sized spot, revisit the joint before you put cleaning supplies back. Label the shutoffs with hot and cold tags so anyone in the house can react quickly if they see water. If your home has elderly angle stops or you’re not sure how to turn off the main, put that on your weekend list. Peace of mind is worth more than any sink caddy.

A leak under the sink is one of the most common home repairs, and also one of the most forgiving if you take a methodical approach. Start with accurate diagnosis, respect how each joint is designed to seal, and replace tired parts instead of forcing them. Know when a fix crosses into risk. And if you’d rather hand it off, find a nearby pro who fixes water leaks every day, communicates clearly, and stands behind the work. Whether you search for the nearest plumbing contractor or ask your neighbor for a referral, you’ll feel the difference when the cabinet stays dry, the valves turn smoothly, and your kitchen goes back to being a kitchen, not a wading pool.

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Pub: 27 Sep 2025 19:41 UTC

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