How to Fix a Leaking Sink Drain or P-Trap

A leaking sink drain almost always traces back to a single weep point, and the repair is fast once you find it. The water you see pooling in the cabinet or dripping off the trap is rarely coming from where it lands. It runs along the underside of the pipe and falls from the low spot, so the puddle tells you very little about the actual source. The fix that works is to stop guessing, dry every fitting, run water, and watch for the first bead to form. That one bead points at the joint you need to repair, and on the drain side of the sink that repair is almost always a worn slip-joint washer, a failed strainer seal, or a cracked length of trap, none of which is dangerous to work on.

This guide stays on the drain assembly: the tailpiece, the trap, the slip-joint connections, and the basket strainer or drain flange. The drain side carries no pressure, which is what makes it a safe homeowner repair. Before you start, one boundary matters more than any tool. If the wet line is a supply line, the smaller pressurized hot or cold tube feeding the faucet, that is a different and higher-stakes job covered in our guide on replacing a sink sprayer or supply line (post 195). The section below shows you how to tell the two apart in under a minute.

Find the Exact Weep Point: Trap, Slip Joint, Strainer, or Tailpiece

To pin a drain leak to one joint, dry the entire assembly with a paper towel, then run the tap and watch where the first drop reappears. Water tracks downhill along the pipe, so the visible drip is downstream of the real leak. Drying everything first removes that confusion. Wrap a dry paper towel or tissue around each connection one at a time and watch which one darkens first. The order in which the weep points fail is predictable, so check them in this sequence.

Start at the slip-joint nuts, the threaded collars where one tube slides into the next. They are the most common drain leak because the washer inside compresses over time. Next look at the trap itself, the U-shaped bend, for a hairline crack or a corroded spot, especially on older metal traps. Then check the tailpiece, the straight pipe dropping from the sink, where it meets the strainer above and the trap below. Finally, look up at the basket strainer or drain flange, the metal ring in the bottom of the sink basin. A flange that leaks shows up as water on top of the trap or running down the tailpiece only when the basin holds standing water, which is your clue to test it differently. Two tests separate these: a running-water test catches slip-joint and trap leaks, while a filled-basin flood test, described in the last section, is the only way to catch a strainer or flange seal that leaks only under a head of standing water.

Drain Leak vs Supply Leak: Why They’re Fixed Differently

A drain leak is unpressurized and sealed with washers and putty, while a supply leak is pressurized and sealed with compression fittings or threaded connections, and that single difference decides the entire repair. Knowing which one you have keeps you from buying the wrong parts and from underestimating a more serious leak.

The supply lines are the two small tubes, often braided steel or chrome, that run from the shutoff valves up to the faucet. They are always under pressure, so a supply leak sprays or runs steadily even when no tap is open. The drain side, by contrast, only gets wet when water is actually flowing down the drain, because nothing in it is pressurized between uses. A simple confirmation: dry everything, close the faucet, and wait. If a connection gets wet while the faucet sits off, you are looking at a pressurized supply leak, and that belongs to our guide on supply lines and sprayers (post 195), not this one. If the fittings stay dry until you run water down the drain, you have a drain leak you can fix with the steps here. Leaks behind the wall, in a copper pipe, or under a slab are a separate matter entirely and are covered in our guides on pinhole leaks in copper (post 105) and slab leaks (post 111). A faucet weeping from its base or handle on top of the counter is also its own problem, addressed in our guide on a faucet that leaks at the base or handle (post 026).

Re-Seating or Replacing Slip-Joint Washers and Nuts

If the leak is at a slip-joint nut, loosen it, slide back the nut and washer, and either reseat the washer square or drop in a fresh one, then hand-tighten and add no more than a quarter turn. Slip-joint connections seal with a soft washer compressed by the nut, not by brute force on the threads. Most slip-joint leaks come from a washer that has hardened, slipped off center, or was overtightened until it deformed.

Put a bucket under the work, then unthread the leaking slip nut by hand or with channel-lock pliers, turning counterclockwise. Slide the nut and the washer back along the pipe to expose the joint. Inspect the washer: a good one is soft and round, while a flattened, cracked, or brittle one needs replacing. Slip-joint washers are inexpensive and sold in the common one-and-a-quarter and one-and-a-half inch tubular sizes, so matching the pipe diameter is the only choice you need to make. If your washer is beveled rather than flat, the tapered edge points into the joint so the nut squeezes it into a tight ring as it threads down. Seat the pipe back so it sits straight and fully engaged, not cocked at an angle, then thread the nut on by hand until it stops. Snug it with pliers no more than a quarter turn past hand-tight. Oatey, a major fitting manufacturer, warns that excessive force can crack plastic fittings or strip the threads, which turns a small drip into a part you have to replace. Run water and check before you tighten any further.

Repairing a Leaking Basket Strainer With Plumber’s Putty

A basket strainer or drain flange that leaks needs to be unthreaded from below, cleaned of old sealant, and re-bedded with a fresh ring of plumber’s putty or silicone under the rim, depending on your sink material. This is the seal between the metal drain ring and the sink basin, and it fails when the original putty dries out or the locknut beneath loosens. A telltale sign is water that only appears when the basin is filled and sitting full.

From under the sink, loosen and remove the large locknut holding the strainer body to the basin, then disconnect the tailpiece so the whole strainer lifts out from above. Scrape every trace of old putty off both the sink surface and the underside of the flange, and wipe the area dry. The sealant choice depends on your sink. For a stainless steel, porcelain, enamel, or cast iron sink, roll plumber’s putty into a rope about half an inch thick and press it in a complete circle under the flange rim. For a granite, marble, quartz, or other natural or composite stone sink, do not use standard plumber’s putty: its oils can leach in and leave a permanent stain, so use a stain-free putty made for stone or a pure silicone sealant instead, per Oatey’s own guidance. Silicone is also the right call on most plastic and acrylic sinks, where standard putty can attack the material. Set the flange into the opening, press it down so putty squeezes out evenly all around, then tighten the locknut from below to draw it tight. Wipe away the squeeze-out. If you used putty, you can run water right away to test, while a silicone seal needs to cure undisturbed, typically about 24 hours, before it holds against standing water. Replacing the entire pop-up or strainer assembly rather than just resealing it is a separate job covered in our guide on replacing a sink drain or pop-up stopper (post 035).

Replacing a Cracked Trap or Tailpiece

If the trap body or tailpiece is cracked, corroded, or split rather than just loose, no amount of tightening will seal it, and the only fix is to swap the part for a matching new one. A hairline crack in a plastic trap often opens just enough to weep under flow, and an old metal trap can corrode through at the bottom of the bend where water sits. Tightening a cracked fitting only stresses it further.

Trap kits are sold by pipe diameter and material, so take the old part to match it, or measure the tube before you buy. A standard sink trap is one-and-a-quarter inch for a bathroom lavatory or one-and-a-half inch for a kitchen, in PVC, ABS, or chrome-plated brass. Unthread the slip nuts at both ends, remove the damaged piece, and dry-fit the replacement before sealing anything to confirm the bend reaches both the tailpiece and the wall outlet without strain. Reuse the slip nuts and fresh washers, and assemble in the same hand-tight-plus-a-quarter-turn way as a slip-joint repair. If the damage continues past the trap into the pipe entering the wall, or the drain opening itself is corroded, that crosses into drain-line work that may need a licensed plumber rather than a tubular trap kit. If you only want to clean a trap that is not actually leaking, that is a different task covered in our guide on cleaning a sink P-trap (post 031), and a refresher on what the trap does lives in our guide on what a P-trap is (post 004).

Testing the Repair With a Filled-Basin Flood Test

Confirm the repair with two tests in order: run water for a full minute watching every joint, then plug the basin, fill it, and release the whole load at once. A repair that passes a thin stream can still fail under a real volume of water, which is why the flood test matters. Many strainer and flange leaks only reveal themselves when the basin holds standing water and puts a head of pressure on the seal.

First, with the assembly dry and a dry paper towel laid under the trap, run the tap hard for sixty seconds and watch and feel each connection for any bead of moisture. The dry towel will catch a slow weep your eye might miss. Then close the stopper or plug the drain, fill the basin to the overflow, and pull the plug so the full basin drains in one rush. This loads every joint the way daily use will and tests the strainer seal under standing water. Recheck each fitting and feel the underside of the trap with a finger. If a joint weeps, tighten that nut a small amount, no more, and test again before assuming the washer is bad. Leave a dry sheet of paper towel under the trap and check it after a day of normal use, since the slowest leaks announce themselves only over time. When a full basin drains clean and the paper stays dry overnight, the drain is sealed and the job is done.

This article is general information about a common household repair and is not professional or safety advice. The drain assembly under a sink is unpressurized and generally safe for a homeowner to work on, but if the leak is on a pressurized supply line, behind the wall, or in a pipe you cannot fully reach, or if you are not comfortable doing the work, consult a licensed plumber.

Sources

• Oatey, How Do You Install a P-Trap? Complete DIY Guide (position a slip joint nut and washer on the tailpiece, seat the washer against the fitting, tighten snugly by hand then firm with pliers; do not overtighten, because excessive force can crack plastic fittings or strip threads and cause leaks; run water 30 to 60 seconds and inspect every connection). https://www.oatey.com/resources/project-guides/how-install-p-trap
• Oatey, How to Properly Use Plumber’s Putty (clean and dry the surface; knead the putty and form a half-inch rope in a complete circle under the flange; avoid standard putty on plastic, acrylic, and ABS, where silicone is recommended; use stain-free putty made for porous stone such as granite, marble, and quartz). https://www.oatey.com/faqs-blog-videos-case-studies/blog/how-properly-use-plumbers-putty-tips-mistakes-avoid-and-when-use
• Oatey, Stain-Free Plumber’s Putty (specially formulated for porous stone surfaces like granite, marble, quartz, sandstone, and Corian to deliver a secure seal without staining the finish). https://www.oatey.com/products/oatey-stainfree-plumbers-putty–934748176