A drip under the sink when you fill the basin past halfway — that’s usually the overflow pipe connection failing, not the main drain. The overflow is a backup system: it routes excess water to the drain before the sink spills over the rim. When the gasket dries out or the threads corrode, water leaks where the pipe connects to the drain assembly instead of going where it should.
I’ve replaced this twice in my house. Once on a pedestal sink where I had to wedge myself behind the basin with a flashlight in my mouth, and once on a vanity where I had room to actually see what I was doing. The job costs $8–25 in parts and takes about 45 minutes if you’ve got access. But before you pull anything apart, make sure the overflow is actually your problem.
Is it the overflow pipe or something else?
Run this check first:
- Water drains slowly during normal use → that’s a clog in the P-trap or aerator, not the overflow
- Water backs up when the basin is less than half full → main drain blockage
- Water leaking from underneath only when the basin fills past the overflow hole → overflow connection is failing; this is your job
- Visible cracks, corrosion, or mineral buildup on the overflow pipe under the sink → replace it
The overflow isn’t part of the main drainage path. It only activates when the basin fills high enough to reach the hole near the rim. If your sink drains poorly even when the water level is nowhere near the overflow, you’ve got a different problem.
Why overflow pipes fail
Most failures come down to three causes:
Hard water mineralization — Calcium and lime deposits build up inside the pipe and around the gasket over time, eventually blocking the overflow path or breaking the seal at the connection point. If your area has hard water and you’re seeing white crusty buildup on the pipe, the replacement will eventually fail again unless you address the mineral issue. This Old House recommends periodic descaling in hard-water areas.
Improper initial seating — If the overflow was installed without the gasket, or with the gasket crooked, it’ll leak from day one or fail within a year. I’ve seen this on new-build homes where a plumber rushed the job.
Corroded threads — The compression nut that holds the overflow to the drain assembly is usually brass or plastic. Brass corrodes if it sits in standing water (common in rarely-used guest bathrooms); plastic threads strip if someone over-tightened them. Either way, the connection loosens and water drips.
Replacement fixes the immediate leak, but if you’ve got hard water or a habit of over-tightening, the new pipe will fail the same way. I’ll cover prevention in the steps below.
What you’ll need
Tools:
- Adjustable wrench or 1.5” slip-nut wrench
- Flathead and Phillips screwdrivers
- Flashlight
- Bucket
- Old towels
- Plastic putty knife (for scraping old putty)
Materials:
- New overflow pipe and gasket kit ($8–15)
- Plumber’s putty or silicone caulk (optional, only if the rim seal needs reinforcement)
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About “universal” kits and compatibility
Most hardware stores sell universal overflow kits from manufacturers like Keeney or Danco that fit standard bathroom sinks. These work for the majority of drop-in vanity sinks and modern pedestal sinks. But there are exceptions:
- Older pedestal sinks (pre-1990s) — Rim diameters and hole spacing vary. Measure the overflow hole diameter and the distance from the overflow to the drain before you buy. If the kit collar doesn’t cover the hole, you’ll need an OEM replacement from the sink manufacturer.
- Vessel or bowl sinks — Many have non-standard overflow placement or no overflow at all. Check your sink model before assuming a universal kit will work.
- Cast-iron drain systems — Older homes with cast-iron drains may need adapter fittings to connect a modern plastic overflow pipe. If your drain pipe is black cast iron (not chrome or white PVC), bring the old overflow assembly to the hardware store and ask for compatibility.
If you’re not sure, measure the old pipe diameter where it connects to the drain (usually 1.25 inches) and take a photo of the assembly under the sink. Most hardware store staff can match it. Kohler and Moen both publish spec sheets for their sinks that list compatible overflow assemblies.
Prerequisites:
- Access under the sink (you’ll be lying on your back)
- Clearance to turn a wrench — pedestal sinks are tight
- Water shutoff valves that actually turn (test them before you start)
Before you start
Turn off the hot and cold shutoff valves under the sink. Turn them clockwise until they stop. Don’t force them if they’re stiff — old valves can crack, and then you’ve got a bigger problem. If the valves won’t budge or you don’t have shutoff valves under this sink, shut off water at the main.
Place a bucket under the drain assembly. Even if you drain the sink, water will spill when you loosen the compression nut.
Step 1: Empty and dry the sink
Open the drain and let the basin empty completely. Dry the inside with a towel, especially around the overflow hole. Water left in the basin will run down your arms when you’re lying under the sink. I learned this on a bathroom floor in a rental.
Step 2: Remove the sink stopper assembly
Most bathroom sinks have a pop-up stopper connected to a horizontal rod behind the drain. You’ll need to disconnect this to access the overflow pipe.
Under the sink, find the rod that connects to the back of the drain pipe. There’s a small nut or spring clip holding it. Unscrew the nut by hand (or use pliers if it’s tight) and slide the rod out. Pull the stopper straight up from above.
If your sink has a lift-and-turn stopper or a basket strainer, you may not need this step. Vessel sinks often have no pop-up assembly.
Step 3: Disconnect the overflow pipe from the drain assembly
Under the sink, locate where the overflow pipe connects to the main drain pipe — usually a compression nut about 2 inches below the basin. Turn it counterclockwise with your wrench. It should come loose without much force. If it’s corroded or stuck, spray it with penetrating oil and wait 10 minutes.
Once the nut is off, slide the rubber gasket off the overflow pipe. Set the nut aside; you’ll reuse it with the new pipe. Inspect the gasket — if it’s cracked, flattened, or crusty with mineral deposits, replace it. This gasket is the #1 reason replacements still leak.
Step 4: Remove the old overflow pipe from the sink
Look inside the sink basin at the overflow hole. You’ll see a collar or decorative cover holding the pipe in place. Some unscrew counterclockwise; others have a small setscrew you’ll loosen with a flathead screwdriver.
Once the collar is loose, push the pipe down from above (or pull it from below). The old pipe should slide out. If there’s dried putty or caulk around the rim, scrape it away with a plastic putty knife. Don’t gouge the porcelain.
Step 5: Install the new overflow pipe and gasket
Wipe the overflow hole clean — no old putty, no debris. Slide the new rubber gasket onto the overflow pipe. Check that it’s seated evenly, not crooked or bunched up.
Push the new pipe up through the hole from underneath until the collar seats against the sink rim. From above, thread the collar nut hand-tight, then give it a quarter-turn with the wrench. Stop.
Over-tightening is the most common mistake here. Plastic threads strip easily, and excessive force can crack the porcelain around the overflow hole. I’ve seen this happen twice — both times the sink had to be replaced because the rim cracked. According to Kohler’s installation guidance, overflow assemblies should be snug but not torqued. The gasket does the sealing work, not the threading.
Step 6: Reconnect the overflow to the drain assembly
Slide the rubber gasket onto the bottom of the overflow pipe where it meets the main drain. Use a new gasket if the old one looked worn or crusty. Thread the compression nut back on by hand, then tighten with the wrench — about a half-turn past hand-tight. Test it by tugging gently on the pipe; it shouldn’t move.
Reconnect the pop-up stopper rod if you removed it in Step 2.
Step 7: Test for leaks
Turn the water back on slowly — open the shutoff valves halfway first, then all the way. Air trapped in the lines can spray if you go too fast.
Plug the drain and fill the sink until water reaches the overflow hole. Let it sit for a minute, then check under the sink with your flashlight. Look at the compression nut connection and at the collar where the pipe meets the rim. Any drips mean you need to tighten the nut another quarter-turn or reseat the gasket.
If it’s dry after 5 minutes, you’re done.
Sink type variations
Pedestal sinks: Access underneath is terrible. You’ll be wedged between the pedestal and the wall. If the nuts are frozen or you can’t get a wrench in there, consider removing the basin entirely. This turns a 45-minute job into a 2-hour job, but it beats stripping threads because you couldn’t get leverage.
Vessel or bowl sinks: Usually the simplest case. Most have no pop-up assembly, and the overflow pipe (if present) is a straight vertical connection. Follow Steps 3–7.
Wall-mounted vanities: Check your wrench clearance before you start. Standard adjustable wrenches don’t always fit in tight spaces; a basin wrench helps.
Still leaking after replacement? Troubleshooting common issues
Drips at the compression nut connection
The gasket is either missing, crooked, or damaged. Disconnect the nut, inspect the gasket, and reseat it evenly. If it’s torn or flat, replace it — they’re $2 at any hardware store and reusing a bad gasket wastes the entire job.
Water seeping from the overflow hole rim inside the sink
The collar isn’t seated properly, or you over-tightened it and cracked the porcelain rim. Loosen the collar nut, check for cracks with a flashlight, and retighten gently. If the rim is cracked, you’ll need a sink replacement — the crack will spread.
Overflow pipe won’t stay seated in the hole
The collar threads are stripped, or the hole itself is damaged. Apply a thin bead of plumber’s putty around the rim as a temporary seal while you source a wider-collar replacement kit. This isn’t a permanent fix.
Water still backs up or drains slowly
The overflow was never your problem. Check the P-trap for clogs, or clean the faucet aerator.
Leak appears at the wall or floor below the sink
That’s not the overflow — it’s a supply line, a vent issue, or a cracked drain below the P-trap. Shut off the water immediately and call a plumber.
Preventing future failures
If you’re in a hard-water area, descale the overflow pipe every 6–12 months with a vinegar soak. Pour a cup of white vinegar into the basin, let it sit for 30 minutes, then flush with hot water. Purdue Extension recommends this for all drain components in mineral-heavy water systems.
Replace the gasket preemptively every 3–5 years, even if it’s not leaking. Gaskets are $2 and take 10 minutes to swap. Waiting until it fails means water damage to the cabinet.
Don’t over-tighten anything. Hand-tight plus a quarter-turn is the standard for compression fittings. More force doesn’t make it seal better — it just breaks things.
When to call a professional
The compression nut is corroded solid and won’t budge. Forcing it can strip the threads on the main drain pipe, which turns a $15 job into a $300 drain assembly replacement. A plumber has heat guns and better penetrating tools.
The sink is cracked around the overflow hole. Replacing the pipe won’t fix a structural crack. You’ll need a new sink, and depending on the type (pedestal, wall-mount, undermount), that’s often a two-person job.
You’ve got a pedestal sink and the basin won’t lift off. Some pedestals are bolted or caulked to the wall in ways that aren’t obvious. If you’re not confident removing it, hire it out.
Water is leaking from inside the wall after you reassembled everything. That’s a vent pipe issue or a hidden crack in the drain stack. Shut off the water and call someone immediately.
FAQ
What is a bathroom sink overflow pipe?
It’s the small tube connecting the overflow hole (the slot near the top of the sink basin) to the main drain pipe. It prevents the sink from spilling over the rim if you leave the water running with the drain closed. Most bathroom sinks have one; kitchen sinks usually don’t.
Can you replace an overflow pipe yourself?
Yes, if you’ve got access under the sink and the compression nut isn’t corroded. The job requires basic hand tools and costs $8–25 in parts. Pedestal sinks are harder due to tight clearance, but it’s doable.
How much does it cost to replace a sink overflow pipe?
DIY costs $8–25 for a replacement kit. A plumber will charge $150–250 for the same job, mostly labor. If the overflow is part of a larger drain assembly failure, expect $300–400.
What size is a bathroom sink overflow pipe?
Most are 1.25 inches in diameter where they connect to the drain assembly, but the actual pipe diameter varies by sink model. Measure your old pipe or bring it to the hardware store. Universal kits fit most standard bathroom sinks, but check compatibility for older or non-standard models.
How do I know if my overflow pipe needs replacing?
Water leaking under the sink when the basin fills past the overflow hole, visible cracks or corrosion on the pipe, or mineral buildup blocking the overflow path. If the sink drains normally when the water level is below the overflow hole, the pipe is probably fine.
The gasket is the thing that fails, and it’s the thing most people reuse because it looks fine. It’s not fine. A dried-out gasket doesn’t seal, and a new one costs $2. Every time someone’s called me over to look at a sink they just fixed that’s still leaking, it’s because they skipped the gasket. Don’t skip the gasket.