Leaks only when the sink drains
The cabinet stays dry until you run water, then drips show up on the trap or horizontal drain section.
Start here: Check the P-trap, trap arm, and tailpiece for chew marks, splits, or pinholes.
Plumbing
Mice Chewed Drain Pipe Under Sink
Direct answer: If mice chewed a drain pipe under the sink, the usual fix is replacing the damaged section of the sink drain assembly, not patching it. Most often that means a sink P-trap, trap arm, or drain tailpiece with visible tooth marks, pinholes, or a split seam.
Most likely: The most likely problem is a chewed plastic sink P-trap or nearby slip-joint section that only leaks when the sink drains.
Start by proving exactly which piece is damaged and whether the leak happens only during drainage. Under-sink drips can fool you because water runs along the bottom of the pipe and falls somewhere else. Reality check: even a tiny chew hole can soak a cabinet fast. Common wrong move: replacing the whole sink drain when the damage is only one removable trap section.
Don’t start with: Don’t start with tape, caulk, or glue over a wet chewed spot. Those are short-lived at best and usually fail once the pipe flexes or the sink fills and dumps.
Leaks only when water runs?That points to the drain assembly, not the supply lines.
See tooth marks on plastic tubing?Plan on replacing that section instead of trying to seal it.
Last reviewed: 2026-04-21
What you’re seeing under the sink
Bad odor but not much visible water
You smell sewer gas under the sink, especially after the cabinet doors stay closed.
Start here: Look for a chewed hole above the bottom of the trap or a loose slip-joint connection opened up by damage.
Water shows up at the back of the cabinet
The drip lands near the wall even though the sink drain is farther forward.
Start here: Trace the first wet point with a dry paper towel from the tailpiece down to the trap arm entering the wall.
The damaged pipe is thin corrugated or flexible tubing
Instead of a normal smooth trap assembly, you see bendable plastic that has been gnawed or crushed.
Start here: Confirm whether the chewed piece is an add-on flexible drain section that should be replaced with a proper sink drain assembly part.
Most likely causes
1. Chewed plastic sink P-trap
This is the most common target because it sits low, often holds a little moisture, and is usually easy for rodents to reach.
Quick check: Run water for 30 to 60 seconds and watch the curved trap body for beads, pinhole spray, or a wet line starting at tooth marks.
2. Chewed sink drain tailpiece or extension tube
If the damage is higher up under the sink bowl, water may run down the pipe and make the trap look guilty.
Quick check: Dry everything, then run a slow stream first and watch the vertical pipe directly below the sink drain.
3. Slip-joint washer or nut disturbed by chewing or movement
Sometimes mice chew nearby plastic and the pipe gets bumped out of line, leaving a connection that leaks even if the pipe wall is intact.
Quick check: Look for a crooked joint, cracked nut, or drip forming exactly at a slip-joint connection instead of through the pipe wall.
4. Improper flexible drain section damaged by rodents
Corrugated add-on drain pieces trap grime, sag, and are easy to chew through.
Quick check: If you see ribbed flexible tubing under the sink, inspect the low spots and bends for chew holes and staining.
Step-by-step fix
Step 1: Confirm it’s a drain leak, not a supply leak
You want to separate a drain problem from a faucet supply line or shutoff leak before taking anything apart.
- Empty the cabinet so you can see the full drain assembly and both water supply lines.
- Dry the pipes, shutoff valves, and cabinet floor with towels or paper towels.
- Do not run the sink yet. Check whether anything becomes wet on its own.
- Turn on the faucet briefly and watch the supply lines first. Then plug the sink, fill it partway, and release the water while watching the drain assembly.
Next move: If the leak appears only when water drains, stay on the sink drain repair path. If water shows up even when the sink is not draining, the problem may be a supply line, shutoff valve, or another nearby leak source.
What to conclude: A leak tied to drainage points to the under-sink drain assembly. A constant or pressure-side leak is a different repair.
Stop if:
- Water is dripping onto an electrical outlet, disposal wiring, or extension cord.
- The cabinet floor is swollen, soft, or already holding standing water you can’t control.
- You cannot tell whether the water is coming from the drain or the supply side after drying and testing.
Step 2: Find the first wet point and the exact damaged piece
The drip location is often not the failure location. You need the first wet spot, not the final drip.
- Dry the tailpiece, slip-joint nuts, P-trap, and trap arm completely.
- Use a flashlight and a dry paper towel to wipe each section from top to bottom while a helper runs water.
- Look for tooth marks, tiny punctures, a split seam, or a drip forming at one exact joint.
- Pay special attention to the underside of the trap and the top side of horizontal tubing where small chew holes can hide.
Next move: If you find one clearly damaged section, you can plan a targeted replacement instead of changing the whole assembly. If everything looks dry but the cabinet still gets wet, check the sink drain flange area above, the disposal connection if present, and the wall stub-out area.
What to conclude: Visible chew marks with active seepage confirm the failed part. A dry pipe wall with a wet joint points more toward washers, alignment, or a cracked nut.
Step 3: Decide whether you need a pipe section or just the slip-joint hardware
A chewed pipe wall needs replacement. A joint that was knocked loose may only need the correct sink drain washer and nut setup.
- If water comes through the pipe wall itself, mark that section for replacement.
- If the leak forms at a slip-joint nut, loosen it carefully and inspect for a cracked nut, missing washer, backward washer, or misaligned tubing.
- Check whether the damaged piece is a sink P-trap, trap arm, tailpiece extension, or flexible add-on drain section.
- If the assembly is an odd mix of old brittle plastic and improvised pieces, plan on replacing the affected trap assembly parts together so they line up cleanly.
Next move: If the failure is clearly one removable section, you can replace only that section and reuse the sound pieces. If multiple sections are chewed, warped, or mismatched, replacing the full under-sink trap assembly is usually the cleaner fix.
Step 4: Replace the damaged under-sink drain section
Once the bad piece is confirmed, replacement is more reliable than patching and usually straightforward on a standard slip-joint sink drain.
- Place a small container or towels under the trap to catch water.
- Remove the slip-joint nuts around the damaged section by hand or with light pliers pressure if needed.
- Take the damaged piece with you to match diameter, style, and approximate length.
- Install the new sink P-trap, trap arm, tailpiece extension, or slip-joint hardware in the same orientation, making sure washers face the correct direction and the tubing is not forced sideways.
- Hand-tighten first, then snug slightly more only if needed. Reassemble any removed cleanout or branch connection before testing.
Next move: If the new section sits straight and dry through a full drain test, the repair is likely complete. If it still leaks, stop and recheck washer direction, pipe alignment, and whether a second damaged section was missed.
Step 5: Test hard, then deal with the rodent entry problem
A drain repair is not finished until you prove it stays dry and address how the mice got there in the first place.
- Run a slow stream, then a full sinkful of water, and watch every joint and pipe surface with a flashlight.
- Wipe each connection with a dry paper towel after the test to catch tiny seeps.
- Check the cabinet floor and the wall penetration behind the pipes for gaps, droppings, nesting material, or rub marks.
- Seal accessible non-drain entry gaps around the cabinet or wall opening with an appropriate rodent-resistant method, and set up pest control if activity is current.
A good result: If the assembly stays dry and the access gaps are addressed, you’ve handled both the leak and the reason it happened.
If not: If you still have odor, recurring moisture, or fresh chewing, bring in a plumber or pest professional before the damage spreads.
What to conclude: A dry retest confirms the plumbing repair. Ongoing rodent signs mean the pipe may get damaged again unless the entry route is closed.
Replacement Parts
Repair Riot may earn a commission from qualifying purchases, at no extra cost to you.
FAQ
Can I patch a mouse-chewed drain pipe under the sink with tape or epoxy?
You can sometimes slow a drip for a very short time, but it is not a dependable repair. A chewed sink drain section usually needs replacement because the plastic is already weakened and the pipe flexes during normal use.
How do I know if mice chewed the pipe or if it just cracked on its own?
Mouse damage usually leaves small paired tooth marks, rough gnawed edges, or a localized puncture. Age cracks tend to look cleaner, longer, or follow a seam or stressed joint.
Is this an emergency if it only leaks when the sink drains?
It is usually not a full emergency if you can stop using that sink and dry the area, but it should be fixed soon. Drain leaks can rot the cabinet floor, create odor, and attract more pests.
Do I need to replace the whole sink drain assembly?
Not always. If only one removable section is chewed, like the P-trap, trap arm, or tailpiece extension, you can often replace just that piece. Replace more of the assembly only when several parts are brittle, mismatched, or leaking.
Why does it smell bad even when I don’t see much water?
A small chew hole above the water line in the trap or on a nearby drain section can let sewer gas escape without making a big visible puddle. A loose slip-joint connection can do the same thing.
What if the damaged piece is flexible corrugated tubing?
That is usually a sign of an improvised drain setup. If it has been chewed or is leaking, replace it with the correct rigid sink drain parts that fit the trap and wall connection properly.