Direct answer: If rats chewed a drain pipe under the sink, the usual fix is replacing the damaged drain section or trap, not patching it. First confirm whether the damage is on the sink drain assembly, a dishwasher branch tailpiece, or a nearby hose that only looks like the drain pipe.
Most likely: Most often the bite damage is on thin plastic sink drain parts under the cabinet, especially the P-trap, trap arm, or a dishwasher branch tailpiece.
Start by drying everything and tracing the first wet point while running a small amount of water. A lot of under-sink leaks drip off the lowest bend, but the actual damage is higher up. Reality check: once rodents have chewed through drain plastic, replacement is usually faster and cleaner than trying to save it. Common wrong move: replacing the whole sink drain when the only damaged piece is the trap or branch tailpiece.
Don’t start with: Do not start with tape, caulk, or glue over a wet chewed spot. Those are short-lived at best and usually fail the first time the sink drains hard.
Water shows up under the cabinet only when you run the faucet or dump a basin. The pipe may have visible tooth marks or a pinhole on the trap or horizontal drain arm.
Start here: Dry the whole drain assembly, then run water slowly and watch the highest damaged-looking spot first.
You can see gnaw marks, flattened plastic, or a thinned area on the drain pipe, but it has not opened up fully.
Start here: Treat that section as failed-in-waiting and inspect the exact piece so you replace only the damaged drain part.
Water drips off the trap, but the first wet point is actually above it at the sink basket, tailpiece nut, dishwasher branch, or a supply connection.
Start here: Wipe everything dry and trace the first wet point, not the final drip location.
You found chew damage and also see a gap where the drain passes through the cabinet or wall. There may be droppings, nesting material, or a sewer smell if the drain is open or cracked.
Start here: Confirm the drain still has a proper trap seal and plan to close the entry gap after the plumbing repair is finished.
The trap is low, exposed, and usually made of thin tubular plastic that rodents can damage quickly. Leaks often show up at the bottom bend when the sink drains.
Quick check: Dry the trap completely, then run a small stream of water for 30 to 60 seconds and look for beads forming on the trap body itself, not just at the nuts.
The straight horizontal or vertical drain sections are common chew targets, especially where they pass near stored items or cabinet openings.
Quick check: Look for tooth marks, flattened spots, or a slit on the straight tube between the sink tailpiece and wall connection.
If the sink has a dishwasher drain connection, that branch tailpiece is another thin plastic part rodents can damage. It can leak only when the sink or dishwasher drains.
Quick check: Find the small side inlet where the dishwasher hose connects and inspect the tailpiece body around that branch for chew marks or cracks.
A damaged-looking pipe can distract you from the real leak. Sink basket leaks and loose slip-joint nuts often wet the whole assembly below.
Quick check: Place a dry paper towel around the sink tailpiece, basket area, and each slip joint while running water. The first towel that gets wet marks the source.
Before you replace anything, make sure the leak actually happens during drainage. Under-sink supply leaks can drip onto the drain and make the wrong part look guilty.
Next move: If the leak appears only while water drains, stay on the sink drain next step. If water appears with the sink idle, stop chasing the drain pipe and inspect the supply lines or shutoff valves instead.
What to conclude: This tells you whether you have a true drain-side problem or a lookalike leak landing on the drain assembly.
Rodent damage under a sink is usually limited to one replaceable section. Naming the exact piece keeps you from buying the whole assembly unnecessarily.
Next move: If you can point to one damaged section, replace that section and any slip-joint washers disturbed during the repair. If the pipe body looks intact but a joint leaks, the fix may be a misaligned tube, loose nut, or worn washer rather than rodent damage.
What to conclude: You are narrowing this to a trap-body failure, a straight-tube failure, a dishwasher branch tailpiece failure, or a simple joint reseal.
A leaking nut connection and a chewed pipe body are not the same job. One may need only realignment and a washer, while the other needs a new drain section.
Next move: If the leak was only a crooked joint or bad washer, reassemble it squarely and retest before buying more parts. If the pipe wall is chewed through or too thin to trust, replace that section.
Once the failed piece is identified, replacement is the durable fix. Under-sink tubular drain parts are meant to come apart and go back together without patching.
Repair guide: How to Replace a Sink Drain P Trap Kit
Related repair guide: How to Replace a Drain Trap
A gentle test can miss a small leak. You also do not want to finish the plumbing and leave the same cabinet opening available for more chewing.
A good result: If the cabinet stays dry through a heavy drain test and the entry gap is addressed, you are done.
If not: If the leak only shows up under a heavy drain load or the wall opening is damaged, bring in a plumber or pest-control pro before the cabinet gets worse.
What to conclude: You have confirmed both the plumbing fix and the likely access point that let the rats into the cabinet in the first place.
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You can sometimes slow a drip for a very short time, but it is not a dependable repair on a chewed drain section. Under-sink tubular drain parts are usually easier and more reliable to replace than to patch.
Follow the first wet point while the sink drains. A sink drain pipe is usually a larger rigid tube or trap assembly. A dishwasher drain hose is smaller and flexible. The branch tailpiece sits between them and can confuse the diagnosis, so inspect that area closely.
Not usually. If the damage is limited to the P-trap, trap arm, extension tube, or dishwasher branch tailpiece, you can often replace just that section and any washers you disturb.
Water runs down the pipe and collects at the lowest point before dripping, so the final drip location is often not the source. Dry everything first and watch for the first place that turns wet.
Yes. If the chewed section has opened the drain path or the trap is not holding water, you may notice sewer odor. Once the damaged section is replaced and the trap is reassembled correctly, that smell should stop.
If the damaged piece is the removable trap arm, that is still a normal repair. If the fitting inside the wall is cracked, loose, or glued in a way that is not easily serviceable, that is a good place to stop and call a plumber.