Direct answer: Most of the time, rats are using an oversized wall opening around the sink drain or supply lines, not chewing straight through solid pipe first. Start by finding whether you have a plain gap, a chewed plastic drain piece, or a damaged water line before you seal anything.
Most likely: The most likely problem is an open pipe penetration where the drain stub-out or supply lines pass through the cabinet wall or floor, often hidden behind stored items or a loose escutcheon plate.
Pull everything out from under the sink, dry the area, and look at the exact opening around each pipe. If the pipe itself is intact, this is usually a sealing job. If you see chew marks, cracks, drips, or sewer odor, treat the plumbing damage first and then close the entry point. Reality check: if a rat is showing up under the sink, the opening is usually bigger than it looked from the front.
Don’t start with: Don’t start by stuffing the hole with soft material alone or foaming over an active leak. That hides the real problem and usually gets chewed back open.
There’s an open space where the drain pipe or supply lines pass through the cabinet wall or floor, but the pipe itself looks intact.
Start here: Start with the visible opening size and check whether the wall material is broken, loose, or missing around the pipe.
The pipe, escutcheon, or surrounding wall opening has tooth marks, rough edges, or shredded material.
Start here: Look closely for an actual crack or pinhole in the drain or supply line before planning any sealing.
There’s a sewer smell, staining, or moisture under the sink along with the rodent entry point.
Start here: Separate a drain leak from a supply leak first, because sealing over a wet opening can trap damage in the wall.
You hear scratching in the wall or cabinet back and the opening is behind the trap or supply valves.
Start here: Clear the cabinet, remove any loose trim plate if needed, and inspect the full penetration around each pipe with a flashlight.
This is the most common setup. The drain passes through a rough-cut hole that was never sealed well, or the old patch broke loose over time.
Quick check: Look where the drain arm enters the wall. If you can see open wall cavity, daylight, insulation, or a gap wider than a fingertip, that opening needs repair.
Small copper, PEX, or braided supply lines often pass through sloppy holes in the cabinet floor or wall, especially at kitchen sinks.
Quick check: Check around both hot and cold lines near the shutoff valves. If the pipe opening is larger than the line by a lot, rats can widen and use it.
Rats and mice will chew thin plastic trap parts, tailpieces, or nearby trim when they’re already using the opening.
Quick check: Run a dry paper towel around the trap, tailpiece, and wall tube. Any damp spot, crack, or rough chewed edge means the plumbing repair comes before sealing.
Sometimes the visible gap looks small from the cabinet front, but a loose trim ring or thin patch is covering a much bigger opening behind it.
Quick check: Gently slide or lift any loose trim plate and inspect behind it with a flashlight. If the wall edge is crumbled or open, you’ve found the real entry point.
You need the real opening before you decide whether this is just a sealing job or a plumbing repair too.
Next move: You can point to one main opening or one damaged pipe area instead of guessing at the whole cabinet. If you still can’t see the opening, the access may be behind a loose trim plate, behind the trap, or lower in the cabinet floor around the supply lines.
What to conclude: Most under-sink rodent entries are at the wall or floor penetration, not through the cabinet box itself.
A simple opening can be sealed after cleanup. A cracked drain or chewed water line has to be repaired first.
Next move: You’ll know whether you’re dealing with an intact pipe passing through an open hole, or a plumbing part that needs replacement. If water appears but you can’t tell where it starts, stop and trace the first wet point before sealing anything.
What to conclude: An intact pipe with a dry opening is usually a straightforward exclusion repair. Any leak changes the job and raises the priority.
The visible edge often lies. A trim ring or rough patch can hide a much larger hole that needs a sturdier closure.
Next move: You’ll know whether a simple closure around the pipe is enough or whether the wall opening itself needs rebuilding first. If the opening disappears behind finished wall material or the pipe is loose in the wall, this is no longer a simple under-sink fix.
If a rat chewed a plastic drain part or a nearby water line, sealing the hole first just traps an active plumbing problem.
Next move: You’ve removed the leak source and can close the opening without hiding a failure. If the replacement part still leaks or the damage extends into the wall, the repair has moved beyond a simple under-sink fix.
Once the plumbing is confirmed dry and intact, closing the penetration is what stops repeat entry at this spot.
A good result: The opening is closed, the plumbing stays accessible, and there are no fresh signs of entry at that spot.
If not: If rats are still appearing, there is likely another entry point nearby or a larger wall or floor opening outside the cabinet.
What to conclude: You fixed the under-sink access point, but whole-house rodent control may still be needed if activity continues.
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Yes. If the wall or floor opening around the drain or supply lines is oversized, rats can use that route, especially if the cabinet hides it from view.
Fix the pipe first if anything is cracked, chewed, loose, damp, or leaking. Seal the opening only after the plumbing is confirmed dry and sound.
If it is an exposed plastic trap, tailpiece, or wall tube under the sink, replace the damaged section and retest for leaks. If the damage continues into the wall, call a plumber.
That usually means the drain-side opening is not sealed well, the trap connection is leaking, or there is damage at the drain tube entering the wall. Don’t just cover the smell with a patch.
Only if the opening is small and the surrounding wall is solid. A loose decorative plate over a large broken hole is not enough by itself.
Then this cabinet was only one access point. Check nearby plumbing penetrations and have the broader rodent entry problem tracked down, especially if you still hear movement in the wall.