Direct answer: If rats are showing up under a sink or inside a cabinet, the usual problem is an oversized plumbing cutout that stays open all the way into the wall or floor cavity. Start by confirming whether the gap is only around the pipe trim or whether you can see a deeper opening behind the cabinet, then close the actual entry path with rodent-resistant material.
Most likely: Most often, the cabinet hole was cut too large and never properly blocked where the pipe passes through the wall, floor, or foundation area behind it.
Look for rub marks, droppings, greasy smears, gnawing, and daylight or cold air movement around the pipe. Reality check: if a rat is using that hole, the cabinet opening is usually just the last visible part of a bigger path. Common wrong move: making the cabinet look sealed while leaving the wall cavity open behind it.
Don’t start with: Do not start with spray foam alone, loose rags, or caulk over a deep opening. Rats chew through soft fillers fast.
Droppings, shredded material, or scratching sounds inside the sink base cabinet, usually near drain or supply lines.
Start here: Empty the cabinet and inspect the pipe penetrations with a flashlight before sealing anything.
The cabinet back or bottom has a sloppy cutout, and there may be a dark void behind it.
Start here: Check whether the opening stops at the cabinet panel or continues into the wall or floor cavity.
Cold air, musty smell, or crawlspace/basement odor comes through the same opening the rats use.
Start here: Assume the gap connects to a larger hidden space and inspect deeper before using any finish trim.
Foam, caulk, or stuffing has been chewed out, pushed aside, or bypassed nearby.
Start here: Remove the failed soft patch and rebuild the closure with metal-backed, rodent-resistant material at the actual entry point.
This is the most common setup. The cabinet hole looks ugly, but the real problem is the unsealed pipe opening in the wall or floor behind the cabinet.
Quick check: Use a flashlight and mirror or phone camera behind the pipes. If you can see framing voids, insulation, masonry, or open darkness, the gap goes deeper than the cabinet skin.
Rats often travel along foundation walls, sill areas, and utility runs, then come up through a loose floor penetration under the sink or vanity.
Quick check: Look at the cabinet floor around supply lines and drain pipes for a ring-shaped gap, gnaw marks, or air movement.
Spray foam, caulk, paper, or fabric may hide the opening for a while but usually will not hold up against rats.
Quick check: Probe the patch gently. If it feels soft, hollow, or chewed, it is not a lasting rodent block.
If activity is heavy, the cabinet hole may only be the indoor exit point from a bigger opening at the rim area, foundation edge, or utility entry outside.
Quick check: Check the basement, crawlspace, or exterior directly below and behind that cabinet for gaps, droppings, rub marks, or daylight.
You need to separate a cosmetic cabinet cutout from a true wall, floor, or foundation entry. Sealing the wrong layer wastes time.
Next move: You now know whether this is a simple cabinet trim issue or a deeper opening that needs real blocking. If you cannot see the full opening because the cabinet is built tight to the wall or the area disappears below the floor, inspect from the basement, crawlspace, or adjacent access area next.
What to conclude: Most successful repairs start by finding the deepest reachable opening, not the prettiest visible one.
Rats usually follow utility paths. If the cabinet opening lines up with a basement, crawlspace, or exterior penetration, that lower opening is often the real source.
Next move: You have identified the route feeding the cabinet, which is the place to block first. If there is no visible lower or exterior opening, return to the cabinet and focus on sealing the wall or floor penetration you can reach from inside.
What to conclude: When the lower opening is left open, rats often keep testing the area and may find another weak spot even if the cabinet face looks sealed.
A good seal needs a firm edge to bear against. Soft, chewed, or dirty material keeps patches from staying put.
Next move: You now have a clean, solid edge for a lasting rodent-resistant repair. If the area is crumbling, wet, or too damaged to hold a patch, the opening needs carpentry, masonry, or plumbing correction before sealing.
Rats can chew soft sealants. The durable fix is a mechanical barrier at the true penetration, with sealant used only to tighten and finish around it.
Next move: The route is physically blocked and the cabinet opening is finished neatly without relying on chewable filler alone. If you cannot create a secure metal-backed closure because the opening is too large, too deep, or missing solid edges, you likely need a carpenter, pest exclusion pro, or mason to rebuild the area first.
One sealed hole helps, but rats often use a second gap a few feet away if it is left open.
A good result: No fresh signs after monitoring usually means you blocked the active route.
If not: Ongoing activity points to another opening nearby or a larger entry path outside the cabinet area.
What to conclude: Finish by confirming the repair held in the real world, not just that the hole looks closed on day one.
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Not by itself. Foam may help air-seal, but rats can chew through it. Use a rodent-resistant metal barrier at the real opening first, then use sealant only to tighten the edges.
That is common. Check whether the drain passes through an oversized wall cutout behind the cabinet or through a loose floor penetration below. Seal the deeper opening first, then cover the cabinet-side gap if needed.
Loose steel wool by itself is usually a short-term patch. It can rust, shift, or get pulled out. A more durable repair uses galvanized mesh or sheet metal fitted tightly against solid edges.
That usually means the opening connects to a wall cavity, basement, crawlspace, or exterior gap. The draft is a good clue that the cabinet hole is not the whole problem.
Call for help if the opening is large, tied to damaged masonry or framing, hidden behind finished walls, or part of a bigger infestation. Also call if plumbing has to be disconnected or the area is wet and deteriorated.