Last Updated: March 2026 | Reviewed by The PCC Research Team
The Norway Rat (Rattus norvegicus), often referred to as the brown rat, sewer rat, or wharf rat, is a heavy, ground-dwelling rodent. Unlike the agile, climbing Roof Rat, Norway rats prefer to burrow in the soil and navigate through underground plumbing systems.
In California’s urban centers and agricultural valleys, Norway rats are responsible for severe structural damage. Because they burrow under concrete slabs and foundations, a severe infestation can actually cause your walkways or garage floors to sink and crack.
How to Identify a Norway Rat
Norway rats are built for digging and swimming, making them distinctively stockier than other species. Here is how to positively identify them:
- The Tail Test: Unlike the roof rat, a Norway rat’s tail is thick, hairless, and noticeably shorter than its head and body combined.
- Body Shape: They are heavy and robust, weighing between 10 to 16 ounces, and measuring up to 16 inches in total length.
- Head & Ears: They have a blunt, rounded snout and small ears covered in tiny hairs. If you pull their ears forward, they will not cover their eyes.
- Coloring: Their coarse fur is usually brown or dark grey on top with a grey or yellowish-white underbelly.
Norway rats are excellent swimmers and frequently inhabit city sewer lines. They can enter homes by swimming up through broken sewer pipes or even coming up through unsealed toilets on the ground floor. If you have chronic, unexplained rat issues in a ground-floor bathroom or basement, a plumber may need to perform a smoke test on your drain lines.
Habitat & Behavior: The Ground Invasion
Norway rats construct complex underground burrow systems, typically digging next to solid structures like home foundations, concrete patios, or garbage storage areas. These burrows usually feature one main entrance and at least two hidden “bolt holes” for escape.
When entering a home, they rarely climb higher than the ground floor. They will exploit damaged crawlspace vents, gaps under exterior doors, and unsealed garage thresholds. Once inside, they tend to nest in wall voids, basements, and the spaces beneath heavy appliances.
Eradication & Prevention
Because Norway rats live at ground level and require a constant water source, your eradication strategy must focus on perimeter defense and moisture control.
| Strategy | Application for Norway Rats |
|---|---|
| Trap Placement | Set snap traps along the walls in dark, ground-level areas (like the garage perimeter or behind the stove). Rats have poor eyesight and navigate by using their whiskers against vertical surfaces. |
| Burrow Baiting | Professional exterminators often utilize specialized restricted-use tracking powders or baits placed directly into the outdoor burrows to collapse the colony at the source. |
| Foundation Exclusion | Seal all gaps around plumbing and gas pipe penetrations on the exterior of your home. Use heavy concrete patch or copper mesh, as Norway rats will easily chew through standard expanding foam or caulk. |
Protect Your Foundation & Plumbing
Norway rats can quickly undermine concrete slabs and chew through PVC piping. Connect with a licensed structural pest control operator for professional trapping and permanent ground-level exclusion.
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