Rats

This page is about the two commensal rat species present across the United States — the Norway rat, Rattus norvegicus, and the roof rat, Rattus rattus — as eight university extension and federal publications describe them. It covers identification and telling a rat from a mouse, signs of an infestation, where rats nest and how they get in, exclusion, trapping and control, sanitation, and when the sources say to call a professional. It does not cover rodent-borne disease as a sourced subject; the section below explains why that is a scope decision rather than an oversight.

Which Rat? Norway, Roof, and Telling a Rat From a Mouse

UC IPM Pest Notes 74106 names the two species this page covers and the term for both together: “Two introduced species of rats, roof rats (Rattus rattus, Figure 1) and Norway rats (Rattus norvegicus, Figure 2), are present in almost all cities across California and are commonly referred to as commensal rats.” It gives a comparison table for telling them apart. General appearance: “sleek, agile” for a roof rat against “large, robust” for a Norway rat. Body weight: “5 to 10 ounces” against “7 to 18 ounces”. The tail, pulled over the back toward the head: a roof rat’s “extends at least to snout, uniformly dark with fine scales”; a Norway rat’s “extends to the middle of the skull; dark above and pale below, scaly”. The head: “pointed muzzle” against “blunt muzzle”.

Oregon State University’s solvepest page states the same distinction independently, in its own infographic text: “Roof rats’ tails are longer than the rest their body. They have a slender body shape. They have big ears and eyes and a pointed snout. Norway rats’ tails are shorter than the rest of their body. They have a sturdy body shape. They have small ears and eyes and a slanted snout.” (the grammar in the first sentence, “longer than the rest their body,” is the source’s own)

Where a young rat gets confused with a mouse

UC IPM gives a live-animal test: “very young rats have large heads and feet in proportion to their bodies, while those of adult mice are proportionately much smaller (Figure 5).” It gives a gnawing test too: “rats leave larger tooth marks of 1/6 inch (4mm) versus a mouse’s ‘scratchy marks’ of only 1/12 inch (2mm).” (UC IPM’s own text sets “scratchy marks” in double quotation marks; this page renders it in its own typography, as every quotation mark on this page does) Nebraska Extension’s own comparison table gives the identical pair of figures independently: a gnawing width of “1/12 inch” for a house mouse against “1/6 inch” for a Norway rat. The same table gives feces size, “1/8 to 1/4 inch” for a mouse against “3/4 inch” for a rat, and a minimum access hole, “3/8 inch” for a mouse against “1/2 inch” for a rat.

Cornell CALS states why the distinction matters at the trap, not just the inspection: “Rat traps may be too slow to kill a mouse, whereas a mousetrap will not inflict enough force to kill a rat. Identify your target pest to select the correct trap.” The National Pesticide Information Center gives the same instruction as the first step of control: “Identify your rodent first. Young rats may look similar to mice but certain features can tell them apart.”

Signs of an Infestation

UC IPM lists the physical evidence a yard or building inspection should look for, several of which are not what a homeowner expects: “Rat droppings around dog or cat dishes or pet food storage containers”; “Noises coming from the attic or drop ceilings”; “Rat nests in your firewood stack”; “Dead rat carcasses scavenged by pet cats and dogs”; “Smudge marks caused by the rats rubbing their fur against beams, rafters, pipes, and walls”; “Rat or mouse droppings in your recycling bins”; and “Drowned rat(s) in your swimming pool or hot tub”.

Oregon State gives a largely overlapping list independently, adding two signs UC IPM does not name: “Rats traveling along utility lines or on the tops of fences” and “Interest on the part of dogs in holes, small gaps, and climbable structures”. The National Pesticide Information Center states the same smudge-mark clue in plainer language: “Look for brown or gray smudges along walls and other surfaces. These marks indicate heavily used paths or rat entrances.” NPIC adds a scale most inspections miss: “Check a large area when evidence of chewing is found. Rats may travel hundreds of feet per day for food or water.”

Cornell CALS adds a caution about reading too much into a dropping count, under its own heading, “Number of Droppings”: “the sudden appearance of a large number of droppings can be alarming, but doesn’t prove you have a full-blown infestation.” It also names a monitoring method neither of the other sources mentions: “Rodent baits are available without a rodent-killing chemical, but instead have a product that makes droppings and urine fluoresce under UV light. Monitoring with non-toxic blocks can provide information about rodent movement. For example, if non-toxic blocks are used on the exterior of the building, and fluorescent droppings are found indoors, this indicates that an entry point is present.”

Where They Nest and How They Enter

UC IPM gives each species its own nesting habit. The roof rat: “typically nest in enclosed and elevated spaces, such as attics, wall voids, ceiling voids, and cabinets.” It adds that roof rats “usually live and nest aboveground in shrubs, trees, or dense vegetation such as ivy.” The Norway rat, by contrast, is “a burrowing rodent” whose “burrows may be found along building foundations, beneath rubbish or woodpiles, and around gardens and fields (Figure 3).” Oregon State states the same split independently: “Norway rats build burrows near the ground along building foundations and beneath woodpiles and debris piles. Norway rats adapt to crawl spaces. They use sewer/utility passages to move through areas.” Roof rats: “nest above ground in vegetation or in buildings above ground level.”

On how rats actually get inside, UC IPM states: “Rats can gain entry to structures by gnawing, climbing, jumping, or swimming through sewers and entering through toilets or broken drains. Norway rats are more powerful swimmers, but roof rats are more agile and are better climbers.” The Internet Center for Wildlife Damage Management gives the sewer route in more detail: “Rats enter sewers at outlets and through manholes, catch basins, broken pipes, and drains” and states a figure most readers will not expect: “Rats are excellent swimmers, capable of staying afloat up to 72 hours.”

Nebraska Extension describes what a den itself looks like once established: “Den entrances will be 2 to 3 inches in diameter, with the ground worn smooth by frequent use (Figure 2).” It adds a detail neither UC IPM nor Oregon State names: “Rats may also dig additional openings, called ‘bolt holes’ for emergency escape.” (Nebraska’s own text sets “bolt holes” in double quotation marks; rendered in this site’s own typography)

Exclusion: Two Figures, Two Questions

What a rat can physically squeeze through, and what the sources recommend sealing to, are two different figures answering two different questions — and this page keeps them apart rather than merging them into one number.

What a rat can fit through

UC IPM states it plainly: “Rats of either species, especially young rats, can squeeze beneath a door with only a 1/2-inch gap.” Nebraska Extension’s comparison table gives the same figure independently, as a minimum access hole for a Norway rat: “1/2 inch”.

What to seal to, which is stricter

UC IPM states the combined standard: “All holes larger than 1/4 inch should be sealed to exclude both rats and mice.” Nebraska states the identical standard in its own words: “Seal any openings larger than 1/4 inch to exclude both rats and mice.” The National Pesticide Information Center gives the same combined figure a third time, independently: “Exclude rats and mice by sealing cracks and crevices larger than a quarter inch.” ICWDM gives its own figure, framed only for rats and not merged with the combined standard above: “seal all holes and openings larger than ½ inch across.” Oregon State also gives the quarter-inch figure, framed only for rats: “Use welded wire ‘hardware cloth’ with ¼-inch openings to exclude rats.” (Oregon State’s own text sets “hardware cloth” in double quotation marks; rendered in this site’s own typography)

On material, UC IPM’s own checklist is specific about gauge and mesh size: “Ventilator grills (vent covers) should be enclosed with 18-gauge, 1/2-inch (13 mm) mesh, galvanized expanded metal or 16-gauge, 1/2-inch (13 mm) mesh, galvanized wire cloth.” For low windows: “Windows that open near the ground can be protected with 19-gauge, 1/2-inch (13 mm) mesh, galvanized wire cloth.” And a warning about the obvious wrong choice: “Never use lightweight ‘chicken wire’, since rats can easily pass through wire openings.” (UC IPM’s own text sets “chicken wire” in double quotation marks, with a stray space its PDF renders before the closing mark; rendered here in this site’s own typography, with that spacing artifact silently corrected) ICWDM gives the same instruction about material quality rather than just size: “Rodent-proofing should be done with heavy materials that resist gnawing, including concrete mortar, 24-gauge galvanized sheet metal, and 18-gauge hardware cloth.” Oregon State gives a starting point for the smallest gaps: “Fill entry points as small as a dime with sealant and/or steel or copper wool.” For exclusion overall, UC IPM calls it, in its own words, “building them out” — the most successful and longest-lasting strategy for rat control around structures, in UC IPM’s own assessment.

On roof rats specifically, exclusion takes longer to get right. UC IPM states: “Because roof rats are excellent climbers, openings above ground level must also be plugged. Rodent proofing against roof rats, because of their greater climbing ability, usually requires more time to find entry points than for Norway rats.” For rats traveling utility lines specifically, UC IPM does not recommend a do-it-yourself fix — see the professional section below.

Trapping and Control Methods

UC IPM describes what is available and the tradeoff against poison baiting: “Rat-sized snap traps are available in a variety of styles. They can be made of plastic or wooden, have larger or expanded triggers, or have bait reservoirs.” “Because snap traps can be used over and over, the materials for trapping can be less expensive than poison baiting but trapping will likely be more labor intensive.” On bait: “Nuts, dried fruit, bacon, or a piece of dry pet food can be attractive bait for traps.” On the amount: “it is important to use just a pea-sized amount of bait.”

On placement, UC IPM states: “For Norway rats, install traps close to walls, behind objects, in dark corners, and in places where rat signs (droppings, smudge marks, burrows, etc.) have been seen. Position traps along a wall so that they extend from the wall at right angles, with the trigger end nearly touching the wall (Figure 6).” For the climbing species: “For roof rats, traps often need to be installed both on the ground and aboveground (e.g., ledges, shelves, branches, fences, pipes, or overhead beams) (see Figures 7 and 8).” The UC ANR snap-trap article (by the same author) gives the specific placement patterns: “Effective placement of snap traps for rodents. Clockwise from top left: side-by-side, three in a row with alternating triggers, end-to-end.” It adds a caution about nontarget animals: “Set traps only during the night, but leave them where they are during the day, making sure they are not set. This greatly reduces trapping animals that are mostly active during daylight hours.”

Neophobia: why rats ignore a trap for days

UC IPM names the behavior: “They quickly detect and tend to avoid new objects and new foods. Thus, they often avoid traps and baits for the first several days (and maybe weeks) following the initial placement. This neophobia (fear of new objects) is thought to be more pronounced in roof rats than in Norway rats.” Cornell CALS states the same behavior in its own words and gives a prebaiting protocol: “Rats exhibit this behavior, which is a ‘fear of new.’ To overcome this fear, put out traps with plenty of bait, but do not set the trap. Eventually, most rats will approach the trap and take bait. This should be repeated until rats have taken bait about three times.” (Cornell’s own text sets “fear of new.” in double quotation marks; rendered here in this site’s own typography) The UC ANR article states the same practice: “When using snap traps for rats, it is good practice to prebait your traps.” It names the consequence of skipping the step: “Failure to prebait may result in the capture of juveniles only. The breeding adults are the most important targets.” Nebraska Extension states the same finding as a timeframe: “It is common for rats to wait two weeks before interacting with traps and toxicants.” Cornell names a related failure mode neither of the others states, under its own heading, “Trap Fatigue”: “This can occur when snap traps are left in the same place for a long period of time. Rodents, especially mice, become accustomed to the trap and do not explore, or actively avoid it.”

Glue boards and live traps: cautioned against for homeowners

UC IPM is direct about glue boards: “Glue traps are also available options for rat management. However, they are difficult to use and are not generally recommended for nonprofessional use.” ICWDM states why they underperform on rats specifically, compared to mice: “They are less effective for capturing rats than mice, as rats frequently extricate themselves from the glue.” On live traps, UC IPM states a legal obligation most readers will not expect: “Live trapping of rats is not recommended for nonprofessionals because, legally, trapped rats must be humanely euthanized or released immediately in the area where it was trapped”. It adds why releasing elsewhere is not a solution: “Releasing rats is not recommended, as they will continue to pose health concerns to the residents and their neighbors.”

On devices that do not work, ICWDM states plainly: “Ultrasonic devices do not control rats”. No ultrasonic or vibrational device is named on this page.

Sanitation and Food Storage

UC IPM states the principle and its limit in the same breath: “Sanitation is fundamental to rat control and must be ongoing. If sanitation measures aren’t properly maintained, the benefits of other measures will be lost, and rats will quickly return.” On garbage specifically: “Collect garbage, trash, and garden debris frequently, and ensure all garbage receptacles have tight-fitting covers.” On pet food, which several sources single out: “Where dogs are kept and fed outdoors, rats can become a problem if there is a ready supply of dog food. Feed your pet only the amount of food it will eat at a feeding, and store pet food in rodent-proof containers.” UC IPM adds a caution about pet waste specifically: “Clean up feces from dogs, cats, and other animals since rats will eat feces. Feces can harbor pathogens, so always handle with gloves and be careful not to breathe in any aerosolized fecal material.”

Nebraska Extension gives the same principle as a checklist: “Reduce food, water and harborage necessary for rat survival. Whenever possible, clean up food spills, fix leaky faucets and modify bird feeders.” “Stack packaged foods on pallets with adequate space left around and under stored articles to allow inspection for rat sign.” ICWDM states the same storage instruction independently and adds a detection method neither UC IPM nor Nebraska names: “Stack foods that are in sacks or boxes in rows on pallets in a way that allows thorough inspection for evidence of rats. Keep stored materials away from walls. A 12-inch, white band painted on the floor adjacent to the wall will aid in detecting droppings of rodents and other signs.” Oregon State gives the same list in plainer form for a residential reader: “Use Trashcans with Secure Lids”; “Store Pet Food and Bird Seed in Robust Containers”; and “Don’t Attract Rats with Food Scraps in Your Compost”.

ICWDM adds one figure sanitation alone will not solve, stated as context for why exclusion still matters: “One rat eats approximately 20 to 40 pounds of feed per year and probably contaminates 10 times that amount with urine and droppings.”

When the Sources Say to Call a Professional

UC IPM, on roof rats traveling overhead utility wires, does not recommend a do-it-yourself fix: “If roof rats are traveling on overhead utility wires, contact a pest professional or the utility company for information and assistance to prevent this.” Oregon State states the same recommendation for rodent-proof construction generally: “Consider hiring a pest control company or a contractor familiar with rodent-proof construction and exclusion methods.”

ICWDM names two specific tools it reserves for licensed applicators. On fumigants: “Aluminum phosphide is registered as a burrow fumigant, although it is a highly restricted pesticide. Fumigation Management Plans must be established prior to use of aluminum phosphide. Do not use fumigants where the occupants of a building might be exposed to the fumes. Only licensed pest control operators may use fumigants.” On tracking powders: “Tracking powders are available only to licensed pesticide applicators.” UC IPM draws the same line for live trapping, covered above: the law requires a trapped rat to be humanely euthanized or released on the spot, which UC IPM frames as a job the sources say belongs to a professional rather than a homeowner.

Rodent-Borne Illness: What This Page Does Not Cover

This page does not carry a sourced treatment of rodent-borne disease. That is a scope decision made before this page was built, not a source that could not be found. Cornell CALS’s own rodent management page states: “The Centers for Disease Control and Prevention (CDC) provides clean up recommendations that reduce the risk of exposure to rodent pathogens.” That is as far as this page goes: readers with a contamination concern are pointed to CDC.gov, by name, for cleanup guidance this page does not reproduce.

A specific CDC page was identified and one fetch was attempted, so that a live link could be given rather than a bare name. It returned an access-denied response, not the document. Under this estate’s standing rule, a dead or blocked citation is worse than none, so no cdc.gov link appears on this page. The attempt is recorded, with its outcome, in the sources section below.

What This Page Does Not Carry

A sourced treatment of rodent-borne disease. Covered above: this is a scope decision, and the reader is pointed to the CDC by name rather than through a citation this page could not verify.

Woodrats (pack rats). UC IPM names them as a separate, often native group told apart by a furred tail, but does not give them the depth it gives Norway and roof rats, and neither does this page.

A ranking. Eight publications were read and none compares any two rat control products against one another. Nothing here is called best.

An ultrasonic or vibrational repellent device. ICWDM states plainly that these do not control rats, and none is named.

A named rodenticide brand. UC IPM and Nebraska describe a regulated product class rather than a single tool meeting one stated criterion, and Nebraska Extension’s bait-station specification is not matched against any listing this page fetched. No rodenticide and no bait product is carded — only the station hardware that holds one.

Hardware cloth. Every source above names it as an exclusion material at a stated mesh size. Five distinct product listings were attempted for this page and every one returned an anti-bot interstitial on the first fetch. That is an honest sourcing gap, recorded rather than worked around.

Questions

Sources

Eight publications, each named in the sentence that uses it. Every quotation on this page was located, byte-exact, in the publication it is attributed to. One phrase that could not be reproduced byte-exact because of a PDF extraction artifact — a stray space UC IPM’s PDF renders before a closing quotation mark — is quoted only up to the point the artifact begins, noted above where it occurs.

One further URL was attempted and is not a source on this page: a CDC page on rodent control, fetched once on 31 August 2026, which returned an access-denied response rather than the document. No quotation, citation or link to that page appears here. It is named in this paragraph, under this estate’s standing rule that a failed fetch is recorded rather than substituted, and never quietly dropped.

1. UC Statewide IPM Program (Quinn N). Pest Notes: Rats, Publication 74106 — the source of the species identification table, the two gap figures, the mouse-versus-rat gnaw and feces comparison, the signs-of-infestation list, the exclusion checklist, and most of the trapping material. Revised June 2024; read 31 August 2026. https://ipm.ucanr.edu/pdf/pestnotes/pnrats.pdf
2. Oregon State University, Solve Pest Problems. Rats — the source of the Norway-versus-roof-rat infographic corroboration, the signs-of-activity list, the exclusion mesh figure, and the professional-installer recommendation. Undated on the page; read 31 August 2026. https://solvepestproblems.oregonstate.edu/rodents-wildlife/rats
3. Internet Center for Wildlife Damage Management. Norway Rat Damage Prevention and Control Methods — the source of the single-figure exclusion standard, the sanitation detail, the trap-trigger research finding, and the licensed-applicator statements for fumigants and tracking powders. Undated on the page; read 31 August 2026. https://icwdm.org/species/rodents/norway-rats/norway-rat-damage-prevention-and-control-methods/
4. University of Nebraska–Lincoln Extension. G1737, Controlling Rats — the source of the comparison table (feces, gnawing, access hole), the den and bolt-hole description, and the independent corroboration of both gap figures. 2007; read 31 August 2026. https://extensionpubs.unl.edu/publication/g1737/2007/html/view
5. UC Statewide IPM Program (Quinn N). Rats and Mice: How to Manage Using Snap Traps — the source of the neophobia and prebaiting instructions, the trap-placement patterns, the nontarget-wildlife caution, and the carcass-disposal steps. 15 January 2020; read 31 August 2026. https://ucanr.edu/blog/pests-urban-landscape/article/rats-and-mice-how-manage-using-snap-traps
6. University of Illinois Extension. Norway Rat — the source of the identification and management-summary paragraphs used on this page. Undated on the page; read 31 August 2026. https://extension.illinois.edu/invasives/norway-rat
7. National Pesticide Information Center, Oregon State University. Rats — the source of the identify-your-rodent-first instruction and the quarter-inch combined exclusion figure. Last updated 27 July 2026; read 31 August 2026. https://npic.orst.edu/pest/rat.html
8. Cornell Cooperative Extension, Cornell Integrated Pest Management. Managing Mice and Rats — the source of the trap-fatigue finding, the rat-versus-mouse-trap statement, the glue-board mechanism, and the sentence naming the CDC’s own cleanup guidance. Undated on the page; read 31 August 2026. https://cals.cornell.edu/new-york-state-integrated-pest-management/outreach-education/whats-bugging-you/rodents/managing-mice-and-rats