Color:  Can vary from gray to brown to black.

Size: External measurements average: total length, 440 mm; tail, 205 mm; hind foot, 46 mm.

They may burrow to make nests under buildings and other structures, beneath concrete slabs, along stream banks, around ponds, in garbage dumps, sewers, basement, and at other locations where suitable food, water, and shelter are present.

Although they can climb, Norway rats tend to inhabit the lower floors of multistory buildings.

Norway rats are primarily nocturnal. They usually become active about dusk, when they begin to seek food and water. Some individuals may be active during daylight hours when rat populations are high.

Rats have poor eyesight, relying more on their hearing and their excellent senses of smell, taste, and touch. They are considered color-blind. Therefore, for safety reasons, baits can be dyed distinctive colors without causing avoidance by rats, as long as the dye does not have an objectionable taste or odor.

Rats use their keen sense of smell to locate food items and to recognize other rats. Their sense of taste is excellent, and they can detect some contaminants in their food at levels as low as 0.5 parts per million.

Norway rats will eat nearly any type of food. When given a choice, they select a nutritionally balanced diet, choosing fresh, wholesome items over stale or contaminated foods. They prefer cereal grains, meats and fish, nut, and some types of fruit. Rats require 1/2 to 1 ounce of water daily when feeding on dry foods but need less when moist foods are available. Food items in household garbage offer a fairly balanced diet and also satisfy their moisture needs.

Using their ever-growing incisors and strong jaws, these rats cause structural damage by burrowing underneath buildings and walkways and gnawing through walls, pipes, and electrical wires. These rats have even started fires by gnawing matches and have caused floods by tunneling through dams. They contaminate crops and food, and may also restrict plant growth by eating large amounts of seeds. More importantly, they transmit diseases directly by biting people and contaminating food, and indirectly by carrying lice and fleas. Historically, they have been vectors for bubonic plague, leptospirosis, typhus, spotted fever, tularemia, salmonella food poisoning, infectious jaundice, and other serious diseases.

[A] Sanitation:

In addition to the above-mentioned techniques of excluding rodents from sources of food and shelter, sanitation can play an important role in controlling rat populations. Poor sanitation is one of the basic reasons for the continued existence of moderate to high rat populations in urban and suburban areas.

• Sanitation involves good housekeeping, including proper storage and handling of food materials, feed, and edible garbage. Warehouses, granaries and grain mills, silos, port facilities, and similar structures may provide excellent habitat for rats. Store bulk foods in rodent-proof containers or rooms. Stack sacked or boxed foods in orderly rows on pallets in a way that allows thorough inspection for evidence of rats. In such storage areas, keep stored materials away from walls. A 12-inch (30-cm) white band painted on the floor adjacent to the wall will aid in detecting rodent droppings and other rat sign. Sweep floors frequently to permit ready detection of fresh sign.
• Pet foods often are a source of food for rats in and around homes. Keep all such materials stored in metal rodent-proof containers. Feed pets only what they will eat at a single time.
• Garbage and rubbish from homes, restaurants, farms, and other such sources should be properly stored and subsequently removed for disposal. A proper refuse storage container is heavy-duty, rust-resistant, rat- and damage-resistant, and equipped with a tight-fitting lid. Galvanized steel trash containers in good condition are better than those made of vinyl or plastic. Racks or stands prevent corrosion or rusting of containers, reduce rat shelter under containers, and minimize the chance of containers being overturned.
• Bulk storage containers for refuse, such as those used at apartments, businesses, and housing projects, should be similarly rodent-proof. Large metal refuse containers (dumpsters) sometimes have drain holes to facilitate cleaning. These drain holes should be fitted with a wire mesh screen or a removable plug; otherwise, the container becomes a huge feeding station for rodents.
• Refuse should be collected regularly and before refuse storage containers become filled to excess. Sanitary landfills and incinerators seldom have conditions that will allow rat populations to exist. On the other hand, open refuse dumps are often infested by Norway rats. At a properly operated sanitary landfill, garbage and rubbish are compacted and covered with earth daily. Modern incinerators completely burn refuse, and the resulting residue does not provide food for rats.
• Sewers are inhabited by Norway rats in some towns and cities. Rats may enter at outlets and through manholes, catch basins, broken pipes, or drains. Since Norway rats are excellent swimmers, water traps do not impede their movement; in fact, they can travel upstream against a current. The problem of rats in sewers is usually greatest in places where sanitary sewers are interconnected with storm sewers, thus providing multiple entry points for rats. The domestic sewage of an average community provides enough food to sustain a large number of rats; this problem has increased as a result of the recent prevalence of garbage disposal units in most new homes.
• Regular removal of debris and control of weeds from around structures will reduce the amount of shelter available to rats. In some instances, a strip of heavy gravel placed adjacent to building foundations or other structures will reduce rat burrowing at these locations. Gravel should be at least 1 inch (2.5 cm) in diameter and laid in a band at least 2 feet (0.6 m) wide and 1/2 foot (15 cm) deep. In any event, keep the perimeter of buildings and other structures clean of weeds and debris (including stacked lumber, firewood, and other stored materials) to discourage rat activity and to allow easier detection of rat sign.

[B] Rodent Proofing (Exclusion):

Physical barriers can prevent rats from gaining entry to structures where food and shelter are available. “Rat proofing” is an important and often neglected aspect of rat control. It is a relatively permanent form of rodent control that prevents damage from occurring.
To exclude rats, seal all holes and openings larger than 1/2 inch (1.3 cm) across. Rodent-proofing should be done with heavy materials that will resist rodent gnawing. These include concrete mortar, galvanized sheet metal, and heavy-gauge hardware cloth.
Holes and Openings:

• To prevent rodent entry, seal all such holes with durable materials. Steel wool, copper gauze (Stuff-it brand) or screen wire packed tightly into openings is a good temporary plug. For long-term or permanent repair, mix a quick-drying patching plaster or anchoring such as Fix all into a wad of Stuff-it before pushing the material into the hole, and smooth over the outside. If steel wool is used, rust stains are likely to result. Holes 3 inches (8 cm) or more in diameter should be covered or backed with 1/4-inch (0.6-cm) woven/welded hardware cloth prior to filling with a good patching compound. Another backing material available is Strong Patch TM (D. P. Wagner Mfg. Inc.), a 6 x 6-inch (15 x 15-cm) sheet metal patch to cover holes up to 5 x 5 inches (11 x 11 cm). It has a self-adhesive backing and a mesh on the surface for better adhesion of the patching compound or other texture.                                                         

• To close larger openings or protect other areas subject to gnawing, use materials such as Hardware cloth, if not woven, breaks easily. The woven/welded hardware cloth maintains its shape when cut to fit around pipes or other objects. Hardware cloth used to cover gaps and holes can be filled with foam caulk, Fix-all, Quick-Fix, or other fast-drying interior patching compounds. When used on the exterior, concrete mortar, plaster, or Concrete Patch can be used to provide longer-term rodent-proofing. These are just a few of the many products available.

• Close openings around augers, pipes, and electric cables where they enter structures with Portland cement mortar, Concrete Patch, masonry, or metal collars. Even a small unprotected opening can be an invitation to rodents.

Vents and Windows:

• Use only metal window screening materials where windows or doors are accessible to rodents. Avoid unnecessary ledges outside windows. When necessary, screen ventilation openings and windows with woven/welded galvanized hardware cloth. Such screening is critical in commercial and farm buildings and where high rodent pressures in residential areas are found. For large openings or where the screen may be subject to abuse, add crossbars to support the hardware cloth. If the opening is an access route, install the screen on a hinged frame. All vents and duct openings for heating and air conditioning should be screened or raised and/or guarded with an excluder device to prevent rodent entry. Residential cold air return grills can easily be mouse-proofed by placing 1/4-inch (0.6-cm) hardware cloth behind the grill where it is not unsightly. In some applications, power vents can be covered with hinged metal plates (louvered) that open with air flow and close when fans are off.

Exterior Doors:

• Doors should fit tightly, the distance between the bottom of the door and the threshold not exceeding 1/4 inch (0.6 cm). In some instances, it is possible to build up the threshold rather than modify the door. Metal thresholds can be fastened to floors. Mechanical door-closing devices save time and help overcome human negligence. Equip doorways used for ventilation with rodent-proof screen doors, or if the door surface is too slick for rodents to climb, modify the existing door so the upper half can be left open for ventilation. Always use a heavy kick plate and solid frame on screen doors in commercial and agricultural buildings. Light-framed screen doors easily get bent out of shape, allowing rodent entry.

Foundations and Floors:

• Gaps or flaws along building exteriors where the wall framing or siding meets the foundation provide easy entry for rodents. Such openings can be prevented by well-formed and finished concrete work and installation of tight wall framing and siding, or installing metal screed-type flashing between the siding and the foundation. Use of rodent-proof exterior surface materials such as concrete, plaster, or metal sheeting is also effective if properly installed so that all ribs or corrugations are closed. Rodents can gain entry into buildings with piers or shallow foundation walls by burrowing beneath the floor or foundation. To prevent rat entry by this route, extend foundation walls below ground at least 36 inches (91 cm). This also reduces damage from frost. A horizontal footing extension also may be added to deflect burrowing rodents away from the foundation.

• Ideally, install floors, slabs, and sidewalks with deep footings or with curtain walls of concrete or 1/4-inch (0.6-cm) mesh wire. The choice between concrete and wire mesh depends on the expected life of the structure. Though wire mesh costs considerably less than concrete, its usefulness generally lasts only 5 to 10 years. Repair cracks in foundations and floors with concrete or masonry grout.

• Maintaining a clean, 3-foot-wide (1-m) weed-free area around building foundations, concrete slabs, and footings often discourages rodents from burrowing as well as eliminates a food source and attractive harborage. Where erosion of bare soil is likely, this buffer can be maintained by regular, close mowing of vegetation or by installing heavy gravel. To discourage burrowing, install a strip of 1-inch-diameter (2.5-cm) or larger gravel laid in a band at least 2 feet (60 cm) wide and 1/2 foot (15 cm) deep.

Interior Rodent-Proofing:
When rats or mice are present in a building, attention must be given to interior as well as exterior rodent-proofing to remove all sources of shelter. A combination of actions is required in such instances, as no single effort is likely to yield the desired result.

• Concrete floors are preferred to wooden floors. An attempt should be made to seal off rodents. Use traps to remove the rodents, or place poison bait packets through openings in the floor or wall and then seal the openings with galvanized metal or hardware cloth and patching plaster as previously discussed. Promptly treat new openings as they are found. In occupied buildings, always trap the rodents before sealing interior walls to avoid odors, stains, and an influx of insects that feed on decaying rodent carcasses. Eliminate rodent hiding places beneath and behind equipment.

Drains and Pipes:

• Both rats and mice use drainage pipes or sewage systems as routes to enter buildings. Equip floor drains with metal grates held firmly in place. Grate openings should not exceed 1/4 inch (0.6 cm). Maintain 1/2-inch (1.3-cm) hardware cloth over sewer roof vents in rat-infested areas. If the sewer system is known to be rat-infested, a “Rat Guard” one way flap valve may be placed in toilets. Sewer laterals should be checked for openings that could allow rodent entry. Smoke producing leak detectors are often used by agencies checking sewer lines for leaks or openings. If openings are detected, replace the pipe or wrap the pipe break with 1/4-inch (0.6-cm) hardware cloth and use concrete patching material to seal the area. Rain gutter downspouts are often used by rats to gain access to roofs. It may be possible to screen over openings at the base of downspouts with 1/2-inch (0.6-cm) hardware cloth or a grate, but this will require continued maintenance to remove accumulated debris, particularly where leaves and small sticks are washed from roofs into the gutter system. Flap valves have been used here too—swinging shut except when water is flowing. Openings to floor or driveway drains should have covers. Gutter and other drain covers must be kept clean of debris to prevent water backup.

Food Handling and Storage Areas:

• Even when all of the holes are plugged, rodents seem to find a way into food storage and handling areas. Sometimes rodents come in with supplies, or they run in through open doors or windows. Often, one or more openings remain undetected. These hidden holes are often below sinks, behind equipment, in false or suspended ceilings, and behind or under cupboards. Once in an environment having all the basic needs, rodents quickly establish viable populations. The solution is to eliminate harborage and exclude rodents from food and water sources inside the building. All equipment such as large refrigerators, freezers, counters, dishwashers, and sanitizers should be raised and easily movable, enabling cleaning underneath and behind them. Insulated walls and closed areas should be tightly closed off to avoid use as harborage. Openings are commonly seen in new stainless-steel work counters in supports under the work surface, or in areas provided for drains.