An adult roof rat is about 6 to 8 inches long from head to the base of the tail. It has a tail length of 7 to 10 inches and is smaller and more slender than the Norway rat. Roof rats weigh only 5 to 9 ounces. The coloration tends toward dark gray or brown to black, including the tail. The belly may be dull white to gray. It has a more pointed nose than the Norway rat and large naked ears that can be folded to cover the large eyes. The longer tail will reach or go beyond the tip of the nose when extended back over the body. Droppings are up to 1/2 inch long and tapered at the ends.

Female roof rats have 4 to 6 litters per year with 6 to 8 pups each. Roof rats reach maturity at 2 to 5 months and adults live an average of 9 to 12 months total. Their diet consists of seeds, fruit, and nuts but varies with what is available. Roof rats will eat snails and slugs, insects, or even shellfish if they are close to the water. Only about an ounce of food and 1 to 2 ounces of water (including that from food) is needed for survival. The roof rat is not adapted to cold seasons and therefore is uncommon for Pennsylvania. This species may turn up in the warmer southeastern part of the state but is unlikely to persist.

Roof rats damage and destroy materials and property and cause structural damage by gnawing. They also feed on and contaminate stored foods because rodents urinate and defecate constantly as they move about. Roof rats carry and vector diseases, particularly bubonic plague, which is associated with rat fleas on roof rats. Plague is absent in the northeastern United States but occurs in the Southwest. Other diseases include murine typhus, leptospirosis, and rat bite fever, and foodborne illnesses such as trichinosis and salmonella. Parasites of rats that affect humans include fleas and tropical rat mites, both of which can become problems when rats die in buildings.