Norway Rats...
prefer to live close to people. They burrow and make their nests under and around structures where food, water, and shelter is available. Our household garbage provides quite the balanced diet and generally meets their moisture needs. These rats cause considerable damage to food stuffs and animal feeds. They'll damage crops as well as the containers they're stored in. Their burrowing can weaken foundations of structures and they'll gnaw both wiring and water pipes. They also destroy insulation and anything else that they can line their nests with.
Roof Rats...
are much more aerial than Norway Rats. They often live in trees or on vine covered items, such as fences. They'll follow trees or power lines into structures and easily make their home there. They'll even live on the second floor or attic area of a structure infested with Norway Rats on the first floor. Often their damage is similar to that of Norway Rats, although in homes, it may be limited to wiring, pipes, and insulation. Roof Rats can cause significant damage to fruit trees and other tree crops. Like the Norway Rat, Roof Rats can transmit diseases to both animals and humans, such as murine typhus, leptospirosis, salmonellosis (food poisoning), rat-bite fever, and plague.
Trapping & Exclusion
Because rodent poisons create the risk of a rat dying inside a wall or other void area, and the fact that live traps expose the "handler" to rodent excrement, bites, and disease, kill traps are the optimal choice in rat control. Traps are checked every 1-2 days until exclusion has been completed and the population has been eliminated from the structure. Proper Exclusion or rodent-proofing requires that all openings 1/2 inch or larger be sealed off, using flashing, wire mesh, repellent foam sealants or a combination of these to permanently close all access points. Modification to the exterior of the structure may be necessary as well, including trimming tree limbs and removing vines attached to it.