Rats usually occupy the lower floors of the building because they still need access outside for food or water sources. But as the population grows, they will start to invade higher floors to fulfill nesting requirements. They nest in wall voids, under floors, crawlspaces, and among stored items.
Rats are often found in burrows alongside or beneath some object (boulders, cement slabs, sidewalks, trees, foundations, streets, ect.) for protection from predators and weather conditions. Burrow systems have one entrance hole and either one or two exit points for the rats to escape in an emergency situation.
Rats will travel 300 ft or more from their nesting sites to obtain food and water.