Roof rats breed throughout the year, with two peaks of production: In February and March and again in May and June. The period of least activity is in July and August. The gestation period is approximately 21 days, and the number of young per litter averages almost seven.
Roof rats are largely commensals and live in close association with man. They seldom become established as feral animals as do the Norway rats; however, in Lavaca County they have been found throughout the county, in the towns, and on the farms. They inhabited grocery and drug stores, warehouses, feed stores, and poultry houses and were very common in cotton gins and associated grain warehouses. On the farms they lived in barns and corn cribs. They may live near the ground, but usually they frequent the attics, rafters, and crossbeams of the buildings. They make typical runways along pipes, beams or wires, up and down the studding, or along the horizontal ceiling joists, often leaving a dark-colored layer of grease and dirt to mark their travel-ways. Like the Norway rat, the roof rat is largely nocturnal and only where populations are relatively high does one see them frequently in the daytime. There is some indication that the larger and more aggressive Norway rat is supplanting the roof rat in many parts of the United States. In the southern United States, however, the roof rat is by far the more common of the two.