Roof rats breed in attics for 3 main reasons:

1). Warm Temperature:
Rats are tropical animals and crave warmth. When nighttime temperatures fall below 70 degrees, Los Angeles rats are driven to seek a warmer home to get out of the wild.

2). Mating: Rats are hyper programmed to reproduce.
They constantly scent out other rats instinctively to find mates. An attic that has ever had a rat problem will hold on to an attractive rat scent. New rats in the area will be driven to join these rats to continue breeding.

3). Safe From Predators:
Attics are free of predators. Like all critters, rats avoid danger for themselves and their offspring by seeking escape from predators such as hawks, cats, snakes, and dogs.

Potential Rat Damage to sensitive attic wiring and duct work is the main reason to eliminate and remove the rats quickly and permanently. Rats innately chew and gnaw all the time just like other rodents, including mice and squirrels. Rats in the attic can fray electric wires, cause leaks in air ducts, and explode plumbing pipes. This threat is a serious fire-hazard, and many unexplained fires are attributed to uncontrolled rat problems.