Rats need food and water , once that is found they infest prime nesting sites. Unfortunately that sometimes ends up being your home.
Attics, wall voids and sub areas are commonly infested.
   The common rat of Marin and Sonoma Counties is the Roof Rat
(Rattus Rattus).  It is easily identified by its long tail, big ears, pointy nose,
dark upper fur and a white underbelly. An adult weighs in at 12 to 16 ounces. These rats hoard away food in large amounts thru out their nesting areas. Common urban food sources consist of acorns, bird feed, pet food, fruit trees, vegetable gardens, and snails. A rats territory can be up to a 1/4 mile in diameter.                                                                                   Once these rats mate, the gestation period is 23 days. A litter of pups is typically 6-8. 
These pups are helpless for the first 4 weeks, then they are able to leave the nest and explore their territory. This group of juveniles is known as a "rat pack". Within 6 to 8 weeks the juveniles are sexually mature and the cycle continues. Rat populations grow in direct proportion to available food sources. Each rat can leave 300-500 droppings every 5-7 days. Huge amounts of droppings and urine accumulates where ever they infest.
The rats constantly build new nests, destroying insulation, heater ducting etc. Rats gnaw on just about everything and their teeth are harder than iron.
  Some Rat