Roof rats prefer to nest in locations off of the ground and rarely dig burrows for living quarters if off-the-ground sites exist. When rodent-proofing against roof rats, pay close attention to the roof and roof line areas to assure all accesses are closed. Roof rats entering homes are generally found in raised or secure enclosures such as walls, cabinets, attics, and false ceilings. Also, carry out a visual examination of the attic to find the holes they were using to get in and out of the attic. Bubonic plague was a scourge in Europe several times throughout history. Hundreds may be nesting in a city block-in underground burrows, in sewers, on roofs, inside buildings-with few people in the area realizing it. Rats usually begin searching for food shortly after sunset. METHOD OF CONTROL: First of all, mothballs or ammonia won't make them leave, nor will ultrasonic sound emitters or strobe lights.