Stains- The tell tale sign of infestation
When performing a home inspection I am on the lookout for obvious entry point for rodents and other critters.
Squirrels, rats, mice, bats and raccoons can have a health impact as well as destroying insulation and wiring.
Here we have an obvious entry point for a rat. Rats will leave oil stains on their trails and entrance holes. They only need an opening larger than ½-inch in diameter to gain access. This means spaces around pipes or conduits, under the door (especially garage doors that are left open, or do not seal properly), through a hole in the screen or floor, or through a gap between a window and its frame are all potential entry areas.
Rats will make holes in walls or floors soon after invasion. Rat holes are usually circular, averaging 2-3 inches in diameter, and are usually just inches off the floor.
Rats (and their cousin the squirrel) have a distinctive musky odor they will leave behind, particularly if it is in a confined to a small space.
Remember the best way to control rodents is to keep them out of the home in the first place. All rodents like to hide in vegetation. One of your first lines of defense is to trim the vegetation close to your home.