Why do I hear scratching in my walls at night?

Scratching, scurrying or gnawing inside a wall after dark is usually a rodent. Rats and mice usually search for food at night,^[1] and the space inside walls gives them shelter, warmth and a protected path between rooms. Flying squirrels are the other common night-time noisemaker, mostly higher up. The time you hear it, where in the wall it is, and what the animal leaves behind narrow it down quickly.

Why mice and rats end up in walls

House mice nest inside appliances, in wall and floor voids, in false ceilings and behind counters.^[1] Rodents prefer to travel along edges, using their whiskers as guides, including along pipes, utility wires and rafters,^[1] and they can run up almost any vertical surface, including wood, sheet metal, metal pipes and cables.^[1] The run of pipes and wires inside a wall is a ready-made highway, which is why you hear them moving up and down.

The gnawing is not random. Rats and mice chew on almost everything to keep their large front teeth sharp and short,^[1] so a steady scraping sound in one spot often means one is chewing on wood, wiring or a pipe.

Height is a clue to which rodent you have. Roof rats prefer to nest in elevated places, including walls and attics, while Norway rats generally nest in burrows at or below ground level.^[1] Noise low in the wall near the kitchen often means mice; noise high in the walls and ceiling can mean roof rats or flying squirrels.

Other animals that make noise

• Flying squirrels. The Virginia Department of Wildlife Resources names flying squirrels as the most common culprits when squirrels take up residence in an attic, and they are nocturnal.^[2] Expect light, quick scampering and sliding sounds overhead or high in exterior walls.
• Gray squirrels. Fairfax County's wildlife page describes squirrels as diurnal, with peak activity at dawn and late afternoon.^[4] Noise that starts around sunrise and dies down at night is a squirrel pattern, not a mouse one.
• Raccoons. Fairfax County describes raccoons as nocturnal and 10 to 30 pounds in weight,^[5] so they make heavy thumps and dragging sounds in the attic rather than light scratching inside a wall.

If the noise is overhead rather than in a wall, read what animal is living in your attic.

How to tell which one you have

Droppings. A house mouse leaves rod-shaped, pointed droppings; a roof rat leaves pointed, curved ones; and a Norway rat leaves capsule-shaped ones.^[1] Look under the sink, in the back of cabinets, behind the stove and refrigerator, and along baseboards on the side of the wall where you hear the noise.

Rub marks. Mice leave brown or gray smudges along walls and other surfaces they travel over, which mark heavily used paths and entrances.^[7] Check around holes where pipes come through the back of the sink cabinet.

Tracks. Rodents pass the same spots over and over, so lightly dusting the edge of a floor with chalk dust or talcum powder will show footprints and drag lines made by tails.^[1] Virginia Tech's rodent guidance also suggests temporarily closing a suspected hole with paper or aluminum foil and checking after a few days whether it was removed or chewed through.^[1]

Openings. A mouse can squeeze through any opening ¼ inch across, and some rats through openings as small as ¾ inch.^[1] Rats and mice also get in through the eaves, the roof, attic vents and where pipes and wires go through walls.^[1] Look from the ground; a round, chewed hole at the roofline often points to squirrels.

Why it is worth dealing with now

Mice are mostly active at night, and they reproduce quickly,^[7] so a few become many. The bigger risk is wiring. Mice chewing on wire insulation can cause electrical shorts and fires.^[1] Call a professional if electrical equipment has rodents in it or looks damaged.^[8] Wild animals in the house can damage electrical wiring and destroy insulation, too.^[9]

Droppings carry their own risk. Hantaviruses spread to people mainly by breathing air contaminated with rodent urine, droppings or saliva,^[6] so wet droppings with disinfectant or a bleach solution and wipe them up rather than sweeping or vacuuming them dry.^[6]

What not to do

• Do not seal the hole yet if the animal might be inside. For squirrels, the Department of Wildlife Resources says to make sure no young have been left behind inside before plugging the hole.^[2] A trapped animal may chew a new way out through the drywall.
• Do not cut into the wall to look. Signs outside the wall usually tell you enough, and there may be wiring or plumbing behind it.
• Skip mothballs and plug-ins. Mothballs do not repel animals.^[9] Mice quickly get used to regularly repeated sounds, and there is little evidence that sound, magnetic or vibrating devices drive established mice or rats out of buildings.^[11]

If the signs point to mice, the fix is snap traps against the walls nearest the noise plus sealing the gaps. How to get rid of mice in your house walks through it.

Stop and call a pro if

• You smell burning, see chewed wiring, or lights flicker in the area where you hear the animal.
• The noise is heavy, happens in the daytime, or comes from the attic or roofline. Never corner a wild animal, and contact a professional if an animal seems sick or scared.^[9] The Virginia Department of Wildlife Resources' Trapper Finder Tool lists licensed trappers and Commercial Nuisance Animal Permit holders near you and the species each will trap.^[3]
• You find rat droppings, or the scratching continues despite steady trapping.
• A smell of something dead comes from inside a wall or ceiling.

Getting help on Loclis

Read when to call an exterminator for mice and how to keep raccoons and squirrels out of the attic. Browse rodent control services or wildlife removal services, find pest control around Blacksburg or see more pest control answers. When you are ready, tell local businesses what you are hearing and when so they can quote an inspection.