Mice In The Garage, The Shed And The Camper You Just Parked

Mice move into unheated buildings and stored vehicles for the same three reasons they move into a house: cover, something to eat, and nobody bothering them. A garage, shed, pole barn or parked camper gives them all three with the added advantage that the door stays shut for months. The damage is usually found in spring, by which point a nest has been sitting in the wiring or the upholstery since October.

Why A Cold Building Suits Them Fine

People assume mice head for the warmest spot available. Warmth helps, but being left alone matters more. Cooperative Extension's wildlife damage guidance on deer mice and white-footed mice, the native species that show up in outbuildings and cabins, notes that they sometimes nest in the cupboards and furniture of unoccupied buildings, that they nest at or just below ground level or inside buildings, and that they are excellent climbers that frequently enter through fireplace chimneys. An unoccupied building is not a second choice for these mice. It is the habitat.

House mice are in the same buildings for the stored food. The same guidance recommends keeping food in rodent-proof containers in cabins and rarely used buildings, which covers more than groceries: birdseed, grass seed, dog food, deer corn and the bag of fertilizer in the corner all qualify. If you store any of those in an outbuilding, that building has a food supply whether or not anyone is cooking in it.

October is the month this matters because the building is about to be closed up. Around Hartville and Lake Township, where pole barns, detached shops and stored campers are normal rather than unusual, the camper gets winterized and parked, the mower goes in the back of the shed, and the overhead door comes down for the season. Whatever walks in during the next two weeks has until April undisturbed.

What They Build, And Out Of What

A mouse nest in a stored vehicle is made from the vehicle. University of Tennessee Extension's school IPM guidance describes house mice shredding fabric, paper, cardboard and building insulation into loose ball-shaped nests, and says mice stash food within a small range of travel. Read that list against the inside of a camper: seat foam, headliner, cabin air filter, cardboard boxes of bedding, the fiberglass batt behind a panel. The nest and the food cache both end up within a few feet of each other, in a cavity you cannot see into.

That is also why the first sign is rarely a mouse. It is a smell, a handful of droppings on a seat, shredded material under the hood, or a stash of birdseed somewhere birdseed has no business being.

How Small An Opening Are We Talking About?

Small enough that an outbuilding almost never passes on the first look. University of Nebraska-Lincoln Extension's rodent-proof construction guide puts it in numbers: mice can use holes as small as a quarter inch in diameter, and rats can use holes as small as half an inch. For screening, the same publication specifies 24-gauge hardware cloth with a quarter-inch mesh to exclude mice, against 19-gauge half-inch mesh for rats. It also names the junction where wall framing or siding meets the foundation as easy rodent access, which on a pole barn is a long run of exactly that detail.

Doors are the other standing problem. Nebraska's guidance is to fit flashing or a metal channel on the lower edge of a door, extending to within an eighth of an inch of the edge of the door at the sides and bottom, because a wooden door bottom is chewable and a gap under it is an open invitation. Oregon State University Extension adds the simpler version for an exterior door: a brush-type sweep, with any gap of a quarter inch or more filled and then sealed. A service door with daylight under it and an overhead door with a cracked rubber seal are the two openings worth measuring before the building closes for winter.

• Close the overhead door and look along the bottom. Daylight at the corners is the usual failure, not the middle.
• Check the sill line all the way around. On a post frame building, the gap between the skirt board and the slab is the classic entry.
• Screen vents, not just holes. Gable and soffit vents on an outbuilding are often open mesh or nothing at all.
• Look up. Chimneys, stove jacks and roof penetrations matter, because the native mice around here climb well.
• Move the feed indoors or into metal. A sealed metal container defeats a mouse, a bag leaning against the wall does not.

Cleaning Out A Nest Without Making It Worse

This is the part people get wrong, and it is worth getting right before you open a shed that has been shut since fall. The CDC's guidance on cleaning up after rodents is a wet method: wear rubber or plastic gloves, spray urine and droppings with a bleach solution or an EPA-registered disinfectant until they are very wet, let it soak for five minutes or for the time the product label gives, wipe everything up with paper towels, put the towels in a covered garbage can, and then mop or sponge the area with disinfectant. The instruction people miss is the one before all of that: do not vacuum or sweep urine, droppings or nesting material, because that puts fine particles into the air you are standing in.

For a vehicle, Public Health Seattle and King County publishes a version with the extra steps a car needs. Move it into the open air, open the hood and let the engine compartment air out for twenty minutes, and open the doors and trunk as well. Wear plastic gloves and long sleeves while you inspect for nests, and disconnect the battery cables before pulling nesting material out of the engine bay. Soak the material with a disinfectant, or a one-to-nine bleach and water mix, and give it five minutes before you touch it. A pressure washer is the wrong tool here for the same reason a shop vacuum is.

The reason for the care is a rodent-borne virus, and the National Park Service fact sheet on light infestation cleanup states the exposure route plainly: disturbing contaminated material by sweeping, vacuuming or disturbing nests and then inhaling the dust is the most common cause of exposure. That guidance also says to bring in public health specialists for heavy concentrations of droppings in enclosed, poorly ventilated spaces. A closed shed with a long-running population is that situation, not a wipe-down job.

Traps Alone Leave The Door Open

Setting traps in an outbuilding catches the mice that are already in it. It does nothing about the quarter-inch gap they used, which is why the same building has the same problem next October. Exclusion is the part that lasts, and on an outbuilding it is usually a short list: a door bottom, a sill gap, two vents and the place a conduit or water line enters the wall. Our rodent control and exclusion work is built around sealing those openings rather than only treating what is inside, and the entry point walkthrough we wrote for houses applies to a detached garage almost line for line.

There is a practical order to it. Deal with the activity first, because sealing a building with a population inside it moves mice into walls instead of out of the building, then close the openings, then take away the food. On a farm shop, a commercial garage or a storage building where the mice are coming out of a field, that becomes an ongoing monitoring job rather than a one-visit fix, which is what commercial rodent management is for.

If the outbuilding shares a wall with the house, treat them as one problem. An attached garage is inside the building envelope, and mice that settle in it have a short walk to the kitchen. We went through what that looks like from inside in the guide to mice in walls and attics.

Do This Before You Shut The Door For Winter

Twenty minutes with a flashlight now is worth more than a spring morning spent pulling a nest out of a dashboard.

• Empty the food out. Every bag of seed, feed and pet food either goes into sealed metal or comes inside.
• Strip the camper. Dry goods, paper products and bedding out, cabinets left open so you can see into them in spring.
• Lift the hood before you park it. Then check it again in a month, because a nest built in November is easier to deal with than one built on in March.
• Get cardboard off the floor. Shelving and plastic totes instead of boxes stacked on concrete.
• Measure the two worst gaps. If a quarter inch fits, so does a mouse.
• Mow and clear the edges. Tall grass, brush and stacked wood against an outbuilding wall is the cover mice cross from.

State Termite has worked out of North Canton since 1933, with a second office in Hartville, and serves Stark, Summit, Portage, Wayne and Tuscarawas counties. Houses, garages, pole barns and the outbuildings behind them all count. Get a free estimate or call 330-494-6199, and we will look at the openings as well as the mice.