You keep the doors shut, the windows are closed, and the house looks tight. Then you find droppings under the sink. So how do mice get into your house when there is no obvious hole? Almost always, the answer is a gap you have not noticed yet, smaller and higher up than most people expect.
This guide maps the common entry points, outside and inside, using the CDC's own inspection checklist. Walk through it with a flashlight once before winter sets in, and you will know exactly where to focus your sealing.
The Short Answer
Mice get in through small gaps in the structure of a home. The CDC says a mouse can fit through a hole about the width of a pencil, roughly 1/4 inch. The usual routes are the roof line, gaps around doors and windows, the joint between the foundation and the ground, vents, and the holes where utility lines enter. Because UC Integrated Pest Management (UC IPM) describes house mice as excellent climbers that can run up any rough vertical surface, those gaps can be anywhere from ground level to the eaves.
Why a Tiny Gap Is Enough
The numbers are small. UC IPM says house mice can squeeze under gaps 1/4 inch tall and through openings 3/8 inch wide, and NC State Extension says gaps of 1/4 inch or larger allow adult mice in, including gaps under doors. Mice also have teeth made for gnawing, as the National Pesticide Information Center (NPIC) puts it, so an opening plugged with wood, vinyl, rubber, or foam does not stay closed for long. UC IPM lists all of those as ineffective materials for plugging holes.
Timing matters too. UC IPM notes that house mice frequently enter homes in autumn as nighttime temperatures drop.
How Mice Get Into Your House From the Outside
The CDC's outside checklist covers five areas. Here is what to look for at each one.
The Roof Line, Eaves, and Soffits
The CDC lists the roof, including rafters, gables, eaves, and soffits. UC IPM says mice run along wire cables and ropes and can jump up to 12 inches onto a flat surface, so a utility line or rough siding can lead straight up to a gap under the eaves. NPIC recommends trimming trees and vegetation growing along walls or near roofs. Mice that enter high often end up overhead, which our guide to mice in the attic covers.
Doors, Windows, and the Garage
The CDC flags windows and doors, especially those without weather stripping. NPIC recommends door sweeps and weather stripping to keep mice from crawling under doors, and NC State notes that rodent-proof door sweeps are not the same thing as ordinary weather stripping. The CDC also says to seal garages and outbuildings, since a gap at the garage door gives mice a sheltered space right next to your living area.
The Foundation
Check the gap between the foundation and the ground, plus any cracks in the foundation itself. The CDC recommends caulk for foundation gaps, and for mobile homes, fixing gaps in the skirting.
Attic and Crawl Space Vents
The CDC lists attic and crawl space vents on its outside checklist. NC State Extension suggests 1/4 inch hardware cloth for vents, so air still moves but mice cannot.
Utility Penetrations
Every spot where electrical, plumbing, cable, or gas lines enter the house is a likely route, according to the CDC. NC State adds drainpipes to that list.
How Mice Get in the House From Inside the Walls
Once a mouse is inside a wall, the garage, or the crawl space, it still needs a way into your living space. The CDC's inside checklist is where to look next:
- Under, inside, and behind kitchen cabinets
- Closet floors, especially the corners
- Around fireplaces, windows, doors, and appliances
- Around pipes under sinks and washing machines, and pipes to water heaters and furnaces
- Around floor vents and dryer vents
- Around electrical, water, gas, and sewer lines
- The attic, basement, and crawl space
- Around floor drains and where the floor meets the wall
If you are finding droppings in one room, start with the items on this list closest to that spot. NPIC says mice usually stay within 30 feet of their nests, so the way in is rarely far from the signs.
What Draws Mice to Your Door
Gaps are the how; food and shelter are the why. NPIC says uncovered garbage, pet food, and birdseed can attract mice. The CDC recommends keeping woodpiles 100 feet or more from the home and raised at least one foot off the ground, keeping compost bins at least 100 feet away, keeping grass and shrubs within 100 feet well trimmed, and placing bird feeders away from the house.
How to Close the Routes You Find
The CDC recommends steel wool held in place with caulk or spray foam for small holes, and lath screen, hardware cloth, metal sheeting, or cement for larger ones. NC State Extension does not recommend expanding foam by itself, since mice may chew through it. For materials and technique, see our guides to steel wool for mice and how to mouse proof your house.
Cheat Sheet
- Any gap about 1/4 inch or larger is a possible way in.
- Outside: roof line, doors and windows, foundation, vents, utility lines.
- Inside: cabinets, closets, appliances, pipes, vents, floor drains, wall edges.
- Look high as well as low, because mice climb.
- Add door sweeps and weather stripping, including on the garage.
- Screen vents with 1/4 inch hardware cloth.
- Plug with metal or cement, not foam, wood, or vinyl alone.
- Move woodpiles, compost, and bird feeders away from the house.
Where BioGuard Fits
We sell BioGuard Natural Rodent Repellent, a pack of 12 scent balls infused with peppermint and cinnamon essential oils. Once you have sealed the gaps on this checklist, it can add a scent layer near entry points, in the garage, and in sheds or storage areas. It does not close a gap and it is not a removal product. Replace it every 4 to 6 weeks, keep it away from children, pets, and food, and expect results to vary.
Frequently Asked Questions
How can mice get in your house if there are no holes?
There are almost always holes, just small or hidden ones. A gap about the width of a pencil is enough, according to the CDC. Check under doors, around pipes and wires, at the foundation line, and up at the eaves.
Can mice climb walls to get inside?
Yes. UC IPM says house mice can run up any rough vertical surface and along wire cables, so brick, stucco, and utility lines can lead them to gaps near the roof.
Can mice get in under a door?
Yes, if the gap is about 1/4 inch or more. NPIC recommends door sweeps and weather stripping, and NC State notes that rodent-proof sweeps differ from standard weather stripping.
Why do mice come inside in winter?
UC IPM says house mice frequently enter homes in autumn as nights get colder. A warm house with food and nesting material is an easy place to spend the winter.
Can mice get into the house through the garage?
They can. The CDC recommends sealing garages and outbuildings, not just the house. Check the garage door seal and any gaps around pipes or wires in the wall the garage shares with the house.
Sources
- CDC, How to Seal Up to Prevent Rodents
- UC Integrated Pest Management, House Mouse
- National Pesticide Information Center (Oregon State University and U.S. EPA), Mice
- NC State Extension, Surveillance and Management of Common Structure-Invading Mice