Mice get into your house through gaps and cracks far smaller than most people expect. An adult house mouse can squeeze through an opening roughly the width of a pencil, around 6 to 8 millimeters. That means any crack in your foundation, any unsealed pipe opening, or any warped piece of siding is a potential front door. Understanding exactly where and why mice enter makes it much easier to keep them out.
How Mice Fit Through Tiny Openings
A mouse’s skull is the widest rigid part of its body, and even that is surprisingly narrow. The rest of its skeleton is flexible, and its soft tissue compresses under pressure. Controlled studies have found that mice consistently pass through circular holes as small as 16 millimeters (about 5/8 of an inch), and some individuals manage openings closer to 13 millimeters (1/2 inch). Juvenile mice can slip through even smaller gaps, sometimes as tight as 3 to 6 millimeters.
This means you can’t judge a gap by whether it “looks” big enough for a mouse. If you can slide a pencil into a crack, a mouse can likely work its way through. Gaps alongside pipes, where caulk has pulled away from siding, or where a foundation meets the framing are common culprits that homeowners overlook precisely because they seem too small to matter.
The Most Common Entry Points
Mice don’t pick locks. They exploit the structural shortcuts already built into your home. The most frequent entry points fall into a few categories.
Foundation and slab gaps. Older homes commonly have cracked foundations, crumbling mortar, or gaps where the wall framing meets the foundation. Even newer construction can have flaws at these junctions. Wood and masonite siding is especially prone to warping and cracking near corners and around the base of the building, opening pathways right at ground level where mice are already foraging.
Utility penetrations. Every pipe, cable, gas line, and electrical conduit that enters your home creates a hole in the exterior shell. These openings almost always have excess space around the utility line, and even in new buildings, they’re rarely sealed tightly enough to stop a mouse. Water pipes, electrical conduit at meters or breaker panels, and communication cables are repeat offenders.
Doors and garage openings. Mice frequently enter under exterior doors, particularly when weatherstripping is worn or missing. Garages are a major staging area. Mice slip under or beside poorly fitting garage doors, then travel deeper into the house along electrical lines, furnace ducts, laundry drains, or gaps in fire wall sheathing. If your garage connects to your living space, it’s essentially one more exterior wall you need to seal.
Dryer vents and HVAC returns. Cold air return ducts on forced air furnaces, especially those located in outside cabinets or garages, give mice a straight shot into your ductwork. Dryer vents with broken or missing flaps are another easy access point.
How Mice Reach Your Roof and Attic
Mice are strong climbers. They scale textured walls, brick, siding, downspouts, and gutters without much difficulty. They can jump 12 to 18 inches vertically, and they’ll walk along wires and narrow ledges to reach higher entry points. This means your roofline is not safe just because it’s off the ground.
Tree branches that touch or overhang your roof act like ladders. Mice climb them, land on the roof, and then probe for any weakness. Common upper-level entry points include attic vents without wire mesh (or with damaged screens), loose or cracked soffits, worn seals around roof vents, and gaps along flashing or shingle edges where materials have shifted over time. Gutters that have pulled away from the fascia board create both a climbing path and a gap at the roofline.
Attic invasions often go unnoticed for weeks because homeowners rarely inspect soffits and roof vents from below. By the time you hear scratching overhead, mice may have already nested in insulation.
Why Mice Come Inside
Mice are warm-blooded and relatively small, which means they lose body heat quickly. When outdoor temperatures drop in fall, they actively seek warmer shelter. The peak invasion window runs from October through February, and an estimated 21 million homes in the U.S. experience rodent intrusions during this stretch. Food also becomes scarcer outside in winter, making the crumbs, pet food, and pantry items inside your home irresistible.
But cold weather isn’t the only driver. Mice will enter homes year-round if they detect food, water, or safe nesting material. A leaky outdoor faucet, an open bag of birdseed in the garage, or accessible trash cans near the house all broadcast an invitation. Once a single mouse finds a reliable food source inside, it establishes a scent trail that other mice follow.
What Mice Can Chew Through
Sealing gaps only works if you use the right materials. Mouse incisors grow continuously, so mice gnaw constantly to keep them worn down. They chew through drywall, wood framing, insulation, cardboard, thin plastic packaging, and expanding spray foam with little effort. They can also damage electrical wiring inside walls, which creates a real fire risk, and gnaw through plastic plumbing pipes.
This is why materials like spray foam or caulk alone won’t stop a determined mouse. They’ll chew right through it within a day or two. Effective exclusion requires a physical barrier they can’t get their teeth around.
How to Find Active Entry Points
Before you seal anything, it helps to know which openings mice are actually using. Look for greasy, dark smudges along walls, pipes, or edges of holes. These “rub marks” come from the oils and dirt in a mouse’s fur transferring onto surfaces as the animal repeatedly travels the same path. You’ll also find small, dark droppings (about the size of a grain of rice) clustered near entry holes, along baseboards, and inside cabinets.
Fresh gnaw marks around a gap confirm active use. The marks will look light-colored on wood; older gnaw marks darken over time. Check both the interior and exterior sides of your foundation, around all utility penetrations, under doors, and along the roofline if you can safely access it.
Sealing Entry Points Effectively
The goal is to stuff or cover every gap larger than a quarter inch with something a mouse can’t chew through, then seal it in place.
- Copper mesh is the most durable option for exterior gaps. It resists rust, holds up in rain and humidity, and doesn’t degrade over time. Use mesh made from at least 20-gauge wire, tightly woven, and pack it firmly into the opening.
- Stainless steel wool works well for indoor gaps or irregular spaces that need flexible packing. Standard steel wool rusts and becomes brittle when exposed to moisture, so stick with stainless steel if there’s any chance of dampness. It costs more but lasts far longer.
- Silicone caulk or expanding foam should go over the mesh or wool to lock it in place and prevent mice from pulling it out. Use waterproof, exterior-rated sealant for outdoor repairs. On its own, foam or caulk is not enough, but layered over metal mesh it creates a barrier mice can’t defeat.
For larger openings, like where a dryer vent exits or where a section of soffit has failed, hardware cloth (metal screen with 1/4-inch openings or smaller) screwed into place is a more practical solution. Around doors, replace worn weatherstripping and consider adding a door sweep with a metal or heavy rubber edge.
Trim any tree branches to at least three feet from your roofline. Reattach loose gutters. Replace damaged vent screens in the attic and along the foundation. These steps don’t just fix the current problem. They remove the pathways mice will test again next season.