How do mice get in the house, and why they keep coming back

How do mice get in the house through such tiny gaps?

Any gap about 1/4 inch wide or bigger will do. A mouse can fit through a hole the width of a pencil, the CDC says. If a pencil fits, figure a mouse can too.

UC IPM says a house mouse can squeeze under a 1/4-inch gap or through a 3/8-inch hole. They're excellent climbers and can jump up to about a foot onto a flat surface. So check your eaves, not just the ground.

Where do mice get into an Austin house?

Where something goes through your wall, plus doors, vents and the roof. Here's the CDC's list of where to look:

• The roofline. Rafters, gables, eaves and soffits.
• Windows and doors. Especially ones without weather stripping.
• Where the foundation meets the ground.
• Attic and crawl space vents.
• Holes for utility lines. Electric, plumbing, cable and gas.

Texas A&M adds brush or baffle sweeps on outside doors, and sealing around pipes.

Are weep holes letting mice in?

An open one can be wide enough for a mouse. Weep holes are small gaps in the mortar of a brick wall, just above the flashing at the base and over windows and doors. They let water out, the Brick Industry Association says.

Got a brick house? You've got them. By our count of Census data, about half of new houses finished in 2025 in Texas and its three neighbor states were mostly brick. Nationally, it was 16%.

About half of new Texas-area houses are mostly brick

Our count: the share of new single-family houses whose main outside wall is brick or brick veneer.

A weep must be at least 3/16 inch across, under the 1/4 inch rule. But mortar joints are commonly 3/8 or 1/2 inch wide, so an open head joint, with the mortar left out, is roughly 3/8 to 1/2 inch. A house mouse can get through 3/8 inch, so an open joint can be wide enough.

Just don't seal them shut. They're how your wall drains. Screen them with copper mesh or a rust-proof metal weep screen. Texas A&M's scorpion guide lists copper mesh for weep holes too, though it's about scorpions, not mice. For those, see our scorpion control.

Why do mice come inside in the first place?

Colder nights are a big reason. Where mice live outdoors, they often move into homes in fall as nights get colder, UC IPM says, writing about California. Janet Hurley, a Texas A&M pest expert, said in 2015 that Texans can expect more mice as the weather turns colder.

When do our cold nights come? We counted 30 years of NOAA records for Camp Mabry: about 106 nights a year at 50 F or colder, 92% of them from November through March.

When Austin's cold nights come

Our count of 30 years of NOAA overnight lows at Camp Mabry: the average number of nights each month at or below each mark. The number on top is the 50-degree count.

• Nights at 50 F or colder
• Nights at 40 F or colder
• Freezing nights, 32 F or colder

October averages about 4 of those nights, November about 15. So finish sealing by late October. Our pest calendar covers the rest of the year.

Why do mice keep coming back?

One big reason: they breed fast. A female can have 5 to 10 litters a year, UC IPM says, and her pups can breed as early as 5 or 6 weeks old.

Mice leave more than droppings. In a national survey of 831 U.S. homes, begun in 1998 and published in 2009, researchers found mouse allergen in the dust of 82%. That doesn't mean 82% had mice.

Seal first or trap first?

Do both. UC IPM calls sealing the most successful and permanent way to control house mice. But once they're inside, it says you'll almost always need traps or bait too, plus cleanup.

• Steel wool for small holes. Hold it in with caulk or spray foam, the CDC says. UC IPM calls it a good temporary plug that may rust.
• Metal or concrete for bigger gaps. That's for foundation cracks and gaps around pipes and vents, UC IPM says.
• Traps for mice already inside. Hurley suggested traps over poison bait.

What about bait stations, kids and dogs?

Go easy on bait. Hurley called poison bait a last resort, best left to the pros, since it could be a risk to kids and pets. With young kids or pets at home, she suggested putting the traps in locked box stations.

Dogs are a real worry. ASPCA Animal Poison Control says very small amounts of some baits can be life-threatening to dogs and cats. Dogs will chew open bait stations, and rodents often drag bait away. If you think your pet's eaten some, call your vet or the ASPCA poison line at (888) 426-4435 right away.

That's why our stations are sealed, tamper-resistant and checked every visit. That lowers the risk, but it doesn't remove it.

Can you mouse-proof the house yourself, or should you call?

You can do a lot of it yourself. Grab a flashlight and a pencil, walk around your house and plug what you find. Then set traps inside. Call us when:

• Droppings keep showing up after you seal. You've probably missed a gap, or mice are still inside.
• The gaps are up high. Eaves and soffits mean a ladder.
• You hear them in your walls or attic.
• You'd rather not deal with it. That's fair.

That's what our rodent control is for. Plans cover 40+ pests, mice included, from $66 a month. An active infestation is a separate job from $449, and sealing is quoted on top once we see your house. See our services and cost guide.

Rather watch? Cornell's Matt Frye, who helped write UC IPM's mouse guide, covers keeping rodents out in about five minutes.

Common questions