How Do Mice Get Into Your House?

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Mice get into houses through openings most people walk past every day, like the corner gap under a garage door or the seam where a cable enters a wall. A mouse fits through a gap about a quarter inch wide, roughly the width of a pencil, so it doesn’t have to look like an obvious hole. Cold overnight temperatures in October drive them to look for a way in. Traps clear out what is already inside, but sealing the openings is the only part that keeps the next one from walking in.

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A house mouse needs an opening about the width of a pencil. Homeowners rarely believe that until we walk the outside of the house together and point to the spot. By mid-October around Des Moines, cold overnight lows send mice looking for those openings. So how do mice get in the house with every door and window shut? Almost always low, at a corner, or wherever something was run through a wall and never sealed behind.

The Gaps Mice Use to Get Inside

Start at the ground and work up. We inspect in that order because most entries sit below knee height, and the ones above are often gaps along the roof edge that a squirrel opened first.

• Where the siding meets the foundation at the base of the house, especially on older homes where the sill plate (the board resting on top of the foundation) has settled
• Seams around utility lines, anywhere a cable, gas line, or dryer vent was cut in and packed with nothing but foam
• If the rubber seal on your garage door is worn, the two bottom corners open up before anything else
• Brick homes add weep holes, the small drainage slots near the base, which sit wide open unless someone screened them

What surprises people is how little work a mouse puts in. They rarely chew a fresh hole through solid material. They find something already open and widen it slightly. So the entry point you are hunting for is a small gap that was already there, and it will not look like much.

Why the Garage Is Usually First

People write off mice in garage spaces as normal, and that is the mistake. Most garages in Ankeny and Urbandale sit level with the driveway, with a rubber seal along the bottom of the door that ice and a snow shovel have scraped for a decade. The corners curl first.

From there, a mouse is standing in a space warmer than outside, with a door into the house, cardboard to shred, and a bag of grass seed or dog food in the corner. The garage is one of the most common first entries we find. If the traps in your kitchen keep producing and nobody has looked at the garage, you are clearing mice rather than stopping them.

What Attracts Mice to Your House

Food is the obvious answer and it is half of it. Warm air leaking from a dryer vent signals shelter before any food smell does, and a mouse will run along a wall where it can feel heat.

Bird seed left in a paper bag in the garage is a reason to linger against that wall. So is a wood pile stacked flush to the siding. The longer a mouse spends there, the more likely it is to find the opening. A winter mouse problem in the house starts in October, not January. By the time you hear something moving in a wall in December, that opening has likely been in use for weeks.

How to Mouse-Proof Your Home

Sealing comes down to using the right materials. Mice chew through expanding foam, steel wool rusts apart, and a bead of caulk across a gap wider than a pencil will not hold up to chewing for long.

• Copper mesh packed tight into the opening, then sealed over so it cannot be pulled back out
• Hardware cloth, a stiff metal screen, fastened over vents and weep holes so nothing can push past it
• Replace the full garage door seal rather than patching the two corners
• Every exterior door needs a door sweep, the rubber or brush strip along the bottom, including the door between the garage and the house

The habits side of this matters too, and we wrote that up separately in our mouse prevention tips for winter. What is here is the sealing work.

What Exclusion Work Looks Like

Exclusion is our term for sealing a house against rodents and wildlife, and it is a big part of what a rodent job involves. A visit starts outside. We walk the full exterior, including the roof edge where it meets the walls, and mark every opening instead of stopping at the first one. Seal one gap while three stay open, and all you have changed is which side of the house the traffic uses.

Inside, we set traps to clear out the mice already in the house, and we check them on our schedule. No Trap Check Fees. No Per Animal Fees. No Trap Reset Fees. Dan Nielsen started this company in Treynor back in 2014, and we still run it as a family-owned business.

Not sure yet whether what you are hearing is a mouse? Our guide to the signs of a rodent problem in the house covers droppings, sounds, and the grease marks mice leave on the routes they reuse. Our mouse and rodent control service page covers trapping and removal, damage repairs, and prevention.

Call Before the Overnight Lows Settle In

October is the easiest month to fix this. A sealed gap now is a maintenance item. The same gap in January can come with droppings in the insulation and mice that may have already had a litter inside. If you hear anything in the walls, or you want us to check the outside of the house before you hear anything, call our Des Moines area office at (515) 985-0772.

We handle mice and wildlife for homeowners across central Iowa, including:

• Des Moines, IA
• Grimes, IA
• Ankeny, IA
• Urbandale, IA
• Waukee, IA
• Altoona, IA
• Bondurant, IA
• Norwalk, IA
• Van Meter, IA
• Saylorville, IA
• Pleasant Hill, IA
• Carlisle, IA
• Indianola, IA