Slide a pencil into the gaps under your sink before winter: Why? Anywhere it fits, this animal can get in
It looks like nothing: a thin gap around the pipe under the kitchen sink, a crack beside a door frame. But as temperatures drop, those are exactly the openings mice are looking for. There’s a quick way to check them before winter, and all it takes is a pencil.
Why fall is mouse season
When it gets cold, rodents move indoors. Each winter, they invade an estimated 21 million homes, according to the National Pest Management Association (NPMA). The group also warns, on its PestWorld site, that rodents gnaw through wallboard, wood and electrical wiring, which raises the risk of a house fire, and that their droppings can spread hantavirus and trigger allergies.
The pencil test
The reason gaps matter is size. The CDC puts it plainly: “Mice can fit through a hole the width of a pencil (1/4 inch or 6 millimeters in diameter).” The NPMA describes it as an opening roughly the size of a dime.
So the test is simple. Take a pencil and try it in every gap you find. If it slides in, treat that opening as a door for mice, and seal it.
Where to check
The CDC lists the places to inspect.
Inside: under kitchen cabinets, around pipes under sinks, around windows and doors, and around floor drains in the basement or laundry room.
Outside: the roof (rafters, gables, eaves and soffits) and the gap between the foundation and the ground.
Where utilities enter: the NPMA adds cracks around pipes and utility lines, vents and chimney openings, loose mortar, and worn weather-stripping around windows and the foundation.
Window frames deserve a close look anyway, especially if you’re already cleaning the window tracks before winter.
How to seal what you find
The CDC’s method depends on the size of the hole:
Small holes: fill them with steel wool, then caulk around it to keep it in place, or use spray foam.
Larger holes: use lath screen or metal, cement, hardware cloth or metal sheeting.
The NPMA adds a few habits that make a home less tempting: store food, including pet food, in airtight containers, clean up crumbs quickly, and declutter basements, attics, crawl spaces and garages, where rodents like to hide. A well-organized garage is easier to inspect, too.
The limit
The pencil test finds entry points; it doesn’t deal with mice that are already inside. Droppings, gnaw marks and unusual noises in quiet areas like attics and basements are the signs to watch for, according to the NPMA, and at the first sign the group recommends calling a licensed pest professional. In a 2024 NPMA and Harris Poll survey, only 17% of Americans said they would do so. Sealing also won’t help if food is left out, and roof or soffit gaps are best handled by a pro.
In a nutshell
Rodents invade an estimated 21 million US homes each winter.
A mouse fits through a hole the width of a pencil, about 1/4 inch.
Check under sinks, around pipes, windows, doors, drains and the foundation.
Seal small holes with steel wool and caulk; call a pro if mice are already in.
The Trendir take: the smallest gap in the house is the one that matters most in winter. If a pencil fits, so does a mouse.
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