How to Keep Mice Out of Your House This Winter: The Long Island Mouse-Proofing Checklist

You keep mice out of your house in winter by closing every opening 1/4 inch or larger on the outside of the building, sealing the doors and vents they use most, and removing the food and nesting material that keeps them interested once they are in. The CDC's guidance is blunt about the size that matters: mice fit through a hole the width of a pencil, 1/4 inch or 6 millimeters. If a pencil fits, a mouse fits.

Here is the hard truth we tell every Long Island homeowner who calls in November. Traps and bait are how you win a mouse problem you already have. Sealing is how you stop having one. Most people do the first and skip the second, then call us again next fall from the same house with the same gap under the same garage door. The National Pest Management Association estimates rodents invade about 21 million U.S. homes each winter, and the entry point is almost never dramatic. It is a pipe sleeve with no sealant, a missing weep hole cover, or a corner of the bulkhead door that never closed right.

The good news is that a house mouse does not need much space. Its whole world is small. One mouse typically works an area only 10 to 30 feet across, according to the Internet Center for Wildlife Damage Management, so the mouse you saw in the kitchen is living within a few yards of that spot, and the hole it used is usually within the same radius on the outside wall. Find that hole and you have found the whole problem.

This is the order we work in on an exclusion job, and it is the order that gets the most protection per hour if you are doing it yourself.

1. Walk the foundation line at ground level. Check where every pipe, cable, gas line, and AC line set enters the house, the gap where siding meets the foundation, and any crack in the block or poured wall. Pack anything 1/4 inch or wider with steel wool or copper mesh and seal it in place with caulk. The CDC recommends steel wool held by caulk or foam for small holes, and lath screen, metal, cement, hardware cloth, or metal sheeting for larger ones.
2. Fix the doors. A garage door with a worn bottom seal is the single most common mouse entrance we find on Long Island. Replace the rubber astragal so it seats flat on the slab, add a door sweep to every exterior door, and make sure the bulkhead or basement door closes tight at the corners, not just the middle.
3. Screen the vents. Dryer vents, bathroom exhaust vents, crawl space vents, and attic gable vents all need intact 1/4 inch hardware cloth or a louvered cover that actually closes. A dryer vent with a broken flap is an open door with warm air pouring out of it.
4. Go up to the roofline. Look at the soffits, the gap where the fascia meets the roof deck, the plumbing vent stacks, and the point where the electrical service comes into the house. Cornell's Integrated Pest Management program notes that deer mice and white-footed mice, the species with white bellies, spend most of their time outdoors but enter structures from fall through spring, and finding one indoors means there is an opening they are using. These are the ones that show up in attics.
5. Cut back the bridges. Trim tree branches so nothing touches the roof, and pull firewood, mulch piles, and stored items at least a couple of feet off the foundation. Anything leaning against the house is a ladder and a hiding spot.
6. Take away the food. Move birdseed, grass seed, pet food, and pantry goods into sealed hard containers, and clean under and behind the stove and refrigerator. A house mouse eats roughly a tenth of its body weight a day and gets by on about 0.4 ounces of water, so even crumbs and a dripping pipe are a full pantry.
7. Clear the clutter in the basement, garage, and attic. Cardboard boxes, insulation scraps, and fabric are nesting material. Get storage into plastic bins on shelves so you can see the floor and spot droppings early.

Mice come inside in November on Long Island because the first hard freeze pushes them off the lawns, fields, and leaf litter where they lived all summer and toward the nearest warm, food-rich structure. Cornell Cooperative Extension puts Long Island's average first 32 degree freeze at three different dates depending on your location: around October 20, around November 1, or after November 10 for the milder coastal stretches. Mice move ahead of that line.

The Internet Center for Wildlife Damage Management states plainly that mice are most likely to invade structures as temperatures cool in the fall. On Long Island that window is later than upstate and it can slide later still in a warm year. The National Pest Management Association's fall and winter forecast, released August 26, 2026, said that if warm, dry conditions persisted into fall in the Mid-Atlantic and New England, pressure from cooler-season pests like rodents may be delayed rather than reduced. Delayed means November, not never. In our experience, a warm October on Long Island just compresses the whole rush into a couple of cold weeks after Halloween.

Mice get into a house through gaps at the foundation and ground level first, then through doors, vents, and utility penetrations, and, for white-footed and deer mice, through the roofline. The CDC's inspection list for the outside of a house covers the roof rafters, gables, eaves, and soffits, windows and doors without weather stripping, the gap between the foundation and the ground, attic and crawl space vents, and the holes where electrical, plumbing, cable, and gas lines come in.

Inside, the CDC's list is just as useful, because a hole on the inside tells you where to look on the outside. Check the backs of kitchen cabinets and closet corners, around the fireplace, behind appliances, around pipes under sinks and behind the washer, at the water heater and furnace connections, at floor vents and drains, and where the floor meets the wall. A grease-colored rub mark along a baseboard or a scatter of droppings 1/8 to 1/4 inch long is a mouse telling you its route.

Steel wool keeps mice out; spray foam by itself does not. Mice chew through cured foam with no trouble, which is why the CDC's guidance is to fill small holes with steel wool first and then use caulk or foam only to hold the steel wool in place. Copper mesh does the same job without rusting, and for anything bigger than a small hole you want hardware cloth, sheet metal, or cement.

We see a lot of Long Island basements with a neat bead of orange foam around every pipe and a mouse problem anyway. The foam looks finished, so nobody rechecks it, and by the second winter there is a clean round hole through the middle of it. A wad of steel wool or copper mesh packed tight, then sealed, is the fix. For the record, the things that do not seal a hole are peppermint oil, dryer sheets, ultrasonic plugs, and mothballs.

If mice are already inside when you seal the house, you have to trap them out, because sealing locks them in with your food and gives them nowhere to go but deeper into the walls. The CDC recommends traditional snap traps, set with the baited end against the wall so the trap forms a T, in the places mice actually travel: behind the stove and refrigerator, in the backs of cabinets and drawers, and in attics, basements, and crawl spaces.

Bait with a small dab of chunky peanut butter, check the traps daily, and keep setting them until you go a full week with nothing. The CDC specifically advises against glue traps and live traps because a frightened mouse urinates, which raises your chance of getting sick from what it leaves behind. Poison baits are for infestations that persist, must be EPA-registered products used exactly by the label, and belong out of reach of kids and pets. This is also where the math works against waiting. A female house mouse has litters of 5 or 6, can produce 5 to 10 litters a year, and her young are breeding themselves within 6 to 10 weeks. Two mice in November is a colony by February.

The order that works is trap first, seal second, then monitor. That is the same sequence we use on every mouse job, and it is why our mouse service comes with a 60-day written guarantee: once the population is at zero and the openings are closed, there is nothing left to guarantee against.

Call a pro when you have already sealed what you can see and still have activity, when the noise is coming from the attic or inside a wall, when you have found droppings in more than one room, or when the house has a crawl space, a finished basement, or an addition where the framing junctions are hidden. Those are the jobs where the entry point is above eye level or behind a finished surface, and where a licensed applicator with a flashlight and a mirror pays for itself.

We will tell you honestly which side of that line you are on. A single mouse in a ranch with an open basement is often a weekend project with a bag of copper mesh and a few snap traps. A colonial in Massapequa or Huntington with scratching over the bedroom ceiling and droppings in the garage is not, because the openings that matter are at the roofline and the population is already established. Either way the inspection is free, the quote is in writing before any work starts, and the sealing work is done with materials that will still be there in five winters.