Quick answer: How do mice get in the house? Through gaps about 1/4 inch wide, the width of a pencil. On Willamette Valley homes the seven openings we find most are garage door corners, crawl space vents and access doors, siding-to-foundation gaps and weep holes, utility penetrations, dryer and bathroom vents, roofline gaps, and doors without sweeps. Rodent Awareness Week, observed each October, is the time to check all seven.
Key takeaways
- A mouse needs a gap only about 1/4 inch wide, according to the CDC, smaller than most people expect.
- The National Pest Management Association estimates rodents invade about 21 million U.S. homes every winter, and October is when valley mice start moving.
- Check each spot with a flashlight and a mirror, looking for daylight, droppings, rub marks and chewed edges.
- Traps remove the mice you already have; only sealing stops the next ones, so exclusion comes first.
What is Rodent Awareness Week and why does it land in October?
Rodent Awareness Week is an October observance promoted by the National Pest Management Association (NPMA) to get homeowners looking for entry points before winter. The timing is not random.
According to the NPMA, rodents invade about 21 million U.S. homes every winter. In the Willamette Valley, the first sustained rains arrive in October, the fields around Woodburn, Keizer and Dallas sit bare after harvest, and mice that spent the summer outdoors go looking for somewhere dry and warm. Once inside a heated house, our mild winters (often 35 to 50 F) never push them back out.
So treat the week as a deadline: one afternoon outside with a flashlight, before the mice make the same trip.
How do mice get in the house? The seven entry points
Mice get in wherever a gap reaches about 1/4 inch. The CDC puts it plainly: a mouse can fit through a hole the width of a pencil. The seven spots below are where we find the actual entry on most valley homes.
Before you start: the flashlight-and-mirror method
Walk the exterior in daylight, then check the garage and crawl space at dusk with the lights off, looking outward for pinpoints of daylight. A hand mirror on a stick shows the underside of siding edges and the back of AC lines. Carry a pencil: if it slides into a gap, so does a mouse. At every spot, look for rub marks, droppings, chewed edges and nesting fluff.
1. Garage door corners and bottom seal
The most common entry we find. The rubber bottom seal flattens with age, and the corners gap where it meets the side weatherstrip. Close the door, kill the lights and look for daylight at both bottom corners.
2. Crawl space vents and the access door
Original screens rust through, sag or were cut for a dryer duct decades ago, and access doors warp or disappear. Every vent needs intact 1/4-inch hardware cloth with no gap at the frame, and the access door must close flush and latch. If you find droppings at the hatch, read our guide to rodents in the crawl space before you go in.
3. Siding-to-foundation gaps and weep holes
Run the mirror along the bottom edge of the siding all the way around. Gaps open at inside corners, behind hose bibs and where an addition meets the original house. Brick weep holes need covers that pass water and block mice.
4. Utility penetrations
The AC line set, gas line, cable and electrical service, hose bibs and the plumbing that rises from the crawl space into the kitchen and bathrooms. The CDC’s advice is steel wool held in place with caulk for small holes, and lath screen, metal, cement or hardware cloth for larger ones. Foam alone is filler.
5. Dryer and bathroom exhaust vents
Louvered flappers stick open with lint, and plastic hoods crack. Check that the flapper closes fully and that the ledge is free of droppings. Skip fine mesh on a dryer vent, which clogs with lint; use a rodent-resistant hood made for dryers.
6. The roofline
Soffit gaps, unscreened gable vents, roof-to-wall joints and loose vent flashing. Roof rats travel fence tops and utility lines, according to UC IPM, and overhanging big-leaf maple limbs or ivy make a ladder to the eaves. Check from the ground with binoculars, then from inside the attic for daylight.
7. Doors without sweeps and sliding door tracks
Worn sweeps on exterior doors, especially the door from the garage into the house, and open ends on sliding door tracks. Daylight under a closed door means a mouse fits.
| Entry point | Check for | Fix |
|---|---|---|
| Garage door | Daylight at bottom corners, chewed seal | New bottom seal, corner pads, side weatherstrip |
| Crawl space vents and hatch | Torn screens, gaps at frames, warped door | 1/4-inch hardware cloth, latching door |
| Siding base and weep holes | Gaps at corners, hose bibs, additions | Copper mesh or steel wool plus sealant, weep hole covers |
| Utility penetrations | Openings around lines and pipes | Steel wool and caulk, metal or hardware cloth for large holes |
| Dryer and bath vents | Stuck flapper, cracked hood, droppings | Rodent-resistant hood, clear lint |
| Roofline | Soffit gaps, open gable vents, limbs touching roof | Repair soffits, screen vents, trim limbs |
| Doors and sliders | Daylight under door, open track ends | Door sweeps, threshold adjustment, track plugs |
What does the first week of a mouse problem sound and look like?
Light scratching in a kitchen wall right after the house goes quiet, a few rice-sized droppings in a drawer or under the sink, and a nibbled corner on the dog food bag. That is the whole first week, and it is easy to dismiss.
Early evidence clusters in one place because, according to UC IPM, house mice seldom travel more than 30 feet from their nest and food. UC IPM also notes a musky odor with mice, which you will notice first inside closed cabinets. Pets often know before you do; a dog that stares at the dishwasher kick plate every evening is telling you something.
Not sure whether the droppings are from a mouse or a rat? Our guide to rat droppings vs mouse droppings shows the differences side by side.
Why don’t traps alone fix a mouse problem?
Because traps deal with the mice already inside and do nothing about the opening they used. UC IPM reports that a female house mouse may have five to ten litters a year of about five or six pups each, so an open gap refills the house faster than a trap line empties it.
The order that works:
- Seal the exterior first, using the seven-point check above.
- Set traps inside the same day, along walls where you found droppings, trigger side toward the wall.
- Check traps daily and move any that go a week without a catch.
- Clean up droppings with gloves and disinfectant, never a broom or vacuum.
- Leave a few traps as monitors for a month, and recheck the seals after the first big storm.
Sealing also protects the rest of the house: mice that cannot reach the kitchen wall do not chew the wiring inside it. For the full weekend version, including insects, see our fall pest-proofing checklist for Salem homes.
Which entry points matter most on Willamette Valley homes?
Crawl space vents and the garage door, in that order. Most valley homes sit on vented crawl spaces, and much of Salem’s housing went up between the 1950s and 1980s, so we see original vent screens, later additions and decades of cable and plumbing penetrations nobody sealed.
Where farmland borders the neighborhoods, the pressure is higher. Around Woodburn, McMinnville and Dallas, harvest sends field mice toward shops, barns and houses at once, and garages holding feed and RVs are the first stop. In older Salem and Keizer neighborhoods near the river, mature oaks, maples and ivy-covered fences put the roofline in play.
Whatever your neighborhood, do the check before the first big October storm. Once the soil is saturated and nights drop into the 40s, the mice are already moving.
When it’s time to call AAA Pest
Call when you find droppings in more than one room, hear activity in walls or ceilings, find chewed wiring, or when you have sealed everything you can reach and droppings keep coming.
A professional visit starts with a full inspection of the attic, crawl space and exterior to find the openings a homeowner cannot see or reach. Our rodent control service seals them with rodent-resistant materials, places traps or tamper-resistant stations along the routes the evidence shows, and returns to confirm the activity has stopped. We use EPA-registered products applied according to the label, offer family- and pet-considerate options, and tell you exactly what to expect, including any re-entry times.
AAA Pest Exterminators has been family-owned and operated in Salem since 1997, with no long-term contracts. If the crawl space is damp as well as busy, our full list of services includes crawl space mold and fungus treatments.
Frequently asked questions
What size hole can a mouse fit through?
About 1/4 inch, the width of a pencil, according to the CDC. That is smaller than most gaps around pipes, cables and garage door corners, which is why a house can look tight and still have mice. Seal anything you can fit a pencil into, and check it again after the first storms.
When is Rodent Awareness Week?
It is observed each October and promoted by the National Pest Management Association. The exact dates shift from year to year, so check the NPMA’s PestWorld site for the current week. The point for Willamette Valley homeowners is the same every year: finish your entry-point check before the October rains settle in.
Does expanding foam keep mice out?
Not on its own. The CDC recommends packing small holes with steel wool and using caulk or foam only to hold it in place, with lath screen, metal, cement or hardware cloth on larger openings. Foam alone gives a mouse something soft to chew through. Copper mesh works like steel wool and does not rust.
Should I seal the house before or after setting traps?
Do both the same day. Seal the exterior first so nothing new arrives, then set traps inside along the walls where droppings are. Mice sealed inside will keep finding the traps, and a few monitors left in place for a month tell you whether you missed an opening. Catches that continue past a few weeks mean a gap is still open.
Pests in the nest? Don’t stress. Call the best.
Family-owned and operated in Salem since 1997. No long-term contracts. Serving Salem, Keizer, Albany, Dallas, Woodburn and McMinnville.
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