How small a gap can be.
A mouse can enter through a seam about as tall as a pencil is thick. A rat needs a hole closer to the width of a thumb, but it will gnaw the edges until it fits. Neither takes long. A soft material, such as caulk alone, plastic or paper, gives way in a single night.
The rule we follow is simple: if you can fit a pencil in, it has to be closed, and it has to be closed with something harder than a rodent's teeth.
Where the openings are.
- The foundation, where mortar has crumbled or a sill plate leaves a seam.
- Utility entries: gas, water, cable, AC lines and electric service.
- Vents for the dryer, bath, attic and crawl space.
- The roofline, soffits, fascia returns and the places where roof meets wall.
- Doors and windows, especially garage door corners and basement window wells.
Most houses show a pattern, with the biggest cluster at the lowest corner and the roofline. Where the house meets an addition, the seam deserves extra attention.
Which material fits which opening.
| Opening. | Material. | Why it works. |
|---|---|---|
| Gap around pipes or cables. | Stainless mesh stuffed tight and topped with sealant. | Teeth cannot drag the mesh out because the sealant grips it. |
| Vent openings. | Hardware cloth in a metal frame. | Air still flows, animals cannot pass. |
| Foundation cracks and mortar joints. | Masonry patch compound. | Hard as the wall around it. |
| Garage door corners and thresholds. | New rubber seal or a rebuilt sill. | Closes the gap that daily use wears open. |
| Roof edges and soffits. | Metal flashing. | Sheet metal outlasts wood trim. |
Order of work.
The sequence matters more than the materials. Sealing too early locks animals in, and they chew a fresh exit. So we trap first, let the catch fall, then seal in two passes: the obvious openings, then the ones the first pass revealed.
When there is doubt that every animal has left, we confirm before the last holes are sealed. The trap setting page describes how that tally works.
Houses that stand shoulder to shoulder.
Where houses stand close, a gap on a neighbor's wall can feed your side. You cannot fix their wall, but you can fix yours. Sealing your own foundation and roofline raises the odds that a problem next door stays there.
Would you rather speak with someone than read an article? A person answers this number day and night.
(551) 274-5838An annual recheck.
Walk the base of the house each autumn with a flashlight. Check that mesh has not shifted, vent screens are intact and sealant has not cracked. A ten-minute walk now is cheaper than a repair visit later, and you can call about anything that looks changed. For damage already done, see repairs after gnawing.
Dust and droppings: a safer way
Most steps here are about stopping dust from drifting through the air.
- Never clean rodent waste dry. Brooms, dust mops and vacuum cleaners all throw invisible particles into the room.
- A room that has sat sealed for weeks deserves a cross-breeze, so prop two openings and walk away for a while.
- Keep a gloved hand away from your face while working, then wash both hands well.
- Wet the waste thoroughly, wait the five minutes, then pick it up with paper towels.
- Use tongs or a gloved hand to lift a carcass, and seal it in a bag at once.
- Questions about a symptom belong with a medical professional, who can decide if rodents are involved.
See the agency pages for yourself
Each title below links to the agency page.
- CDC summary on preventing hantavirus: How blocking access and tidying storage reduce encounters with rodents.
- CDC page: mopping up after mice and rats: The agency's own wording on ventilating, soaking and bagging.