How Small a Hole Can a Mouse Fit Through?

A mouse can get through a gap of a quarter inch, about the width of a pencil. NC State Extension says gaps of a quarter inch or larger let adult mice into a structure, and the same size lets young rats in. Adult rats need about half an inch.

That means sealing a house is detail work. This article covers where gaps that size hide on Asheville homes and how the common sealing materials compare: steel wool, copper mesh, hardware cloth, sheet metal, mortar and foam.

Where quarter-inch gaps hide

Walk the outside of the house with a flashlight and a pencil. If the pencil fits, a mouse fits. Check:

Where pipes, gas lines, cable, internet and electrical lines enter the house.
Around the refrigerant lines from a heat pump or AC unit.
Dryer, bath and range vents, especially if the flap does not close.
Crawl space vents with torn or rusted screens, and the crawl space access door.
The bottom of the garage door and the corners where it meets the frame.
Exterior doors with more than a quarter inch of clearance at the floor, which NC State Extension advises against.
Where siding meets the foundation, and weep gaps along brick or stone veneer.
Where a deck, porch or addition meets the original house.
Roofline gaps at soffits, gable vents and where trim has pulled away.

Inside, check under sinks, behind the washer and dryer, around the water heater and furnace and in closet corners.

Gaps that are common on Asheville houses

Older homes from the 1890s through the 1940s, common in Montford, West Asheville, Kenilworth and Grove Park, have stone or brick foundations with mortar joints that loosen over time, plus decades of pipe and wire holes from repairs. Porch voids often connect to the crawl space.

Hillside houses have a tall foundation on the downhill side and siding close to the ground on the uphill side, with gaps hidden behind shrubs.

Newer homes in South Asheville, Arden and Weaverville usually leak at garage doors, utility entries and veneer weep gaps. See rodent exclusion.

Sealing materials compared

00-grade steel wool: NC State Extension says only 00-sized steel wool deters rodents and that it should be caulked over. It works well packed into pipe holes. It can rust outdoors and in damp crawl spaces.
Copper mesh: packs like steel wool but does not rust. NC State Extension lists copper mesh among recommended materials. Good for damp spots and exterior gaps. Cover with sealant.
Hardware cloth: NC State Extension recommends quarter-inch-mesh hardware cloth for vents. Use it on crawl space vents, behind porch and mobile home skirting and over larger openings.
Lath screen and sheet metal: for larger gaps. NC State Extension notes lath screen does not rust or bleed through caulk.
Mortar and concrete: for gaps in stone, brick and block foundations.
Door sweeps and garage seals: rubber with a metal edge holds up better than plain rubber.
Expanding foam: NC State Extension does not recommend expanding foam by itself, because mice and rats can chew through it. It can fill a void behind mesh, but the mesh does the work.
Caulk alone: fine for hairline cracks, but rodents chew through soft caulk over open gaps. Back it with mesh.

Want every gap found and closed?

Your call goes to an independent rodent control provider serving Asheville and Buncombe County. Describe what you are seeing and ask for the plan and the price in writing before any work starts.

(828) 503-0038Tap to call · Free to call

How to seal in the right order

1. Find the active runways first, where you see droppings and rub marks. 2. Seal the gaps that are not active right away. 3. Trap along the active runways. 4. Seal the active gaps once catches slow down, so you do not trap mice inside walls. 5. Check sealed spots after a few weeks for chewing or pushed-out mesh.

See the best rat and mouse traps for the trapping side.

When to get help

Small pipe holes and door sweeps are good do-it-yourself jobs. Crawl space vents, foundation gaps, rooflines and hard-to-reach spots on a hillside house are where most people call for help. Fixr lists professional exclusion at about $175 to $500 nationally, and Angi at $200 to $600. See rodent exclusion in Asheville and our cost guide.

A simple sealing kit for homeowners

For small jobs, this covers most of what you need:

Copper mesh, for packing pipe and wire holes.
00-grade steel wool, for indoor gaps that stay dry.
Quarter-inch hardware cloth and tin snips, for vents.
An exterior-grade sealant and a caulk gun.
A door sweep with a metal edge for each exterior door with a gap.
A new garage door bottom seal if the old one is cracked or short.
Gloves, a flashlight and a pencil for testing gap size.

Pack mesh firmly into the gap so it does not pull out, then cover it with sealant. Check your work after a week for chewing.

Frequently asked questions

How small a hole can a mouse get through?

NC State Extension says gaps of a quarter inch or larger let adult mice in. That is about the width of a pencil.

What size gap can a rat use?

NC State Extension says half an inch for adult rats and a quarter inch for young rats.

Is steel wool or copper mesh better?

Both work when packed tight and caulked over. Copper mesh does not rust, so it is better outdoors and in damp crawl spaces.

Can I use spray foam to seal mouse holes?

Not by itself. NC State Extension says rodents can chew through expanding foam. Use mesh, hardware cloth or metal, with foam only as a filler behind it.

Get a rodent technician on the phone

Free to call. You are connected with an independent provider serving Asheville and the rest of Buncombe County. Same-day appointments are often available.