Mice in Chimney: How They Get In and How to Keep Them Out

TL;DR

• Yes, mice can climb a chimney. They run up rough brick and concrete, and squeeze through cracks as small as a dime (Health Canada).
• Don't light a fire to smoke them out. Nesting material can ignite or block the smoke, raising the risk of a chimney fire and carbon monoxide in the home.
• Keep them out with a stainless steel cap, a fully closed damper and gaps sealed with steel wool or metal. A cap alone is usually not mouse-proof.
• Never cover the flue with window screening or hardware cloth instead of a cap. It clogs with soot and debris and becomes a fire hazard.
• Check for Chimney Swifts before you cap. Capping a chimney they use can break federal law (Environment and Climate Change Canada).
• Need help? Get a quote from a recommended local exterminator through Pest Agent's referral form.

Scratching behind the fireplace on a cold fall night usually means mice in the chimney, and they did not need an open door to get there. A mouse can climb rough brick and slip through a crack the size of a dime, so the chimney top, the flashing and the damper all work as entrances.

The bigger worry is what they build. Nests of paper, insulation and dry plant material sitting in or near the flue can catch fire or block the smoke from leaving the moment you light the fireplace.

From the ground, a chimney looks far too tall and too smooth for something that small to climb.

Can Mice Climb a Chimney?

Mice can climb a chimney: they run up almost any rough vertical surface, including brick, concrete and wood. A chimney is also a sheltered, enclosed space that leads straight into the house, which makes it attractive once the nights get cold.

How Mice Climb Brick and Masonry

Brick and masonry give mice all the grip they need. Rats and mice can climb many rough vertical surfaces such as wood, brick, concrete and weathered sheet metal, according to University of Nebraska-Lincoln Extension, and they can also climb electrical wires, cables, vines, shrubs and trees to reach a building. Only a slick, smooth finish stops them. The same skill explains how mice climb walls inside your home.

Where Mice Get Into a Chimney

The open top of the flue is the obvious way in, but it is rarely the only one. Mice can squeeze through cracks as small as a dime, according to Health Canada, and through gaps about 1/4 inch (about 6 mm) tall, according to UC IPM. Damaged flashing where the chimney meets the roof, cracked mortar and gaps around the chimney base can let them reach the roof line and the attic. The mice entry points guide shows how to find these openings around the rest of the house.

Why Mice Move Into Chimneys

Mice move into chimneys for shelter as temperatures drop. Canadian pest control companies such as Orkin Canada and Terminix Canada note that mice seek refuge in homes in the fall to wait out the winter. House mice build their nests in undisturbed, enclosed spaces, and a smoke chamber or a fireplace that sits unused for weeks fits that description. More on the seasonal pattern is in the guide to mice in winter.

What Are the Signs of Mice in Your Chimney?

Mice in a chimney give themselves away through sounds, droppings, nesting material and smell around the fireplace. Check the firebox, the hearth and the area around the damper with a flashlight before you assume the noise is wind or birds.

Scratching Sounds at Night

Scratching sounds are the first sign most people notice. Health Canada lists scampering and scratching sounds in the walls at night among the signs of rats and mice. Quiet scratching, scampering or occasional squeaks above the fireplace or in the wall beside the chimney, more common after dark, point to mice.

Droppings in the Firebox

Droppings in the firebox or on the hearth are the clearest proof. House mouse droppings are about 1/8 to 1/4 inch (about 3 to 6 mm) long, similar in size and shape to grains of rice. Look along the edges of the firebox, on the smoke shelf if you can reach it, and behind fireplace tools. The mouse poop guide helps you confirm what you found.

Nesting Material

Nesting material shows where the mice have settled. House mice often shred fabric, paper, cardboard, wool, building insulation and plant material into loose, ball-shaped nests. Shredded material on the damper, in the firebox or falling from the flue means a nest is close.

Musky Smell

A musky smell is commonly associated with mice. A stale odour coming from a cold fireplace, especially with droppings nearby, suggests mice have been living in the chimney for a while.

Are Mice in a Chimney Dangerous?

Mice in a chimney are dangerous mainly when you light the fireplace. Their nests, droppings and gnawing also bring the same health and property risks as mice anywhere else in the house.

Fire Risk

Fire is the most immediate risk. Mice gather flammable nesting material such as shredded insulation, paper and dry twigs near the flue, and that material can ignite once the fireplace is in use. Rodents gnawing on wiring in the attic or walls next to the chimney can also start electrical fires, according to the Government of British Columbia.

Carbon Monoxide Risk

Carbon monoxide becomes a risk when a nest or debris blocks the flue. The Chimney Safety Institute of America links obstructions in chimneys to both chimney fires and carbon monoxide poisoning. In Ontario, the Town of Tecumseh fire service warns that age, weather and animals can damage a chimney in ways that may lead to fires or carbon monoxide, and the Government of British Columbia notes that rodents can start electrical fires. Working smoke alarms and carbon monoxide alarms are your safety net while the chimney is checked.

Disease Risk

Disease risk comes from droppings, urine and nesting material, according to Health Canada, which notes that mice and rats carry disease and can damage property. The Government of British Columbia notes that humans can get hantavirus from the droppings of wild mice, specifically the deer mouse, and it can spread when droppings, urine or nests are disturbed and the dust is breathed in. Deer mice are brown or grey with a white belly and feet.

How Do You Get Mice Out of a Chimney?

Getting mice out of a chimney starts with leaving the fireplace cold. A fire lit over a nest can ignite the nesting material or block the flue, and it does nothing to seal the way the mice came in.

Stop Using the Fireplace

Stopping all use of the fireplace is the first step until the mice and their nest are gone. The Town of Tecumseh fire service advises against using a wood-burning appliance that has sat unused for a long time until a W.E.T.T. (Wood Energy Technical Training) certified technician has inspected it. Apply the same rule after mice: have the chimney checked before the next fire.

Set Traps in the Firebox

Traps in the firebox, near the damper opening, or in the basement or attic spaces connected to the chimney catch mice where they travel. Health Canada says snap and electronic traps work well when they are well positioned and set properly, and that all traps should be checked daily. If you consider poison, use it only in a tamper-proof bait station wherever children or pets can reach it, never as the only control method, and look for a Pest Control Products (PCP) number on the label. The how to set a mouse trap guide walks through each trap type.

Clean Up Nests and Droppings Safely

Cleaning up after mice in the chimney follows Public Health Agency of Canada guidance for droppings and nests:

1. Open doors and windows and stay out of the area for at least 30 minutes before you start.
2. Put on rubber or plastic gloves.
3. Spray droppings and nesting material with 1 part bleach to 9 parts water, or a disinfectant approved by Health Canada.
4. Let it soak for 10 minutes.
5. Wipe it up with paper towels and double-bag the waste.

Never sweep or vacuum dry droppings, since the dust can carry germs. Nests inside the flue are out of reach for most homeowners and are a job for a chimney sweep. The how to clean mouse droppings guide covers the full method.

When to Call a Pest Control Professional

A pest control professional is the right call when traps catch nothing but the sounds continue, when a nest sits inside the flue, or when droppings show up in the attic too. Professionals can use specialized tools to retrieve animals from the flue liner and one-way exclusion devices that let mice out but usually not back in. A typical service inspects the home inside and out, seals gaps with exclusion materials, and cleans and sanitizes the affected areas. Rodent control services cover this work, and you can get a quote from a recommended local exterminator through Pest Agent's referral form.

How to Keep Mice Out of Your Chimney

Keeping mice out of your chimney takes a cap, a closed damper and sealed gaps, since no single measure blocks every route. Pair the steps with the how to keep mice out of your house guide, which covers the rest of the building.

Install a Stainless Steel Chimney Cap

A commercial chimney cap is one of the best ways to keep animals out of the flue, according to the Illinois Department of Natural Resources, and it should be secured to the chimney with bolts or screws. Choose stainless steel: the Internet Center for Wildlife Damage Management notes that galvanized steel caps can break down from rust.

Never cover the flue with window screening or hardware cloth instead of a cap. Screening clogs with soot and debris and becomes a fire hazard, and blocked mesh can trap carbon monoxide and push it back into the house. A cap is not mouse-proof on its own, though. Many caps sold in Canada use an "animal-proof" mesh that is larger than the gap a mouse needs, so the cap mainly stops birds, squirrels and raccoons. Inspect it periodically for damage, since caps may not exclude baby mice.

Keep the Damper Closed

The damper is the last barrier between the flue and your living room, and it must be fully closed whenever the fireplace is not in use. A closed damper also keeps a mouse that gets past the cap from reaching the firebox. Check that it seals all around and replace a warped or rusted damper plate.

Seal Gaps Around the Chimney

Gaps around the chimney need materials mice cannot chew. UC IPM recommends wire screen or coarse steel wool, and warns that plastic screening, rubber, vinyl, expanding foam and wood are often ineffective because mice can gnaw through them. Health Canada suggests stuffing steel wool around pipes before caulking, and the Public Health Agency of Canada lists metal flashing and cement for larger holes. Have damaged flashing or crumbling mortar repaired by a roofer or a mason.

Trim Branches and Vines

Branches and vines give mice a ladder to the roof. The Chimney Safety Institute of America advises keeping tree branches away from the top of the chimney. UC IPM also recommends thinning or removing plants that grow next to or up the walls of a house, since house mice can be excellent climbers.

Book an Annual Chimney Inspection

An annual chimney inspection catches nests, gaps and cap damage before the heating season. The Chimney Safety Institute of America recommends an inspection every year and cleaning as needed by a qualified professional. In Canada, fire services such as the Town of Tecumseh recommend a yearly inspection and cleaning by a W.E.T.T. certified technician, even if the fireplace is used only a couple of times a year, and the City of Greater Sudbury warns never to assume the chimney is clean.

Check for Chimney Swifts First

Chimney Swifts are a protected bird that often nests in chimneys, and capping a chimney they use can break federal law. Environment and Climate Change Canada explains that the Migratory Birds Convention Act, 1994 and the Species at Risk Act both prohibit damaging their nests or residence, and that capping or screening an active chimney can make it unusable for the birds. Swifts are usually in Canada from roughly late April or May until late summer or autumn, so plan cap installation and sweeping outside that period, or ask about a permit.