Mice and Rats: The 6 mm Gap Almost No One Sees Coming

Most people don’t worry about rodents until they see one dash across the kitchen in broad daylight. The problem is that by the time a mouse lets itself be seen like that, the colony is often already well established, sometimes for several weeks.

In the fall, rodents are actively looking for a warm way in well before the first signs become obvious to the people living there.

Why Fall Triggers the Search for Shelter

Mice and rats don’t hibernate. As outdoor temperatures drop and food becomes scarcer, they start actively looking for somewhere warm, dry, and close to a food source to spend the winter.

Homes, garages, businesses, and apartment buildings all become obvious targets. Not because they’re dirty or poorly maintained, but simply because they offer exactly what a rodent is looking for at this time of year.

6 Millimetres: The Measurement That Changes Everything

A mouse can compress its rib cage and squeeze through an opening as small as 6 mm in diameter — about the width of a pencil. A rat needs a bit more room, roughly 1.2 cm, about the size of a dime.

In practical terms, that means most homes already have several points of entry without anyone realizing it: around a pipe, under a garage door, at the dryer vent, or in a foundation crack that looks harmless.

The Signs People Notice Too Late

• Small black droppings shaped like grains of rice, often near the pantry or under the sink
• Scratching sounds in the walls, ceiling, or attic, especially at night
• A persistent ammonia-like smell in a closed or rarely used space
• Grease or dirt marks along baseboards, where rodents always follow the same path
• Food packaging, wood, or insulation that’s visibly gnawed

Mistakes That Delay Solving the Problem

• Setting traps without sealing entry points: new individuals keep coming in as fast as the first ones are caught
• Relying on mothballs or ultrasonic repellents, whose effectiveness isn’t proven
• Sealing a hole with expanding foam alone: rodents chew through it easily, so it needs to be combined with metal mesh or steel wool
• Using poison without knowing where the nests are, which can leave carcasses in inaccessible spots — a source of odours and sometimes secondary insect problems
• Ignoring a rodent report in a neighbouring unit, in an apartment building

The Right Approach: Inspect, Seal, Then Control

An effective treatment relies on three steps, always in the same order.

First, a full inspection of the perimeter and interior to identify every active and potential entry point, not just the obvious one.

Next, sealing openings with materials rodents can’t chew through, such as metal mesh, steel wool, or metal plates.

Finally, strategically placing bait stations and/or traps at confirmed points of activity.

How Long Before You Can Sleep Easy

The first signs of slowing activity often show up within a few days. But confirming complete elimination usually takes two to four weeks, with at least one or two follow-up visits.

Sealing entry points is the step most often overlooked, even though it’s what prevents a new episode the following spring.

What About an Apartment Building?

Like many pests, rodents move easily from one unit to another through shared walls, plumbing, and utility chases.

Treating a single unit without coordinating with the property manager and adjacent units rarely gives a lasting result: a rodent driven out of one unit often turns up a few weeks later somewhere else in the same building.

Hearing suspicious noises or noticing small droppings near the pantry? A professional inspection can identify entry points before the colony settles in for the winter.