Every year, between October and December, calls about mice rise sharply. Nights get colder, food gets scarce outdoors, and heated buildings become irresistible. Traps and bait deal with the mice that are already there. But if the entry points stay open, other mice will take their place.
This is the third step of our Lock-Down Method, the one our whole approach is built around: lock down. And it starts with a number.
Where the 6 mm rule comes from
An adult house mouse can squeeze through an opening of about 6 mm, roughly the diameter of a pencil. Its body is flexible; the width of its skull sets the limit. For a young rat, the usual figure is around twelve millimetres.
Two other traits make the mouse a formidable intruder:
- its incisors never stop growing. It gnaws to wear them down, and it can enlarge a hole that is too tight in plaster, wood, plastic or expanding foam;
- it runs along walls and memorises its routes. Once a gap is found, the whole family uses it.
The practical test is simple: if a pencil goes into the gap, assume a mouse can too.
Where to look for gaps
Grab a strong torch, a pencil and a notebook. In a flat or a house, the most common entry points are always the same.
In the kitchen and bathroom
- pipe openings under the sink, often drilled too wide and never filled;
- behind the dishwasher, the fridge and the cooker;
- kitchen unit plinths: take them off, the space behind is the mice's favourite motorway;
- pipe boxing and access hatches.
In the rest of the home
- heating pipes running through the floor;
- cable conduits, sockets and badly fitted boxes;
- gaps under front, cellar or garage doors;
- air vents and cellar windows with mesh that is too wide or broken.
In the building or outside
- shared service ducts, rubbish chutes;
- cable and pipe entries through the outer walls;
- garage door seals, cracked wall bases and eaves on houses.
To find out whether a hole is in use, plug it with a ball of newspaper and come back 48 hours later. If the paper has been pushed out or nibbled, you have found a route. Droppings nearby confirm the diagnosis: our article on what rodent droppings tell a technician explains how to read them.
Which materials really hold up
Not everything stands up to teeth that grow for life. Here is what we use, and what we avoid.
| Material | Use | Note |
|---|---|---|
| Steel or copper wool + sealant | Holes around pipes and cables | The wool blocks, the sealant holds it. Copper wool does not rust |
| Mortar or plaster | Holes in masonry | Lasting if the surface is sound |
| Metal mesh, gaps smaller than 6 mm | Vents, cellar windows | Screwed in place, never just glued |
| Metal plate | Gnawed door bottoms, large holes | The toughest solution |
| Brush or rubber door sweep | Gaps under doors | Also stops draughts |
| Expanding foam on its own | Avoid | Mice gnaw through it easily; only use it on top of metal wool |
The order of operations matters
This is the mistake we see most often: blocking every hole while mice are still inside. They end up shut in, look for a way out and gnaw a new route, sometimes through an electrical cable.
The logical order is the Lock-Down Method itself:
- Detect: find the signs, routes and entry points.
- Neutralise: remove the mice present, with traps or secured bait in closed stations.
- Lock down: seal once activity has stopped, starting with the main routes.
- Monitor: check for a few weeks that no new signs appear.
Why act before winter
A female mouse can breed from about six weeks old and have several litters a year. A mouse that gets in during October can therefore start a small population before spring. Sealing early means stepping in before that happens, while the problem can still be solved in one or two visits.
In a block of flats, sealing a single home has its limits: mice travel through shared ducts. It is worth raising with the residents' committee so that the common areas are treated at the same time.
Calling us in
Our technicians carry out the entry-point survey, the treatment and the sealing as part of our house mouse treatment, which belongs to our rodent extermination service. To learn all about the animal, see the mouse guide. Prices are shown on the pricing page and you can book a slot.
FAQ
Your questions
Can a mouse really get through a 6 mm hole?
Yes. Its body is very flexible and the width of its skull decides whether it fits. An opening of about 6 mm, the size of a pencil, is enough for an adult mouse. It can also widen a slightly narrower gap by gnawing soft materials.
Is expanding foam enough to block a mouse hole?
No, not on its own. Mice chew through expanding foam with no trouble and reopen the route within days. First pack the hole with steel or copper wool, then close it with sealant, mortar or a metal plate depending on the surface.
Should I block the holes before or after getting rid of the mice?
After, or at least alongside active trapping. Blocking the entrances with mice still inside traps them: they then gnaw new routes, sometimes through cables. Remove the current population first, then seal and monitor for a few weeks.
