"I went down to the basement twice a month, for the freezer. In October, I found droppings on the lid, and the box of empty bottles had been moved." This homeowner in Vitry-sur-Seine was convinced the rats were coming "from the sewer network", through the pipes. On inspection, they were getting in through an air vent whose original cast-iron grille had been replaced with a sheet of plywood by the previous owner, twenty years earlier.
In a house, the basement is the first room to be colonised by a brown rat. It is cool, dark, rarely visited, often cluttered, and it sits at exactly the level where rats move around: the foot of the building, the soil, the buried network. In the vast majority of cases described by pest control professionals in Val-de-Marne, the entry point is neither mysterious nor technical: it is a ventilation opening, an inspection chamber or the bottom of a door that nobody has looked at in years.
This blog has already covered broken pipes, sealing materials and disinfecting a basement after flooding. This article addresses a distinct subject: a complete inventory of the entry points into a house basement, from air vents to meter penetrations, and how to seal them without cutting off the building's ventilation — which would simply create another problem.
Why the basement, and why now
Three factors come together between the end of October and December.
Temperature. A house basement stays between 10 and 14 °C all year round. When outside temperatures drop below 8 °C at night, that difference becomes a powerful signal for a rodent looking for a stable shelter. Rats do not "migrate": they tighten their territory around the most favourable spots.
The sewer network. The brown rat (Rattus norvegicus) lives largely in sewers and in the connections serving blocks of flats and houses. Autumn rains and occasional surcharging of the network — common in areas close to the Marne and the Seine — push populations to look for exits into dry spaces, meaning basements and crawl spaces.
Seasonal clutter. October is the month when garden furniture, boxes, bags of compost, bulbs, apples and squashes all go downstairs. A bag of bird seed or a crate of apples in the basement is enough to turn an occasional visit into permanent residence.
A rat that finds warmth, water (condensation, an inspection chamber, a weeping pipe) and a food source all in one place has no reason to leave. Removing just one of those three factors already reduces the pressure considerably.
An inventory of entry points: what you really need to look at
Equip yourself with an LED head torch — both hands need to stay free — and walk the inside perimeter of the basement, then the outside perimeter, at ground level. Entry points fall into five families.
1. Air vents and low-level ventilation grilles
This is entry point number one. An air vent in an older house typically measures 20 × 40 cm. The classic defects:
- original cast-iron grille corroded, with bars loose or missing;
- grille replaced by a wood, Plexiglas or polystyrene panel (chewed through in a single night);
- plastic insect mesh or garden netting stapled in place, which stops nothing;
- crumbling masonry around the frame: the rat does not go through the grille, it goes around it.
A mouse can squeeze through 6 to 7 mm, a rat through 20 to 25 mm. An air vent grille is only protective if its mesh is smaller than 6 mm and it is made of metal, fixed into sound material.
2. Inspection chambers, covers and yard traps
In houses across Val-de-Marne, it is common to find in the basement, or at its edge, a service connection chamber (foul or surface water) closed by a cast-iron or concrete cover. Points to check:
- cover cracked, broken at a corner, or simply resting in place without a seal;
- chamber whose frame has shifted, leaving a gap of several centimetres;
- basement floor gully that has dried out: with no water seal, there is no longer any barrier between the sewer and the room.
A floor gully that is never used will evaporate within a few weeks. Filling it with water once a month, or fitting it with a stainless steel floor grating with a flap, eliminates one entry point for free.
3. Wall penetrations: pipes, cables, conduits
Every pipe passing through a basement wall was run through a hole larger than itself. In the past, that gap was filled with plaster or simply left as it was. So you will find:
- the incoming water supply and the meter penetration;
- the waste pipe from the washing machine or basement sink;
- electrical and telecoms conduits — remember the residents of Créteil left without fibre for months after rodents severed the cables;
- old oil boiler penetrations, often blocked up in a hurry.
4. The basement door
An external wooden basement door moves, delaminates and splits at the bottom. A rat enlarges an existing defect; it does not drill through a sound panel. The three weak points: the bottom of the door (a gap of more than 1 cm), the damaged threshold, and the jamb in friable masonry. A metal plate or an aluminium threshold bar screwed over the bottom 15 centimetres solves the problem for the life of the door.
5. The base of the façade and terraces
Outside, look for burrows: holes 5 to 8 cm across with clean edges, often under a terrace on pedestals, along a boundary wall, behind a log pile or against a garden shed. An active burrow is clean, free of cobwebs, and sometimes has a "run" leading to it — a trail of flattened grass.
Reading the signs: rat, mouse, or nothing at all
Before starting any work, confirm the diagnosis. Many basements offer "evidence" that is nothing of the sort (bird droppings under an air vent, hedgehog droppings near a door, mortar debris mistaken for droppings).
| Sign | Mouse | Brown rat |
|---|---|---|
| Droppings | 3-6 mm, pointed, scattered | 15-20 mm, like thick grains of rice, grouped |
| Grease marks | faint | black smears along walls |
| Noises | light scratching, at night | heavy running, banging |
| Damage | nibbled packaging | boxes torn open, gnawed wood, insulation pulled out |
| Smell | weak | strong, ammonia-like, lingering |
To settle the question, the most reliable method remains the flour test: sprinkle a thin layer of flour or talc over a 30 cm wide strip in front of suspected runs, and read the prints 24 to 48 hours later. An infrared trail camera set up for one night in a basement gives an even clearer answer, and avoids treating a basement that has been empty for two years.
