Cellars, Garages and Building Car Parks: The Blind Spots That Fuel Infestations

Introduction: the infestation doesn't start in your kitchen

When a resident calls a technician about cockroaches in the kitchen or rodent droppings behind the washing machine, the question to ask is almost never "how long has this been going on?" but "what's underneath your feet?".

In a shared residential building, the basement is a complete ecosystem: a stable temperature all year round, permanent darkness, damp, abandoned food, stacked cardboard boxes, and a web of service ducts linking every stairwell vertically. A rat that gets in through a faulty pipe joint on level -1 can reach the sixth floor without ever passing through a front door.

That's why so many in-flat treatments fail. You treat the symptom upstairs while the population reservoir stays intact twenty metres below. Three weeks later, recolonisation is complete.

This guide explains what actually goes on in cellars, lock-up garages, car parks and bin rooms, how to inspect them without being a professional, what the block management must cover, and the actions that produce a measurable effect.

Why the basement is a pest magnet

An underground space brings together, all at once, the four conditions any commensal species is looking for.

In its work on commensal rodents, ANSES regularly points out that the brown rat (Rattus norvegicus) is a burrowing species that favours lower levels and sewer networks, while the house mouse is more inclined to colonise intermediate volumes. In practice: the rat comes from below, the mouse moves horizontally.

Add to that a frequently underestimated point: the basement is the only part of the building that nobody keeps an eye on. A resident goes down to the cellar four to six times a year. An infestation therefore has several months' head start before anyone notices.

The ten-minute inspection, torch in hand

A proper check doesn't require professional equipment. It requires light and a bit of method. The original cellar lighting — often on a timer and yellowish — hides precisely the clues you're looking for: a powerful rechargeable LED torch completely transforms the quality of the inspection, especially around skirting corners and under stairways.

Signs of rodents

• Droppings: brown rat droppings measure 15 to 20 mm and are spindle-shaped; mouse droppings are 3 to 6 mm, like a black grain of rice. Fresh ones are shiny and soft; older ones are dry and grey.
• Grease marks: dark, polished smears along walls, 3–5 cm above the floor, indicate a regular run. It's the most reliable clue for mapping a route.
• Gnawing: wooden door edges, electrical conduits, pipe insulation, plastic bags.
• Smell: a persistent ammonia note betrays accumulated urine, meaning an established colony rather than a passing individual.

Signs of cockroaches

• Small black droppings, like scattered pepper, around consumer units and pipe penetrations.
• Empty oothecae — brown capsules 6 to 9 mm long — in cracks.
• A distinctive smell, sweetish and greasy, noticeable as soon as a population exceeds a few hundred individuals.
• Daytime sightings: a cockroach visible in broad daylight signals a saturated population, not the beginning of one.

Signs of textile damage

In cellars used to store clothes, rugs and spare bedding, the clothes moth (Tineola bisselliella) thrives. Look for silky cocoons in folds, small irregular holes in wool and cashmere, and whitish larvae at the bottom of boxes.

Mapping entry points: the vertical logic

A block of flats isn't a sum of apartments, it's a volume criss-crossed by networks. Pests use those networks.

The six critical routes

1. Service ducts: vertical shafts grouping water supply, waste pipes and sometimes electricity. A single unsealed hole between two levels creates a motorway. It's the number one vector for German cockroaches in apartment buildings.
2. Pipe penetrations in below-ground walls: the original seal degrades within ten to twenty years. A remaining 2 cm gap is enough for a mouse; 6 cm is enough for a brown rat.
3. Inspection chambers and floor gullies: a cellar gully whose water trap has evaporated becomes a door left wide open onto the sewer network.
4. Car park doors: the gap under the door leaf, often 3 to 5 cm, is crossed without difficulty.
5. Low-level ventilation grilles that are damaged or torn off.
6. Bin rooms: a door that no longer closes, containers without lids, a decommissioned rubbish chute poorly sealed off.

A practical rule technicians go by: a rodent gets through anywhere its skull fits. A mouse squeezes through a 6 mm gap, a young brown rat through 20 mm.

Sealing properly means using materials the rodent can't work through. Expanding foam on its own doesn't hold — it's gnawed away within a few nights. The proven solution is still metal mesh or packed stainless steel wool, then covered with a repair mortar. A roll of stainless steel wool costs a few euros and permanently settles a dozen or so holes.

What the block management must cover

The question of who pays comes up in every case. Cellars, car parks, bin rooms, service ducts and crawl spaces are common parts within the meaning of the French law of 10 July 1965. Their upkeep and sanitary condition therefore fall to the owners' association, funded through service charges.

Three texts frame the subject:

• The model Departmental Sanitary Regulation (circular of 9 August 1978) requires the owner to keep premises free of rodents and insects, and to carry out any necessary rat and insect control operations.
• Decree no. 2002-120 on decent housing, amended by the ELAN law, requires a dwelling to be "free from any infestation of pest and parasitic species".
• The Public Health Code (articles L.1311-1 et seq.) allows the mayor or prefect to order works where public health is compromised.

In practice, the most commonly applied split is:

For a deeper look at the split of responsibilities between landlord, tenant and managing agent, see our dedicated guide: pests in blocks of flats and rented housing, who pays for what.

One decisive legal point: a flat-by-flat treatment has no curative value if the source is shared. Asking the managing agent for a comprehensive intervention on a whole stack — all vertically aligned units treated on the same day — is not an excessive demand, it's the only technically valid method against the German cockroach.

Cleaning up your own cellar: the protocol

1. Empty and sort

This is the step everyone wants to avoid, and it delivers 70% of the result. A healthy cellar is one where you can see the walls and floor all the way round.

• Get rid of cardboard: it absorbs moisture, feeds cockroaches (starch glue is edible to them) and shelters oothecae.
• Replace it with airtight, stackable plastic storage boxes, kept 10 cm away from the wall.
• Never store food, pet kibble or bird seed in a cellar, except in a closed metal or rigid plastic container.

2. Raise and clear

Put belongings on metal shelving, at least 15 cm off the floor. A clear floor lets you inspect at a glance and deprives rodents of cover. A bare wall is a wall a rat has to cross in the open — and it hates that.

3. Deal with the damp

A cellar above 70% relative humidity mechanically attracts oriental cockroaches, woodlice and silverfish. An indoor hygrometer costing a few euros tells you where you stand before investing. Depending on the reading: restore low- and high-level ventilation as a priority, add a moisture absorber as back-up, or an electric dehumidifier if the space is closed and in use.

4. Protect textiles

Anything that has to stay in the cellar should go into vacuum storage bags or airtight boxes, never bin liners or cardboard. Traditional repellents — cedar, lavender — limit egg-laying but don't destroy an established population: washing at 60 °C or freezing for 72 hours remain the only genuinely lethal methods for moth larvae.

5. Monitor

Place two to four glue board monitors in corners, dated with a marker, and check them monthly. It's a diagnostic tool, not a treatment: it tells you about presence, species and active zone. That information is worth its weight in gold when you have to convince a reluctant managing agent.

The bin room: the highest-return spot in the building

If a block of flats could only fix one thing, this would be it. A poorly kept refuse room single-handedly feeds an entire colony of brown rats and acts as a springboard for cockroaches.

What a proper bin room looks like:

• Solid door, working door closer, bottom of the door fitted with a metal rodent-proof door strip.
• Containers with intact lids, always closed again.
• Washable floor, hosed down with detergent at least once a week.
• No loose deposits on the floor: a bag left next to a container is opened within one night.
• Meshed ventilation, 6 mm maximum aperture.
• Working lighting — a dark room is never cleaned properly.
• Decommissioned rubbish chute: sealed with masonry, not with a simple screwed-on plate.

These measures cost next to nothing compared with an emergency rat control contract. They remove the food resource, and without a food resource no population sustains itself.

Baits and rodenticides: what has changed

The use of anticoagulant rodenticides is governed by European biocide regulations, and ANSES has restricted several uses in recent years because of the risk of secondary poisoning — cats, dogs, birds of prey and hedgehogs eating poisoned rodents.

Three rules for private individuals to remember:

• Rodenticide bait is never laid loose. It must be placed in a lockable secured bait station, out of reach of children and pets.
• Stations must be identified, dated, and removed at the end of the campaign. Leaving bait out permanently sustains the genetic resistance now observed in brown rats in several French départements.
• In common parts, laying bait is the job of the contractor appointed by the managing agent: a private individual who puts down their own bait in a shared car park takes on personal liability.

For mice, mechanical trapping is often more effective than baiting: they are only moderately neophobic, explore a great deal, and eat little at each visit — which makes the lethal dose slow to accumulate.

If the problem goes beyond a single private cellar, have a professional assessment carried out: our treatment services cover both common parts and private units, and the shop brings together prevention equipment suited to basements.

The calendar: why September is a turning point

Autumn brings three phenomena together at once:

• Rodents look for shelter as soon as night-time temperatures drop below 12 °C. Cellars and car parks are the first spaces to be colonised.
• Summer house moves have filled cellars with fresh cardboard, often picked up at supermarkets and sometimes carrying oothecae.
• Harvests and gardens are emptying out, reducing outdoor resources and pushing populations towards buildings.

A basement inspection carried out between late August and mid-October is therefore ten times more useful than a spring one. It's the moment when you can still act on a population of a few individuals rather than an established colony.

Key takeaways

• In an apartment building, the infestation reservoir is almost always in the basement; treating the upper floors without treating the base guarantees a relapse.
• Diagnosis rests on three simple clues: droppings, grease marks, smell.
• Shared cellars, car parks, ducts and bin rooms are common parts: their treatment is the owners' association's responsibility.
• Against the German cockroach, only a full-stack treatment, with all vertically aligned units done on the same day, produces a lasting result.
• Getting rid of cardboard, raising storage, ventilating and closing the bin room removes the resource — that's more effective than any bait.
• Autumn is the window for action: intervening in September costs a fraction of what a campaign in February will.

Sources: ANSES (opinions and studies on rodenticides and commensal rodents), model Departmental Sanitary Regulation — circular of 9 August 1978, decree no. 2002-120 of 30 January 2002 on the characteristics of decent housing, law no. 65-557 of 10 July 1965 establishing the status of co-ownership, Public Health Code art. L.1311-1 et seq.