Cockroaches in Apartment Buildings: Why Treating Only Your Own Flat Achieves Nothing (and What to Demand from the Building Manager)

There is one sentence our technicians hear almost every week, and it is always said in the same tone, half weary, half disbelieving: "I cleaned everything, I set out traps, I had someone come in, and three weeks later they were back." The tenant or owner saying it has done nothing wrong. They have simply fought a building-wide problem with flat-sized means.

The German cockroach (Blattella germanica), the species responsible for the overwhelming majority of domestic infestations in France, does not think in terms of individual apartment lots. It thinks in terms of pipe networks, service ducts, suspended ceilings and bin rooms. As long as the fight stays compartmentalised, door by door, it is lost before it starts — and that has nothing to do with the quality of the product used.

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Why the German cockroach is a building-level problem

A biology tailored to shared housing

Unlike the oriental cockroach (Blatta orientalis), which is larger and tends to stay confined to cellars and damp basements, the German cockroach is small — 12 to 15 mm — light brown, and marked with two parallel dark stripes on the pronotum. It climbs smooth vertical surfaces, slips through a 1.5 mm crack and lives exclusively in heated interiors.

That last line explains everything. A female German cockroach carries her ootheca until it hatches, which shields the eggs from contact insecticides. She produces four to eight oothecae in her lifetime. With an egg-to-adult cycle of roughly fifty to sixty days at 25-30 °C, a single fertilised female introduced in January can generate several thousand descendants before Christmas. So we are never talking about an isolated "cockroach": when one individual is visible in daylight, the population generally already runs to several hundred hidden individuals.

The building's vertical motorways

From a cockroach's point of view, a residential building is one continuous habitat. The crossing points are always the same:

• Service ducts that stack kitchens and bathrooms from one floor to the next, often without fire stopping and therefore completely open;
• Pipe penetrations through the floor slabs: around the wastewater downpipe, the water supply, the washing machine trap;
• Suspended ceilings in corridors and cellars;
• Rubbish chutes and bin rooms, reservoirs of both food and warmth;
• Electrical conduits and socket boxes, particularly in adjoining kitchens;
• Poorly maintained mechanical ventilation risers.

A treatment carried out in a single dwelling triggers a phenomenon well known to applicators: dispersal. The insects flee the treated area through the ducts, colonise neighbouring flats, and come back a few weeks later once the product's residual action has dropped off. The next-door neighbour, who had nothing, then discovers their first cockroaches — and the cycle starts again, one flat along.

Operational rule: in an apartment building, you never treat one dwelling alone. You treat the infested flat, both adjoining flats, the one above, the one below, and all the technical common areas. That is the absolute minimum.

Spotting an infestation before it goes building-wide

The useful window is short. Here are the signs, in the chronological order in which they appear:

1. A smell that is slightly sweet and musty, and lingering, in a cupboard under the sink or behind the fridge. It comes from aggregation secretions.
2. Droppings: tiny black specks resembling ground pepper, on cupboard hinges, along skirting boards, behind appliances.
3. Translucent moults and empty oothecae, shaped like small ridged brown capsules.
4. An individual visible at night, scurrying away when the light goes on.
5. An individual visible in broad daylight: this stage signals a saturated population, where competition for hiding places forces the young ones out into the open.

To objectively gauge the level of infestation and, above all, to map the hotspots, nothing replaces setting out pheromone sticky traps in every vulnerable room: under the sink, behind the dishwasher, on top of the fridge, behind the toilet cistern. Checked at 72 hours and then at seven days, they show where the heart of the nest really is — which is not always in the flat that complains the loudest.

The areas to inspect first

German cockroaches look for three things at once: warmth, moisture and food. That combination is found in a handful of specific places:

• The motor and rear grille of the fridge, permanently at 30 °C;
• The base of the dishwasher and its sound-insulating foam;
• The cavity behind the hob and the splashback seals;
• The bath panel and plumbing access hatches;
• Oven handles, wall unit hinges, drawer runners;
• The inside of electrical appliances: microwave, coffee machine, internet router.

What the law says: who pays, who acts

This is the most frequent sticking point, and it poisons entire cases for months on end. Let us restate the framework.

For a rented dwelling

Decree no. 2002-120 of 30 January 2002 on decent housing, as amended by the ELAN Act of 23 November 2018, states that a dwelling must be "free from any infestation by harmful species and parasites". The wording is clear: the absence of pests is a component of a dwelling's decency. The responsibility therefore falls on the landlord, unless they can show that the infestation results from an obvious lack of upkeep attributable to the tenant — a demonstration that is rarely straightforward in a building where several lots are affected.

In practice, the recommended procedure is:

1. Written notification to the landlord or the managing agent, with dated photos and a description of the rooms concerned;
2. If there is no reply within fifteen days, a registered letter with acknowledgement of receipt formally requiring action;
3. A copy to the building manager if the common areas are involved;
4. If the deadlock persists, referral to the municipal hygiene and health service (SCHS) or the regional health agency, and then to the departmental conciliation commission.

For a shared-ownership building

As soon as the infestation affects several lots or the common areas, dealing with it falls to the owners' association. The building manager may take urgent protective measures without waiting for the general meeting where there is a health risk, on the basis of their duty to administer the building. The departmental sanitary regulations, in most départements in the Paris region, require the owner to keep the premises permanently clean and to ensure control of rodents and insects.

It is worth knowing this: an annual rodent- and insect-control contract for the common areas is commonplace, but it almost never covers the private areas. It is precisely that contractual boundary that lets the German cockroach thrive — and the cockroach, for its part, has never read the building's rules.

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The protocol that actually works

Step 1 — Diagnosis at the scale of the riser

Before any product is used, a serious applicator maps the building: identifying the species, setting traps, locating ducts and penetrations, and interviewing the occupants along the vertical riser concerned. Everything else depends on this diagnosis. A job that starts with a general spray-down without any inspection is a bad sign.

Step 2 — Gel bait, not the spray can

The gold-standard method against Blattella germanica is now gel bait applied as micro-deposits inside the harbourages, at a rate of dozens of spots per kitchen. Its logic is indirect and devastatingly effective: the insect eats the gel, returns to its shelter, dies, and its fellow cockroaches — which feed on droppings and on the dead — are poisoned in turn. That is the cascade effect, which reaches the individuals that never come out.

Conversely, "fogger" aerosols or total-release bombs sold in supermarkets are counterproductive in this context: they do not penetrate the harbourages, they scatter the colony towards neighbouring flats and they contaminate the baits. ANSES has repeatedly highlighted the risks associated with these products in uncontrolled domestic use, and has warned about unauthorised biocides circulating online. The rule is simple: any product used must hold a marketing authorisation that can be checked in the SIMMBAD database, and must only be handled with suitable equipment — at the very least disposable nitrile protective gloves and proper ventilation.

Step 3 — Preparation of the dwelling by the occupant

A poorly prepared treatment loses half its effectiveness. The day before the visit:

• Empty and clean out the kitchen cupboards, degrease behind and underneath appliances;
• Pull the fridge and dishwasher out of their recesses if possible;
• Transfer dry goods — flour, pasta, rice, pet kibble — into airtight storage containers;
• Do not wash surfaces with scented detergents on the day itself: they repel cockroaches and drive them away from the gel spots;
• Above all, do not apply contact insecticide yourself just beforehand.

Step 4 — The second visit, non-negotiable

The biological cycle makes a follow-up check at three weeks essential. The oothecae present at the time of the first visit hatch afterwards, and the new generation has to find fresh bait. A single-visit treatment against the German cockroach is, in almost every case, an unfinished treatment. Expect two to three visits over six to eight weeks for a building-wide case.

Step 5 — Physical exclusion, the step nobody takes

This is the step that gets skipped and that explains the reinfestations six months later. After treatment:

• Seal pipe penetrations with sanitary silicone sealant or, better still, mortar around slab crossings;
• Fit adhesive door bottom seals on flat entrance doors and bin room doors;
• Replace degraded traps and downpipe clamps;
• Have the service ducts fire-stopped during maintenance works;
• Insist that the building manager seal off the rubbish chute or bring it up to standard: most shared-ownership buildings in the Paris region that have closed theirs have seen reports of both cockroaches and rodents fall at the same time.

Collective hygiene: the five habits that make the difference

No hygiene measure will make an established infestation disappear. What they do determine is how fast it comes back.

• No standing water overnight: a cockroach survives a month without food, but less than a week without water. Sink wiped dry, shower tray dry, draining rack emptied.
• Closed bin: an airtight-lidded bin in the kitchen, taken out every evening, deprives the colony of its main resource.
• No cardboard: corrugated cardboard is food, shelter and means of transport all at once. Deliveries and moving boxes are the number one route of introduction into a new home. Unpack them on the landing, never in the kitchen.
• Crumbs and grease: the underside of the oven and the back of the hob concentrate most of a colony's food supply.
• Pet kibble: never leave a full bowl out overnight.

When to call a professional

Three situations leave no room for self-treatment:

• Several lots affected in the same building or the same riser;
• Daytime sightings of individuals, a sign of a saturated population;
• Reinfestation after a treatment, which suggests a source in a common area or at a neighbour's.

A reputable company provides a written diagnosis, the list of products used with their marketing authorisation numbers, a bait placement plan, an intervention report and a follow-up date. It will also tell you plainly whether treating your flat alone makes any sense — and in an infested building, the honest answer is often no.

At ProDeratisation, our teams work 7 days a week across Île-de-France for private individuals, tenants, building managers and landlords, with free diagnosis and quotes. On shared-ownership cases, we draw up a treatment plan at the scale of the technical riser along with a follow-up schedule: it is the only format that genuinely puts an end to the cycle.

Sources and references

• ANSES — Opinions and alerts on domestic-use biocidal insecticide products and on unauthorised biocides marketed online.
• Decree no. 2002-120 of 30 January 2002 on the characteristics of decent housing, as amended by the ELAN Act of 23 November 2018 (article 142).
• Departmental sanitary regulations (obligations regarding insect and rodent control in residential buildings).
• Santé publique France — Documentation on cockroach allergens and asthma in shared housing.
• SIMMBAD database — Checking marketing authorisations for biocidal products.