Cockroaches in Apartment Buildings: Why Treating a Single Flat Never Works (and What Service Ducts Reveal)

There's one conversation that comes up, almost word for word, once a week on our switchboard. A customer calls back three months after an intervention: "you came in June, it was perfect for six weeks, and now it's exactly like before. Your product didn't hold." We always ask the same question in return, and it takes people by surprise: "do you live in a shared apartment building, and do you know whether your neighbours were treated on the same day?"

The answer is almost always no. And that, right there, is the whole explanation.

In a detached house, a German cockroach infestation can be dealt with. In a shared building, it cannot be dealt with flat by flat — it is dealt with at the level of the whole riser, or it is not dealt with at all. This isn't a sales pitch to bill more units: it's a direct consequence of the insect's biology and of the way French buildings have been constructed since the 1950s.

Brown rat eating seeds on a stone, long tail visible, blurred green background

The German cockroach isn't a "kitchen" insect, it's a "network" insect

An animal that doesn't live where you see it

Blattella germanica — the small light-brown cockroach, 12 to 15 mm long, with two dark stripes on the pronotum — is the species behind virtually every infestation in shared housing in France. It is neither the oriental cockroach of cellars, nor the American cockroach of sewer networks.

Its defining trait is that it is strictly thigmotactic: it needs to feel physical contact on its back and its belly at the same time in order to feel safe. In other words, it spends 75 to 80% of its time motionless, wedged into a crack 2 to 5 mm wide. The rest of the time, it looks for water, then for food, in that order.

The direct consequence: the individuals you spot at night in the kitchen represent only a tiny fraction of the population. The rule of thumb used by professionals is simple and unpleasant — for every cockroach visible in daylight, count several dozen hidden ones. An adult out in broad daylight usually signals a population that is already saturated, where pressure on hiding places pushes the weakest individuals into the open.

The breeding rate explains everything

A female German cockroach carries her ootheca (the capsule containing the eggs) attached to her abdomen until it hatches — unlike other species, which drop it. That ootheca holds 30 to 40 eggs, and a female produces 4 to 8 of them over her lifetime. At 25-30 °C, the egg → breeding adult cycle takes roughly 6 to 8 weeks.

Do the maths for a heated building, with ducts sitting at 24 °C all year round: a single fertilised female introduced in January can generate several thousand descendants before the year is out. That's why an infestation never "plateaus". It grows, or it gets treated.

The service duct: the motorway nobody looks at

What's really behind your boxing-in

In virtually every building constructed after 1950, stacked flats share a vertical service duct — that plasterboard or brick boxing-in that runs through the kitchen and the bathroom, carrying:

• the cold and hot water risers,
• the soil stack for waste water,
• the ventilation (mechanical extract ventilation or natural ventilation via a shunt duct),
• sometimes the electrical conduits and gas supply lines.

From a cockroach's point of view, this duct is the perfect habitat. It is warm (domestic hot water risers radiate constantly), damp (condensation on cold water pipes), dark, draught-free, and above all vertically continuous from the ground floor to the loft. No obstacles: the holes cut through floor slabs around pipework are rarely sealed properly, and when they are, it's with a polyurethane foam that cockroaches chew straight through after a few years.

The typical recolonisation route

Here's what actually happens when you treat a single flat on an infested riser:

This phenomenon has a name in ecology: recolonisation of a vacated niche. It reflects no failure of the product. It reflects a badly defined treatment perimeter.

Why cockroaches "come out" more in autumn

September and October bring a peak in calls every year. Three factors combine:

1. The communal heating being switched back on. The risers return to permanent circulation, the temperature inside the ducts climbs by 4 to 8 °C, and the development rate of juveniles mechanically accelerates.
2. Windows being closed. Indoor humidity rises, condensation in bathrooms increases, and available water sources multiply.
3. Returns from holiday and house moves. Early autumn is the period of highest residential mobility in France. And the German cockroach travels almost exclusively passively: moving boxes, second-hand appliances, parcels, shopping bags. An oven or a microwave picked up from a private seller is a classic vector — the appliance's thermal insulation makes an ideal shelter.

A simple rule, valid for any second-hand appliance: leave it for 72 hours in an unheated garage and inspect it with a torch before bringing it into your home. Cockroaches flee the cold and reveal themselves when you remove the back panel.

What you can do at your own level — and what you can't

Priority no. 1: cut off the water, not the food

This is the most counter-intuitive point in cockroach control. A German cockroach survives around a month without food, but less than a week without water. "Deep clean" campaigns almost always focus on crumbs; the real lever is moisture.

In practice:

• wipe down the sink and shower tray before going to bed (the population is active at night);
• repair any dripping trap or tap;
• avoid leaving plant saucers full of water;
• empty and dry the pet's water bowl overnight whenever possible;
• hunt down condensation behind the fridge and under the dishwasher — two of the three main hotspots in a kitchen.

Priority no. 2: document before you argue

Before making any request to the managing agent or the landlord, build a file. It changes the speed of response dramatically.

• Dated photos of the insects, with a reference object for scale.
• Sticky traps placed at strategic points: under the sink, behind the fridge, inside the bathroom boxing-in, at pipe entry points. Glue traps for cockroaches with no insecticide, checked every three days and numbered, produce a map of the infestation that is worth more than any argument: they show where the insects are coming from.
• A written log: date, location, number caught. A simple table kept for three weeks establishes a trend.
• A recorded-delivery letter to the managing agent or landlord including these elements.

Priority no. 3: seal the entry points

You can't treat the riser — but you can close your own front door onto it. Cockroaches use:

• pipe penetrations under the sink and behind the toilet,
• access hatches in boxing-in,
• ventilation vents,
• electrical boxes and cable runs.

A sanitary silicone seal applied neatly around pipe penetrations, backed up by finishing collars that conceal the holes, very significantly reduces incoming traffic. For access hatches, adhesive foam strip is enough — without ever blocking access, which must remain openable for the building management.

Air vents must never be sealed shut: fit a fine mesh instead. Fine-mesh stainless steel insect-proof vent grilles let air through while blocking adult insects.

The legal framework: who pays for what

This is the question that stalls half of all cases. Here are the key points under French law.

In rented accommodation. Article 6 of Act no. 89-462 of 6 July 1989 requires the landlord to provide decent housing, free from any infestation of pests and parasites. Since the ELAN Act (2018), this requirement appears explicitly among the decency criteria (amended Decree no. 2002-120). A tenant who reports an infestation that was present when they moved in, or that comes from the common parts, does not have to bear the cost. Conversely, an infestation that was clearly brought in and not reported may be held against them — hence the importance of the inventory of fixtures and of early reporting.

In co-owned buildings. Vertical service ducts, soil stacks, ventilation ducts and rubbish chutes are common parts. Treating them is the responsibility of the owners' association, and therefore of the building's budget. The managing agent can, and according to case law must, arrange the treatments needed to preserve the building's sanitary condition. In the event of prolonged inaction, the local council's Municipal Hygiene and Health Service (SCHS) or the Regional Health Agency can be called upon under the Departmental Sanitary Regulations.

Refusal of access. This is the real operational sticking point. An occupant can refuse access to their home. A riser treatment achieved with 60% access delivers results close to zero in the medium term. That's why any serious operation always begins with an information phase — notices, letters, door-to-door — several weeks before the intervention.

How a treatment that genuinely works is carried out

The current professional protocol

Since indoor pyrethroid spraying was almost universally abandoned — it scatters populations, resistance is well documented, and it exposes occupants needlessly — the benchmark is now bait gel (gel baits based on fipronil, indoxacarb or boric acid depending on the case), authorised and listed by ANSES under European Biocidal Products Regulation no. 528/2012.

The principle: the insect eats the bait, returns to its harbourage, dies, and contaminates its fellows through cannibalism and coprophagy. This is the so-called "cascade" or domino effect, which reaches individuals never directly exposed, including those in inaccessible ducts.

The standard protocol includes:

1. Diagnosis of the entire riser (at minimum the stacked flats, ideally the whole landing), recording water sources and service penetrations.
2. Simultaneous application in every flat on the same day — this is the non-negotiable condition.
3. Specific treatment of the common parts: ducts, rubbish chutes, bin store, boiler room, cellars.
4. Check at 15-21 days with fresh gel applied at any points still active.
5. Second check at 6 weeks, timed to the ootheca hatching cycle — because no gel acts on eggs protected inside the capsule.

A single-visit treatment, with no six-week check, is structurally incomplete.

What gel cannot tolerate

Two mistakes cancel out the treatment's effect:

• Cleaning off the gel spots. The deposits are placed as discreet droplets in corners and on cupboard hinges. Wiping them away removes the bait.
• Using an aerosol alongside it. A repellent contact insecticide drives cockroaches away from the gel zone and scatters them towards neighbouring flats. It's the surest way to turn a localised outbreak into a building-wide infestation. If you want to act while waiting for the intervention, choose professional cockroach gel baits, which are compatible with the protocol, rather than a spray can.

To monitor progress without interfering, a light trap or a simple motion detector adds nothing compared with a well-placed sticky trap. A red-light torch, on the other hand, lets you observe night-time activity without scaring the insects off — cockroaches barely perceive that wavelength.

The calendar to keep in mind

In summary

The German cockroach is a building problem, not a personal cleanliness problem. We regularly work in immaculate homes that are infested because the service duct is. Conversely, a properly run treatment at building scale delivers lasting results, even in badly deteriorated situations.

Three habits, to finish:

• Report early and in writing. An infestation caught at ten individuals is dealt with in one visit; at a thousand, it takes a whole-building campaign.
• Don't improvise with contact products. You'll scatter the population and make the subsequent work harder.
• Insist on a coherent perimeter. A quote that mentions neither the common parts nor a six-week check is only selling you a reprieve.

Our teams carry out riser diagnostics and quotes free of charge, 7 days a week, for private individuals as well as managing agents and landlords. A building is treated once properly, or three times badly.