Introduction: the pest that benefits from your comfort
There is a coincidence that pest control technicians know by heart but that almost no resident ever makes: calls about cockroaches in apartment buildings take off three to six weeks after the communal heating is switched back on. In France, this usually happens between 15 October and 15 November, depending on building regulations and the temperature thresholds chosen by managing agents. By December, callout requests for the German cockroach reach one of their two annual peaks — the other being high summer.
The reason is anything but mysterious. The German cockroach (Blattella germanica), which accounts for the vast majority of infestations in French apartment buildings, is a strictly thermophilic insect: its development speeds up markedly above 25 °C and effectively stops below 15 °C. And the service ducts of a heated building — those vertical shafts carrying hot water, cold water, waste pipes and sometimes cabling — offer exactly what it is looking for in winter: constant warmth, humidity, total darkness and an unbroken network running from the basement to the top floor.
This article is not about a miracle chemical treatment. It describes a specific place — the service duct and its surroundings — and a specific moment: the weeks that follow the boiler being fired up. The aim is to understand how cockroaches move around a building, to spot the points where they get into your home, and to know what falls to your flat and what falls to the building's management.
What warmth actually changes
The German cockroach's cycle speeds up
The benchmark entomological figures are consistent: at 30 °C, a female German cockroach completes the egg → adult cycle in 35 to 45 days. At 20 °C, the same cycle takes 90 to 120 days. Each egg case ("ootheca") holds 30 to 40 eggs, and a female produces four to eight of them over her lifetime.
The arithmetic consequence is brutal: in a service duct kept at 28–30 °C by a poorly insulated domestic hot water riser, a handful of individuals surviving in October becomes a population of several hundred by January. In an unheated building, that same handful would have stagnated.
The duct becomes a motorway
A typical post-war building has one to three vertical service ducts per stairwell. They are closed off by access panels on the bathroom or kitchen wall, and pierced at every floor where the pipes pass through. Those openings are rarely sealed properly: a 3 cm gap around a PVC pipe is enough for an adult to squeeze through, and a 1.5 mm crack is enough for a young nymph.
The result: your neighbour on the fourth floor can feed your second-floor kitchen without anyone crossing a landing. Technicians talk about an infested riser rather than an infested flat — and that is precisely why treating a single home almost always fails.
Key point: in an apartment building, the German cockroach hardly ever travels along corridors. It moves through the networks — ducts, electrical conduits, waste pipes, suspended ceiling voids, refuse chutes.
Other pests follow the same route
The warmth of the ducts also benefits:
- ants (notably Lasius and the pharaoh ant, Monomorium pharaonis, which readily nests in pipe insulation);
- silverfish and other lepismatids, which climb up damp ducts;
- house mice, which use the same vertical openings and skirting board voids;
- less often bed bugs, which prefer partition walls and electrical sockets but can travel through a duct shared between two adjoining bedrooms.
Spotting the entry points in your home: a 30-minute inspection
Do it now, before the heating is running at full tilt. Equip yourself with a torch for raking-light inspection — it is oblique light, not head-on light, that reveals droppings and shed skins — and a pocket mirror.
1. The service duct access panel
Usually in the bathroom or behind the toilet, sometimes in a kitchen cupboard. Open it (it is often simply screwed or clipped on).
What you are looking for:
| Sign | What it means |
|---|---|
| Black specks 0.5 mm across, like ground pepper | Cockroach droppings — recent activity |
| Translucent amber shed skins | A growing population, not just passing traffic |
| A sweetish, slightly rancid smell | Established infestation (aggregation pheromone) |
| Brown bean-shaped capsules 8 mm long | Egg cases: breeding on site |
| Warm pipes + damaged insulation | The favourable microclimate is already in place |
2. The base of the pipes passing through the duct's ceiling and floor
That is where the openings are. Run the beam along the surface of the slab: the gaps show up in contre-jour.
3. The area around the duct on the flat's side
- Underneath and behind the sink unit (trap/wall junction);
- skirting board behind the toilet;
- where the washing machine hose passes through;
- electrical boxes and switches on the partition running alongside the duct.
4. The warm spots in the kitchen
The German cockroach is after warmth + humidity + food. All three come together:
- under and behind the fridge (condenser grille: the number one spot);
- under the dishwasher;
- inside the built-in oven housing;
- in the hinges and drawer runners near the hob.
Making a diagnosis before treating
Monitoring traps, a non-negotiable step
Before buying anything active, you need to know where the problem is and how intense it is. Sticky monitoring traps laid flat and numbered with a marker give you a map of the flat within a week: one per duct, one behind the fridge, one under the sink, one under the dishwasher, one in the bathroom.
A simple reading, drawn from the integrated pest management protocols used in social housing:
| Catches / trap / week | Interpretation |
|---|---|
| 0 | No detectable activity — keep monitoring |
| 1 to 5 | Early establishment or pressure from a neighbour |
| 6 to 20 | Established population, treatment required |
| > 20, with nymphs | Heavy infestation: professional + collective action |
The presence of nymphs (small dark individuals with two pale stripes) is the decisive marker: it proves breeding on site, not simply passing traffic.
Cockroach or not?
Three common mix-ups:
- German cockroach: 12–15 mm, pale beige, two dark lengthwise stripes on the pronotum, does not really fly, lives indoors all year round. This is the one in 80 to 90 % of flat infestations.
- Oriental cockroach (Blatta orientalis): 20–25 mm, very dark brown, slow-moving, associated with cellars, basements, bin rooms and sewer networks. It comes up through the waste pipes.
- American cockroach (Periplaneta americana): 30–40 mm, mahogany red, capable of gliding flight, mainly found in warm basements, commercial kitchens and district heating networks.
Telling them apart is not a detail: the Oriental cockroach is fought mainly by sealing waste pipes and the bin room, the German one by using bait gel inside the home.
