Cockroaches in a London flat: German or Oriental, and why gel bait beats spray

Cockroaches in a flat in London are nearly always one of two species: the German cockroach, a light yellow-brown insect of 10 to 15 mm with two dark stripes behind its head, or the Oriental cockroach, a shiny black or dark brown insect of 20 to 24 mm. German cockroaches live in warm kitchens and bathrooms, Orientals in drains, basements and bin stores. Both are cleared with gel bait placed where they hide and followed up over two or three visits, not with a spray that kills only the few it touches.

Which cockroach is in my flat: German or Oriental?

Look at the size and colour: a German cockroach is small, pale and striped, an Oriental is large and almost black. The British Pest Control Association's advice sheet names these two as the most common UK pest cockroaches.

The German cockroach likes heat and humidity, which earned it the nickname "steam fly". It lives behind appliances with warm motors, such as ovens and fridges, and in the boxed-in pipework of kitchens and bathrooms.

The Oriental cockroach prefers cool, damp places: drains, sewer pipes, basements, bin stores and the ground floor of a block. It takes six to twelve months to mature against six to twelve weeks for a German, so an Oriental problem is usually a drainage or bin-store problem rather than a kitchen one.

Why do heated London blocks spread cockroaches between flats?

Because the service ducts, risers and floor voids of a centrally heated block are warm, damp and connected to every flat. A Building Research Establishment survey of local authorities in England and Wales found that every metropolitan authority that replied reported cockroaches, that tower blocks and low-rise flats were the only dwelling types with infestations rated "very serious", and that 81 per cent of authorities linked infestations to ducted services.

The same paper describes a 19-storey block where service cupboards ran the full height of the building and gaps around the pipes let cockroaches move from floor to floor. The BPCA puts it in one line: both species are often found around the heating ducts and boiler rooms of large, centrally heated buildings. That is why the worst London problems are in purpose-built blocks and estates, in Westminster or Wandsworth alike, and why they come back if only one flat is treated.

Autumn makes it worse: the BPCA ties cockroach activity to heating ducts and pipes as the heating comes on, so late October is when many Londoners first see them.

What are the signs of a cockroach infestation?

The reliable signs are droppings, egg cases, shed skins and, in a heavy infestation, a smell. Droppings look like coarse ground pepper along the backs of cupboards and behind the cooker; shed skins collect in the same places, because the nymphs moult five or six times before adulthood.

Cockroaches are nocturnal. Daytime sightings mean a heavy infestation, in the BPCA's view. A German egg case holds around 30 nymphs on average and hatches about 28 days after it is formed, so a few adults in September can be a great many by November.

Are cockroaches a health risk in a flat?

Yes: they carry the organisms that cause food poisoning and leave them on food, utensils and worktops. The BPCA lists salmonella, staphylococcus and streptococcus among the bacteria cockroaches carry between drains and kitchens, deposited as droppings and regurgitated food. The BRE paper adds that allergy to cockroaches is among the most common allergies in people with asthma, and its sources found cockroach allergens in house dust where no insects had been seen.

For a food business the stakes are higher. The BPCA says Environmental Health Officers can issue enforcement notices to business owners without adequate pest management procedures, with fines and, for repeat offenders, legal action. A cafe on the ground floor of an infested block needs a proactive contract, not a one-off spray; see our restaurant pest control page.

Why does gel bait work better than spraying cockroaches?

Because bait is eaten and carried back to the harbourage, while a spray kills only the cockroaches it lands on. The BPCA's current guidance is that most treatments now rely on gel or powder baits, with residual sprays "less used following the introduction of baits" and kept for the edges of an infestation, and dust for the places a gel cannot reach, such as ducting.

A supermarket spray does nothing to the egg cases either, and German nymphs feed on adult droppings and shed skins inside the harbourage, which keeps them off treated surfaces. The BPCA says the insecticide must have "sufficient persistence to kill the nymphs hatching later from unhatched egg capsules": a professional product placed where the egg cases are, applied to label, not a mist across the floor.

Gel has one known weakness. Some German cockroach strains in the United States learned to refuse the sugars in older bait recipes; the fix was a new bait base and active ingredient, so a good technician rotates formulations. The professional range is also changing: one widely used cockroach gel is being withdrawn from the GB market, with a last date of use of 30 September 2027, because its active ingredient is under regulatory review.

How many visits does cockroach treatment take?

Usually two or three, spaced to catch the nymphs that hatch after the first visit. A German egg case takes around 28 days to hatch and the nymphs six to twelve weeks to mature, so a single visit cannot deal with insects still inside their egg cases on the day. The London councils that still treat cockroaches sell courses of two or three visits for the same reason.

Our own approach is a survey, then gel in the cracks, cupboard edges, appliance voids and service routes, dust in any ducts, and monitors to confirm numbers are falling before we sign off. Every treatment carries a written six-month guarantee, subject to the site survey; the method is on our cockroach control page.

What should I do before and after treatment?

Clean the kitchen thoroughly before the visit, then leave the bait alone afterwards. The BPCA advises a thorough clean, mending broken tiles and sealing cracks with plaster or mastic, because any gap a quarter of a cockroach's body size is a harbourage. After the visit:

• Do not wipe, wash or hoover away the gel spots; they are the treatment.
• Fix drips and dry the sink overnight. Cockroaches can go months without food but only a few days without water.
• Keep food in sealed containers and take food waste out daily.
• Tell the managing agent or housing officer, with dates, so neighbouring flats and the bin store can be checked. Lewisham's service can put a whole block on one programme.

If you rent, a problem that was there before you moved in or that comes through the structure is the landlord's; our landlord guide sets out who pays.

Council or private treatment for cockroaches in a flat?

The council option is patchy in our part of London, and where it exists it is a paid course. Lewisham charges £186 for up to three visits from 1 April 2026, with a £93 concession; Hammersmith & Fulham £208 for three visits, though its own council tenants do not pay; Wandsworth £214 for three treatments from 1 April 2026. Westminster treats its tenants and leaseholders but, as of May 2026, is not taking on private residents or businesses, and Islington says it does not offer a pest control service at all.

A private firm can usually come sooner, often same day, and treat the drains or bin store as well as the flat. Whichever you choose, ask for a course of visits, bait rather than spray, and a written record for the managing agent.

What to do next

If you have seen a cockroach in your flat, especially in daylight, book a free inspection survey and we will tell you which species it is, where it is coming from and what the treatment involves. We work Monday to Saturday, 8am to 7pm, often same day, with RSPH-qualified, DBS-checked technicians. Call 020 3411 2329 or read more on our cockroach control page.