Mice in offices: treating the problem without closing the premises

In an office building, a mouse infestation is never discovered during a technical inspection. It is discovered at 9:10 am, when someone puts their bag down under their desk and sees a grey shape dart along the skirting board. Within the hour, the photo is circulating on the internal chat, three people refuse to leave their lunch in a drawer, and the facilities management team receives a message that begins with "is this normal".

It is one of the most frequent calls we get from companies based in La Défense, in the 8th arrondissement, in Boulogne, Issy or Saint-Ouen. And it is also one of the worst-started jobs, because the spontaneous reaction — setting four shop-bought traps near the coffee machine and waiting — never reaches the place where the animals actually live. On an office floor, the house mouse (Mus musculus) does not live in the open space. It lives in the technical void that surrounds it.

Brick façade of a residential building with metal fire escapes and numerous windows

Why offices are an ideal habitat for the house mouse

A commercial building is a series of heated voids

From a rodent's point of view, a modern office floor is a temperate Swiss cheese. Three voids account for almost every infestation we treat:

• The raised technical floor. Between 8 and 25 cm of void beneath removable panels, criss-crossed with network cables, extension leads and trunking. Dark, dry, ventilated, never disturbed, with continuous travel routes from one end of the floor to the other. This is harbourage number one.
• The suspended ceiling. Mineral tiles, hangers, air-conditioning ducts, insulation: nesting material and a horizontal motorway above the partitions.
• Vertical ducts and slab penetrations. Service risers, electrical ducts, cable runs between floors. This is how a population from a car park or a basement bin room climbs to the third floor without ever crossing the lobby.

Add an indoor temperature held between 19 and 23 °C all year round, and you get an environment where the house mouse breeds without a season: 5 to 8 litters a year, 4 to 8 young per litter, sexual maturity in six weeks. One female and her first month of offspring are enough to turn an isolated sighting into a whole-floor problem within a quarter.

And a food source nobody identifies as one

The house mouse needs very little: around 3 g of food a day, and it draws most of its water from what it eats. In an office, that means it does not need your kitchen. It feeds on:

• crumbs fallen under keyboards and into raised-floor channels;
• leftovers from lunches eaten at the workstation — the spread of desk dining is, in our view, the leading factor behind the rise in reports in commercial premises;
• tea bags, sugar, biscuits and dried fruit stored in personal drawers;
• waste-paper baskets not emptied daily, and meeting-room bins;
• pet kibble and treats, in "pet friendly" companies.

During an audit, we always ask to open ten drawers at random. We almost invariably find opened food. A floor with 60 workstations is potentially 60 pantries spread over 800 m².

Spotting a mouse infestation before it becomes visible

Seeing a mouse during the working day in an occupied office is not the beginning of the story: it is a sign that the population is already established, or even competing for space. The early indicators are more discreet.

One detail that avoids a lot of pointless debate: fresh droppings are black, soft and shiny; dry and grey ones are several days old. Photographing and dating every indicator means that, three weeks later, you can tell whether the treatment is working. A shared reporting log between facilities management and the contractor is worth more than ten text-message exchanges.

The risk is not merely theoretical. Rodents are recognised reservoirs of pathogens — Santé publique France and the Institut Pasteur document in particular leptospirosis, excreted in the urine of rats and rodents, salmonellosis and hantaviruses — and dried urine is a respiratory allergen documented by the INRS among staff exposed to rodents. In an office, however, the most immediate practical danger remains material damage: electrical incidents caused by cables gnawed inside raised floors are not rare, and a colonised server room becomes an operational risk.

Deteriorated façade of an old building with cracked arcades, overgrown with dry vegetation

The diagnosis: lift the floor panels, not the cupboard doors

A serious office survey is not carried out by walking across the floor plate. It is done on your knees, with a floor panel lifting suction cup, and it takes two to three hours for 1,000 m². What we record:

1. A raised-floor panel lifting plan: surveyed zone by zone, prioritising areas beneath reported workstations, along the façades, around service risers and at the server room entrance.
2. Entry points: unsealed slab penetrations, cable runs, deformed ventilation grilles, the bottom of the delivery entrance door, goods-lift ducts. An adult passes through 6 mm; a juvenile, through less.
3. The link with the basement and common areas. This is the point tenant companies underestimate: in most cases in the Paris region, the source is not in the offices but at the level of the bin room, the car park or a ground-floor retail unit, and the offices are merely feeding territory.
4. An inventory of food sources and the real state of the cleaning contracts (emptying frequency, vacuuming under workstations, cleaning of supply air grilles).
5. Traceability: numbered placement plan, safety data sheets, service report. If your company is audited by its clients or holds certification, this is the file that will be asked for, not the number of traps.

What we do not use in an occupied office

Anticoagulant rodenticide baits have virtually no place on an occupied office floor, and we rule them out by default for three reasons: the animal dies in an inaccessible void — a meeting-room suspended ceiling, a duct — and generates a carcass odour that will last three weeks; the risk of exposing third parties (cleaning staff, pets, children at company events) is poorly controlled; and biocide regulations require reasoned use, as a last resort, under the supervision of a Certibiocide-certified applicator. ANSES regularly points out that these products are reserved for controlled uses, with risk-reduction measures.

In offices, the effective strategy is mechanical and structural: intensive trapping, exclusion, hygiene. Baits, where necessary, remain confined to secured bait stations in basements and plant rooms closed to the public.

The treatment protocol in occupied premises

Week 1 — saturation trapping

The principle is counter-intuitive for a client: you have to set far more traps than there are supposed mice. A classic mistake is installing six devices across 900 m²; we routinely set 40 to 80, most of them under the raised floor, with the panel lifted, alongside the cables, because mice travel in contact with surfaces.

• High-sensitivity snap traps, set perpendicular to the runs, baited with peanut butter or chocolate-hazelnut spread (more attractive than cheese, contrary to legend).
• Enclosed electronic traps in areas accessible to staff: no visible capture, no traumatic scene for an employee, and an indicator light signalling a catch. A mouse electronic trap box placed under a marked panel is also an acceptable solution in a server room, where you want neither powder nor bait.
• Detection pads (fluorescent flour or tracking paper) to map routes before increasing density.
• Checks every 2 to 4 days during the first fortnight. An unchecked trap is a trap out of service.

For companies that want to avoid any killing on site, live-capture traps exist, but they require daily checking and release several kilometres away — which, in practice, is only workable for very small infestations.

Weeks 2 to 4 — exclusion: the only part that solves the problem

Trapping reduces the population; exclusion prevents its replacement. This is the stage that bargain contracts systematically skip.

• Sealing slab penetrations and cable runs with mortar or filled foam; for small openings, fine stainless steel mesh or packed rodent-proof steel wool before sealing, because a mouse gets through polyurethane foam alone in a few nights.
• Fitting brush door strips or metal thresholds on delivery entrances, bin rooms and emergency exits.
• Protecting low-level ventilation grilles with metal mesh.
• Repairing damaged service ducts and soiled insulation — which, depending on their condition, call for removal rather than cleaning.
• Sheathing exposed cable bundles in the identified travel zones.

Weeks 4 to 8 — hygiene, monitoring and sign-off

• A written ban on storing food in drawers, together with a credible alternative: lockable lockers and, for the kitchenette, airtight storage containers for shared supplies (coffee, sugar, biscuits). A rule with no alternative is never followed.
• Daily emptying of bins containing food, vacuuming under workstations at least once a week, cleaning of raised-floor channels in affected areas.
• Cleaning and disinfection of surfaces soiled by droppings and urine: dampen first (never dry sweeping, which puts particles back into the air), detergent-disinfectant, respiratory protection. Beyond a few square metres of soiled plant room, this step calls for a properly equipped contractor.
• Monitoring with non-baited control stations or unset traps, checked monthly and recorded in a register.

A good completion report includes a placement plan, a capture history by point, the list of entry points treated and those still to be dealt with by the landlord or managing agent. This is the document that protects the company in the event of an inspection, an employee complaint or a client audit.

Close-up of a brown mouse with large ears sitting on a concrete floor

Who pays for what: employer, landlord, managing agent

This is the question that stalls most cases for three weeks — during which the population doubles.

The employer has a duty of safety and sanitation

French labour law requires the employer to take the measures necessary to ensure the safety and protect the physical and mental health of workers, and to keep premises in a constant state of cleanliness and sanitation (articles L. 4121-1 and R. 4228-1 et seq.). A rodent infestation in workplace premises falls squarely within this scope: it must be recorded in the single occupational risk assessment document, treated, and brought to the attention of the works council where one exists. The occupational physician and the labour inspectorate may be contacted by staff representatives.

In other words: whatever the origin of the infestation, it is up to the employer to arrange intervention without delay in the premises it occupies. It can then seek redress from whoever is liable.

The landlord and the managing agent

Apportionment depends on the lease and on where the source is located.

• In privately occupied premises (office floors, kitchenette, leased plant rooms): the work is generally borne by the tenant, as routine maintenance, unless the lease states otherwise.
• In common areas (basements, bin rooms, car parks, ducts, lobbies): the work falls to the co-ownership, and therefore the managing agent, and its cost goes into the service charges. It is also for the managing agent to have penetrations in the common areas sealed.
• Where the infestation results from a defect affecting the structure or the building envelope (unsealed ducts, damaged cladding, faulty drainage): the cost falls to the owner or the co-ownership, and the landlord must deliver premises fit for their intended purpose (article 1719 of the Civil Code).
• The departmental sanitary regulations also require owners in the Île-de-France region to keep their buildings free of rodents and insects, and to have the necessary rodent control operations carried out.

Best practice, for a tenant company, is to send a dated letter to the landlord and the managing agent as soon as the first sighting occurs, with photos and the location of the indicators, requesting treatment of the common areas. Without that letter, no claim will stand.

What an office rodent control quote should contain

Be wary of very low annual packages: in reality they cover two inspection visits to basement bait stations, without ever going into the raised floor.

• The area actually treated and a named list of the zones, including the raised floor void and the suspended ceiling.
• The number of devices installed, their type (mechanical, electronic, secured station) and a numbered placement plan.
• The number of check visits during the first fortnight — this is the real marker of a serious contractor.
• Exclusion works: included, priced separately, or excluded. If they are excluded, the quote solves nothing.
• Cleaning and disinfection of soiled areas, and removal of contaminated insulation.
• The operative's qualifications (Certibiocide certificate, service certification where applicable), professional liability insurance.
• Working arrangements: hours outside staff presence, badge access, escorting, server room protocol.
• The expected outcome and the conditions for a free return visit should activity resume.

A 1,000 m² floor with an established infestation is rarely treated in fewer than three to five visits over six to eight weeks, followed by quarterly monitoring. Any promise of resolution in a single visit is sales talk, not technical reality.

In summary

A mouse spotted in an open space is a symptom, not the event. The source is almost always under your feet — in the raised floor — or below your floors, in a bin room or a shared car park. Treatment that works rests on three simultaneous moves: dense mechanical trapping set where the animals actually travel, exclusion work on every 6 mm opening, and food discipline that employees can realistically sustain. Everything else — the four snap traps by the coffee machine, the repellent sachets, the ultrasonic devices — simply costs you the six weeks during which the population renews itself.

For a survey in occupied premises, a costing of exclusion works or the setting up of an annual contract with service reports usable in audits, our teams operate throughout the Île-de-France region.