After a Marne Flood: Rats and a Cellar to Disinfect

"The water only rose fifteen centimetres in the cellar. What really struck us was what we found afterwards: droppings all over the top shelf, and the bag of compost torn open." This account from a homeowner in Saint-Maur-des-Fossés, in the Marne's loop, sums up the phenomenon well. A flood doesn't just soak a cellar: it displaces rats. When burrows and sewer networks fill up, brown rats (Rattus norvegicus) abandon the riverbanks and the lower parts of the sewers, and they climb. Towards crawl spaces, cellars, garages and ground-floor kitchens.

Val-de-Marne is particularly affected: the Seine–Marne confluence, the loops at Saint-Maur, Champigny-sur-Marne, Joinville-le-Pont, Nogent-sur-Marne and Créteil, and the low-lying neighbourhoods of Villeneuve-Saint-Georges and Alfortville. The department is among the areas of Île-de-France most exposed to the risk of flooding by river overflow, and every significant episode on the Marne — even a modest rise, with no evacuation — translates into calls about rodents in the following weeks.

This guide sets out what to do, in order: protect yourself (leptospirosis is the real health risk), empty and clean, disinfect, then seal off access points before the next time the water rises.

Why a flood pushes rats into homes

The brown rat is a burrowing rodent and an excellent swimmer, but it does not live underwater. Its burrows, dug into riverbank slopes, beneath slabs and along sewer networks, drown within a few hours when the water level rises.

Three mechanisms compound each other:

1. The drowning of riverbank burrows. A brown rat burrow generally develops within the top 50 centimetres of soil. A rise of a few tens of centimetres in the Marne is enough to flood an entire network of tunnels along the quays and islands.
2. The surcharging of the sewer network. When the main collectors fill up, the rats living in the upper sections of the pipes look for a way out: inspection chambers, floor drains, the wastewater pipes of individual houses. It's the same route described in our guide on rats coming up through the toilet.
3. The redistribution of territory. Evicted colonies don't disappear: they resettle higher up, often within a radius of 100 to 200 metres. A house with an accessible cellar, a woodpile, a compost bin or a chicken coop instantly becomes a credible option.

One more thing to bear in mind: the water receding settles nothing. Rats established in a dry, quiet cellar have no reason to go back to a sodden riverbank. The risk window runs from the flood itself until 6 to 8 weeks after the water has receded.

The real health hazard: leptospirosis

This is the point on which we compromise least. Leptospirosis is a bacterial disease caused by Leptospira, excreted in rodent urine — the rat is its main reservoir in mainland France. The bacterium survives for several weeks in standing fresh water, mud or damp soil.

According to Santé publique France and the Centre national de référence de la leptospirose (Institut Pasteur), mainland France records on the order of 600 to 900 reported cases a year, with an incidence that has risen markedly since the 2010s. Freshwater activities and contact with floodwater are recognised circumstances of exposure. The INRS classifies it among the occupational biological risks faced by sewer workers and sanitation staff — and a flooded cellar reproduces, in miniature, the conditions of their workplace.

Infection occurs through:

• a wound, however small (a cut, a scrape, cracked skin on the hands) in contact with water or mud;
• the mucous membranes (eyes, mouth, nose) via splashes;
• skin left immersed and macerated for a long time.

The first signs appear 4 to 14 days after contact: sudden fever, severe headaches, muscle pain, sometimes red eyes. It is often mistaken for flu or gastroenteritis. The message to remember is simple:

Any unexplained fever within the two weeks following the clean-up of a flooded cellar should be reported to a doctor, specifying the exposure to floodwater. Early antibiotic treatment changes the prognosis; a severe form can affect the kidneys, liver and lungs.

Two further risks should be added, less talked about but real after a flood: salmonellosis from contact with rodent droppings, and hantavirus carried by the wood mouse, particularly in wooded peri-urban areas.

Protecting yourself before going down to the cellar

Before opening the door, gear up. This isn't excessive: it is exactly what sanitation workers are advised to do.

• Waterproof safety boots that come up high, never trainers or clogs.
• Waterproof protective gloves with long cuffs, thick nitrile or rubber type, not fabric gardening gloves.
• Splash-proof safety goggles, essential as soon as you use a water jet: it's the jet that aerosolises the mud and sprays it onto the mucous membranes.
• An FFP2 or FFP3 mask if the cellar is enclosed, dusty or mouldy after drying.
• Waterproof dressings over any wound, even a two-millimetre cut.

After the work: wash your hands and forearms with soap, rinse your boots outside, and never eat, drink or smoke while working.

The restoration protocol, step by step

Cleaning before the water has gone, or disinfecting a surface still covered in mud, is wasted effort. The order matters.

1. Pump out and drain the remaining water

As soon as the outside level has dropped below that of the cellar, drain it. Beyond a few centimetres, a submersible cellar drainage pump with a discharge hose is more effective than a wet vacuum. Watch out for the point most often overlooked: switch off the electricity for the area at the consumer unit before doing anything, and only switch it back on once you have confirmed the installation is dry.

2. Clear out, sort, discard

Anything porous that has been in contact with floodwater goes: cardboard, chipboard panels, soiled insulation, mattresses, bags of pet kibble, compost, dented tins, bottles whose caps were submerged. Rodents feed and hide in that clutter: readers will find this angle covered in detail in our article on cardboard boxes and storage bins as a vector for pests.

Non-porous items (metal, glass, hard plastic) can be salvaged after washing and disinfection.

3. Clean mechanically

Scrape the mud away with a squeegee and a shovel, remove it in sealed bags, then wash the floors and the first 50 centimetres of the walls with water and detergent, using a brush. No pressure washer in an enclosed space: misting contaminated organic matter into the air is precisely what you want to avoid. If you use one outdoors, wear goggles and a mask.

4. Disinfect

Disinfection only works on a clean surface: organic matter inactivates disinfectants. Once the floor has been rinsed:

• diluted bleach (roughly one 250 ml sachet of 9.6% bleach in 10 litres of water, giving a solution of about 0.5% active chlorine), applied, left to work for 10 to 15 minutes, then rinsed off;
• or a ready-to-use virucidal disinfectant meeting the EN 14476 standard for delicate surfaces;
• maximum ventilation during and afterwards, and never mix bleach with acidic or ammonia-based products (release of chlorine gas).

5. Dry for a long time

The basement of a house takes weeks to dry out. An electric dehumidifier rated for 50 to 80 m², run continuously with connecting doors closed and the tank emptied every day, speeds things up considerably. A basement that stays damp then attracts woodlice, silverfish and house centipedes — a subject covered in our article on damp-loving basement pests.

Checking whether rats have stayed — and taking action

A week after the water has receded, inspect rather than assume. The signs, in order of reliability:

To confirm, the simplest trick remains the tracking patch: a thin layer of flour in front of a suspected route, checked the following morning.

If activity is confirmed, two rules are worth more than a bag of bait blocks scattered at random:

1. Remove the food source and the shelter first (see below), otherwise the colony rebuilds itself within a few weeks.
2. If you set traps, use protected mechanical traps. Rat traps in a secured housing remain the only sensible option in a house with children, a cat or a dog, and around a garden where hedgehogs and birds move about.

As for rodenticides, let's recall the framework: anticoagulants sold to private individuals are regulated biocidal products, to be used only in a secured bait station, never loose, never outdoors without protection, and for a limited period. ANSES and the European Biocidal Products Regulation strictly govern these substances, several of which are classified as reprotoxic. After a flood, the problem is compounded: soaked bait no longer works, and a poisoned rodent that dies in an inaccessible crawl space creates another problem of odour and carrion insects.

Sealing access points before the next flood

This is the work that pays off most, and the only kind that lasts. A brown rat gets through a 20 mm opening, a mouse through 6 mm.

• Inspection chambers and floor drains: replace a plain grating with a screwed-down, fine-mesh grating, and fit a floor drain with a non-return flap. In low-lying areas, a backflow valve on the wastewater outlet is fully justified — it's also what prevents sewage backing up into the house.
• Cellar vents and air bricks: fit a screwed-down stainless steel rodent-proof mesh with a 6 mm mesh, not a plastic insect screen.
• Conduit and pipe penetrations: seal with mortar or tightly packed steel wool behind it, never with expanding foam alone, which rodents chew through in a single night.
• The bottom of garage doors: a metal threshold strip or sill, checked after every flood (rubber strips swell and come unstuck).
• Storage: raise everything onto metal shelving 20 cm off the floor, replace cardboard boxes with rigid plastic airtight storage bins, and never store seeds, pet kibble or compost directly on a cellar floor.

Outside, apply the same logic as in the garden: no woodpile against the wall, a compost bin with a mesh base, no bird feeder at ground level. The details are in our garden plan on rats at the compost bin and chicken coop.

Who to notify, and who pays for what

After a flood, three contacts are useful:

• Your town hall's municipal hygiene and health service (SCHS) or public health department: it handles reports of infestation on public land, municipal riverbanks and street inspection chambers, and often launches targeted rat control campaigns after a flooding episode. Reporting is worthwhile even if nothing happens immediately: that's what builds up the mapping of the neighbourhood.
• The sewer network operator: departmental or inter-municipal depending on the collector. It is the body that works inside the sewers, where no private individual should ever go down.
• Your home insurer: flooding by river overflow falls under the natural disaster scheme once the interministerial order is published in the Journal officiel, with a claim to be filed quickly (typically 10 days after publication of the order; check your policy). Note: rodent damage follows a very different logic, set out in detail in our article on what home insurance covers in the event of rodent damage.

Document everything: dated photos before cleaning, the water height marked in pencil on the wall, a list of the items discarded, invoices for pumping and disinfection.

When to call a professional

Cleaning a 15 m² cellar with five centimetres of water and no sign of rodents is within reach of a well-equipped homeowner. Beyond that, the decision is straightforward. Bring in a company if:

• you find signs of rodents inside the living area (kitchen, pantry, service ducts);
• the water stood for more than 48 hours, or reached insulation and partition walls;
• you discover active burrows against the foundations or under a slab;
• there is wastewater in the mix (sewage backflow): disinfection must then be thorough and documented;
• an occupant is immunocompromised or pregnant, or there are young children and pets.

A full rat control intervention on a house, including a survey, the installation of secured bait stations and follow-up visits, generally costs between €250 and €600; disinfecting a basement depends on the surface area and the degree of soiling. You'll find our benchmarks on the pricing page, and details of our services on the services page. To find out whether your town is covered, the service areas page lists the Val-de-Marne towns we travel to.

Just come through a flood and can hear scurrying above the suspended ceiling, or found droppings on the top shelf? Don't wait for the colony to move from the basement to the ground floor: describe your situation on our contact page for an assessment, or use the emergency page if a rodent is already in the living part of your home.