Rats Under the House: How to Spot a Broken Sewer Pipe

"We trapped eleven rats in three weeks in the pantry. We blocked every hole, put mesh over the air vent, got rid of the bird seed. And after ten days, it started all over again." This homeowner in Saint-Maur-des-Fossés eventually worked out where the flow was coming from: a stoneware wastewater pipe, cracked a metre below the pantry slab, which was acting as a permanent access corridor from the street sewer.

It's the most frustrating scenario in rat control in a detached house: everything visible has been dealt with, and the rats come back anyway. Because the entry point isn't in the wall. It's under the house, invisible, and no amount of bait placed on the ground floor will ever close it.

Earlier articles on this blog have covered the rat that comes up through the toilet bowl and rats arriving from a neighbouring plot. This one tackles a different, more structural issue: how to tell whether your private drainage system has become a front door, which tests can prove it, and who pays for what when the defect sits right on the property boundary.

Why the sewer network is a rat's favourite motorway

The brown rat (Rattus norvegicus), the species you'll meet in Val-de-Marne, is a burrowing, semi-aquatic animal. It swims effortlessly, holds its breath for several dozen seconds, climbs vertical pipes by bracing against the walls, and thrives in the constant damp of a sewer. The network gives it everything: relatively stable warmth in winter, darkness, a continuous supply of organic food, and a grid that connects every house to all the others without ever having to venture into the open.

As long as the network is sealed, that's no problem: the rat moves around in the sewer and doesn't come out into your home. Trouble starts when a break in continuity appears between the pipe and the ground beneath the house.

The classic defects, in the order of frequency observed in Île-de-France housing stock built between the 1920s and 1970s:

• Cracked stoneware or asbestos-cement pipes: mortar joints crumble, socket connections shift by a few millimetres, and a 4 cm gap is all it takes (an adult brown rat can squeeze through an opening the size of a 2-euro coin thanks to its mobile clavicle).
• Abandoned connection left unplugged: a decommissioned septic tank, the old base of a soil stack from a demolished extension, a relocated washing machine outlet. The pipe was cut, not capped. It opens straight into the soil, under the slab.
• A dried-out yard or garage floor trap: a trap whose water has evaporated (a rarely used yard, a dry garage, a hot summer) is nothing more than an open hole into the network. It's the simplest and most underestimated cause of all.
• An inspection chamber with a cracked or unsealed cover: the rat doesn't even need to gnaw.
• A pipe passing through a slab with no sleeve or sealing mortar: the ring of space around the pipe acts as a cat flap.
• Roots that have deformed the pipe: common under gardens planted with prunus, maples or lime trees.

One detail helps explain the persistence: a rat that finds a route marks it with urine and plantar secretions. Others follow the scent trail. As long as the structural defect stays open, you aren't treating an infestation — you're emptying a funnel that keeps refilling.

The clues that point to an underground defect

None of these signs is proof on its own. Three or four together make the hypothesis very likely.

Signs inside the house

• Rats always show up at the lowest level: pantry, cellar, crawl space, utility room, basement garage, under-stairs cupboard with a pipe running through it.
• You can't find any exterior entry point after a thorough inspection of the base of the walls, air bricks, door thresholds and service duct penetrations.
• Noises in the walls where the wastewater stack runs down, often at night, with no scratching in the loft.
• An intermittent sewer smell in a room: it signals a broken water seal or a leaking pipe, and therefore an air connection between your room and the network. Wherever air gets through, so does the rat.
• Droppings on top of a pipe or along a skirting board following the run of a pipe: rats travel alongside pipework, which acts as rails for them.
• A toilet bowl that bubbles or abnormally slow drainage after a storm — a sign of a saturated or partially collapsed network.

Signs outside

• Localised subsidence of paving, terrace or lawn over one or two square metres, often directly above the pipe run: soil washes into the cracked pipe, a void forms, and the rat digs its burrow there.
• Burrow holes 5 to 8 cm across, smooth, with no cone of excavated soil (unlike a mole), near an inspection chamber or along a boundary wall.
• Trails of soil inside an inspection chamber, visible at the bottom when you lift the cover: that soil is coming from somewhere.
• Paw and tail marks in the silt deposits at the bottom of the chamber.

For an initial inspection you don't need specialist kit: a powerful rechargeable torch, thick handling gloves and a telescopic inspection mirror are enough to see the bottom of a chamber and the incoming pipes. What is absolutely off limits is climbing down into a deep chamber: there's a risk of oxygen depletion and hydrogen sulphide — that's a job for properly equipped professionals.

The two tests that settle it

The smoke test

This is the benchmark check: quick and inexpensive. A cold-smoke generator producing non-toxic, non-staining smoke is injected into the network from an inspection chamber or a rodding eye. The smoke takes every path air can take — and therefore every path the rat can take. You'll see it coming out:

• at the base of a façade wall → pipe broken alongside the wall;
• through a crack in the garage floor → pipe broken under the slab;
• in the crawl space through a vent → open abandoned connection;
• through a shower waste or a yard trap → dried-out water seal (a minor defect, to be fixed immediately);
• in the garden, in the middle of the lawn → an outright leak on the private shared run.

The smoke test is not a rodent control method. It's an air-tightness diagnostic, and its value lies in requiring no excavation at all. Expect roughly €200 to €400 for a single-family house, depending on the length of the network. Warn your immediate neighbours: smoke may emerge on their property if the network is shared — which is, incidentally, valuable information in itself.

CCTV inspection (camera survey)

A self-propelled or push-rod camera travels along the pipe and produces a geo-referenced report: material type, gradient, displaced joints, cracks, reverse falls, roots, capped-off connections. This is the document that allows you to price a repair and identify who is liable. Indicative budget: €250 to €600 for a detached house, more if jetting is needed first (a silted-up pipe can't be filmed).

Always ask for:

• the time-stamped video report with distance measurements,
• a drawing of the pipe run with the defects marked,
• the position of the property boundary on that drawing: that's what determines who pays.

What actually solves the problem

1. Refill and secure every water seal

Free, immediate, and it resolves a share of cases. Pour a litre of water into every rarely used floor trap (yard, garage, pantry, utility room, boiler room), and do it again once a month. For traps that dry out systematically, there are anti-odour trap valves with a flexible membrane that lets water through downwards and closes in the other direction. For floor drains that are no longer used at all, a screw-in plug is safer.

2. Cap off abandoned connections

A cut pipe is sealed with a PVC blanking plug, solvent-welded or with a mechanical seal — not with a rag or a loose plate. Rats gnaw through wood, thin PVC, plaster and expanding foam without difficulty. Durable repairs combine sealing mortar with fine-mesh stainless steel mesh embedded in it.

3. Fit a rat flap in the right place

A mechanical non-return flap (a hinged stainless steel flap, sometimes double-flapped) is installed in a chamber, on the outgoing pipe, downstream of all the house connections. It lets wastewater out and prevents rats coming back up. Three precautions:

• it's useless if the leak is upstream of the flap: the rat will come in through the crack, not through the flap;
• it needs an annual check (wipes and grease jam the flap in the open position);
• in flood-prone areas along the Marne, choose a model designed for a surcharged network, and coordinate it with your cellar protection measures.

4. Repair the pipe

Two families of solutions, depending on the camera report:

Lining is often the best compromise under a terrace or a slab, where digging a trench would cost more than the pipe itself.

5. Deal with the population, in the right order

Trapping and baiting only make sense alongside the repair, to remove the individuals already established. The reverse — baiting without repairing — is a subscription. In homes, anticoagulant rodenticides are tightly regulated: private individuals may only use products authorised for the general public, they must be placed in a secure bait station inaccessible to children and animals, and permanent preventive baiting is prohibited. ANSES regularly highlights the risk of secondary poisoning for cats, dogs, hedgehogs and birds of prey. Where there's a dog or children in the house, a mechanical trap inside a protective box remains the soundest choice.

Who pays for what in Val-de-Marne

This is the question that stalls the most cases. The basic rule:

• From the house up to the property boundary: the network is private. Maintenance and repair are the owner's responsibility. Statistically, this is also where most of the defects that let rats in are located.
• Under public land, up to the main sewer: the public section of the connection falls to the public sanitation service. In Val-de-Marne, responsibility is shared between the municipalities, the territorial authorities and the département for the main trunk sewers; your water bill and your municipality's sanitation service regulations will tell you who to contact.
• The street sewer itself: its maintenance and rodent control are the responsibility of the network operator. To report a heavy rat presence in a street chamber, contact your town hall's public health or sanitation department.

In practice, three useful habits:

1. Ask the town hall for the sanitation service regulations and whether a connection compliance check exists: many municipalities in the 94 département carry these out when a property is sold.
2. Have the camera report state the exact distance of the defect relative to the property boundary. Without that measurement, no public liability claim will be processed.
3. Report in writing (email or letter with photos and the report): a verbal report leaves no usable record.

On the compensation side, home insurance almost never covers the repair of a worn-out pipe — that counts as lack of maintenance — but it may cover some consequential damage (water damage, deterioration of insulation or wiring). The details of the cover and how to build your claim are covered in our article on rodent damage and home insurance. If the defect originates on a neighbouring plot or a derelict site, the legal options are those described in the article on rats coming from the street or a neighbouring plot.

What you must absolutely avoid

• Pouring concrete into a pipe to "block the rat": you destroy the network and create a blockage that will back up into the house.
• Tipping rat poison into a chamber or a pipe: instant dilution, environmental pollution, zero effectiveness — and it's a non-compliant use of the product.
• Relying on ultrasonic devices: no independent study demonstrates lasting effectiveness against the brown rat, which habituates within days.
• Screening a trap with thin galvanised mesh: it rusts within months in wastewater. Stainless steel is the only sensible material in a damp environment.
• Treating without checking the crawl space: very often that's exactly where the pipe defect links up with the house. Our guide to crawl spaces and service ducts explains how to inspect them.

Smart timing: October–November

Late autumn is the best window for this kind of diagnostic, for three concrete reasons:

1. Colonisation pressure is at its peak. Rats are looking for a sheltered, temperate spot, so the clues are more numerous and easier to read than in July.
2. Vegetation is low: burrows along walls, chambers and fences are visible, which isn't the case in the height of the growing season.
3. Before the winter floods. A surcharged network during a rise of the Marne backs up and finishes off an already cracked pipe. Repairing beforehand means avoiding rats and water damage at the same time; the subject is covered in detail in our article on the cellar after a flood of the Marne.

Take the opportunity to keep a home maintenance logbook: dates of jetting work, camera report, photos of chambers, records of refilled traps. It's exactly what a buyer, an insurer or the sanitation service will ask for.

In summary

When a rat infestation resists a properly conducted treatment in a detached house, it's time to stop looking for a hole in a wall and start looking for a defect under the slab. The logical order is always the same:

1. refill and secure every water seal;
2. inspect the chambers, look for subsidence and burrows;
3. run a smoke test, then a camera survey if smoke comes out where it shouldn't;
4. cap, line or replace, and fit a flap in the right place;
5. remove the remaining rats — never before.

Are you in Champigny-sur-Marne, Créteil, Vitry-sur-Seine, Nogent or a nearby town, and the rats keep coming back despite your efforts? We operate across the whole département: see our service areas and our rat control services, or request a diagnostic via the contact page. If a rat is already inside the house, the emergency page is the fastest way to reach us.