Are Rats Damaging Your Drains Without You Knowing It? Understanding Hidden Risks and Prevention Strategies

Scratching behind the sink cabinet, a musky smell rising from the floor drain, a toilet that gurgles for no reason: these are not always simple plumbing problems. Rats in drains are more common than most people think, especially in neighbourhoods with ageing pipes. Brown rats travel through sewers, climb up drain lines and use the smallest crack to get into the house, gnawing on pipes along the way. Here is how that link forms, the signs to watch for, the risks to your health and your building, and the measures that actually keep rats out of your plumbing.

Why rats and drains go together

The brown rat, or Norway rat, the dominant species in Montreal, forages in sewers and garbage, digs burrows at ground level along foundations and needs to drink every day. Sewer systems give it exactly what it wants: warmth, moisture, water and plenty of organic waste. It is also a strong swimmer, able to climb a vertical pipe.

To a rat, a pipe is not an obstacle but a hallway. In older homes, clay or cast-iron pipes crack, separate at the joints or corrode. The rat widens those gaps with its teeth and ends up under the floor, inside a wall or, more rarely, in the toilet bowl. All it needs is an opening of more than 12 mm, about half an inch.

The damage rats do to your pipes

Gnawing

A rat’s incisors never stop growing, so it has to gnaw constantly to wear them down. PVC and plastic pipes, pipe insulation and even some softer metals pay the price. A small hole in a drain pipe lets sewer gas escape and gives the rat access to walls and floors.

Blockages

Rats drag nesting material into drain lines: shredded insulation, plastic, paper, leaves. The debris builds up into partial blockages that slow drainage, cause backups or upset the air pressure in the system.

Soil erosion and leaks

When a cracked pipe leaks under the house, the rat digs around it, the water washes away the loosened soil and voids form. Over time this can saturate the ground, shift a slab or weaken a foundation. The damage is often written off as a “collapsed pipe” without anyone asking why it collapsed.

The drain problems that follow

• Breaks and cracks: water infiltration and leaks into the structure;
• Blockages: backups, slow drains, bad smells;
• Leaks: structural damage to the house or building, with costly repairs if left too long.

Signs of rats in your drains

Noises in pipes and walls

Rats are mostly active at night. Listen near drains, vents and the sink cabinet: rhythmic, repetitive scratching or something brushing against a pipe, often in the same spot night after night, is not the plumbing expanding.

Visible traces

• Greasy rub marks around pipe openings where they pass through a wall or floor: the rat’s fur leaves a dark smear from passing the same spot over and over.
• Droppings shaped like capsules, about the size of a raisin, often grouped near floor drains, under sinks or in the utility room. Fresh ones are dark and moist; old ones are grey and crumbly.
• Gnaw marks on plastic fittings, pipe insulation or cabinets near the plumbing.

Not sure whether it is a rat or a mouse? See our guide on how to tell the difference between a rat and a mouse.

A lingering smell

A musky, sour, slightly ammonia-like odour, different from the usual sewer smell, rising from a floor drain, a sink or the base of the toilet. It often gets stronger after rain or after you run water, when the flow disturbs urine, droppings or nesting material.

Plumbing that acts strangely

A bubbling toilet, a slow sink, gurgling when no water is running, a drop in water pressure, unexplained dampness on a basement wall: these symptoms are usually blamed on venting or a clog, but a nest or a gnawed pipe may be the cause. And of course, seeing rats or burrows around the property is a clear sign.

Health risks

Rats are the main carriers of leptospirosis, a bacterial disease spread through their urine. If a cracked pipe leaks into the soil around the foundation, contamination can reach the basement or crawl space without anyone ever seeing a rat. Rats can also spread salmonellosis, and their droppings and urine are involved in the transmission of hantavirus, which spreads in particular when contaminated material is disturbed and becomes airborne.

Never handle a rat, its droppings or its nest with bare hands, and dampen soiled areas with disinfectant before cleaning them.

How to keep rats out of your drains

Install and maintain backwater valves

A backwater valve lets water out but blocks whatever tries to come back up: sewer backups and, often, rats. In Montreal, the City states that installing a backwater valve on each plumbing fixture is mandatory under municipal regulations: it goes in the basement, on the drain line of each fixture or group of fixtures (toilet, sink, shower, washer, garage floor drain). Keep it accessible and check about twice a year that it works and is not blocked. Rat-specific valves designed to stop rodents from climbing the line are also available. Our article on preventing rats with backwater valves goes into more detail.

Have the pipes inspected and repaired

A valve is a mechanical fix; if the line under the house is cracked or separated, rats will find another way. A sewer camera inspection reveals hidden damage in drain lines, and a smoke test (non-toxic smoke pumped into the system) shows cracks, holes and missing caps. Damaged sections then need to be repaired or replaced by a plumber. It is an investment, but so is a recurring infestation.

Seal other entry points

Walk the perimeter of the house at ground level: under the garage door, where pipes and wires come through, unscreened vents, foundation cracks. Plug gaps with materials a rat cannot chew through: steel wool, metal mesh, sheet metal, cement. Spray foam on its own will not stop a rat; it chews right through it.

Cut off food and shelter

• garbage and compost bins with tight lids;
• no pet food left out overnight, bird feeders cleaned and kept away from the house;
• standing water removed;
• shrubs trimmed, grass kept short and piles of wood or debris removed to eliminate hiding corridors;
• basement and garage decluttered, with items stored in plastic bins rather than cardboard boxes.

Monitor regularly

Check the basement and utility rooms once a month: droppings in corners, shredded paper or insulation, holes near baseboards, an ammonia smell. If you set traps, put them along walls on the rat’s runways, and remember that rats avoid new objects for several days. Our article on rat infestations in Montreal offers more pointers.

When to call an exterminator

If you hear rats in the pipes or walls, find droppings near drains or have had a rat come out of a fixture, do not wait. A rat exterminator experienced with sewer-related infestations inspects the property to find damage and entry points, installs rodent barriers such as metal mesh and valves, and sets up control that accounts for the rat’s wariness. They will also tell you which repairs to hand over to a plumber. To choose well, read our tips on choosing the right exterminator for a rat infestation.

Frequently asked questions

How do rats get into a home’s plumbing?

Through cracks, separated joints or damaged sewer lines, especially in older homes with clay or cast-iron pipes. Rats are good swimmers, climb up drain lines and can come out under a sink, inside a wall or, more rarely, in a toilet.

What are the signs that rats are damaging my pipes?

Scratching near drains or in walls, a gurgling toilet, slow drains, greasy marks around pipe openings, a lingering musky smell, droppings near floor drains and gnaw marks on insulation or fittings.

Does a backwater valve keep rats out?

It blocks what comes back up the line, often including rats, and it is mandatory in Montreal on every basement plumbing fixture. It does not fix a cracked pipe under the house, though: that pipe needs repairing and other entry points need sealing.

Are rats in drains a health hazard?

Yes. Their urine, droppings and saliva can spread leptospirosis, salmonellosis and hantavirus. Avoid direct contact and clean soiled areas with disinfectant and gloves.

Who is responsible: the city or the homeowner?

It depends on where the break is. Damage to the public sewer line is usually the municipality’s responsibility; the private part of the connection, often from the property line, is the owner’s. Contact your borough or municipality if in doubt.

Rats in your drains?

The sooner their route is blocked, the less damage they do. Our technicians work across Greater Montreal, from Laval to Longueuil. Call us at 514-677-7526 or request a quote.