Sealing Rodent Entry Points in Apartment Buildings

Four years of contract, two visits a month in the final months, a serious contractor, decent reports. And yet, at every residents' committee meeting, the same item on the agenda: "mice reported on floors 1 and 2, stairwell B". The managing agent changed contractor. The new one installed twelve more bait stations. The reports kept coming.

The diagnosis took a single morning of technical inspection. In the landing riser of building B, the penetration for the hot water column had never been resealed after the pipework was replaced in 2019: a four-centimetre gap running across seven floors, a genuine vertical staircase. In the basement, the ventilation grille of the bike store had been gnawed away over ten centimetres. And three cellar doors didn't seal properly.

No molecule fixes that. It is the most expensive blind spot in rodent control in residential blocks and commercial premises across the Paris region: you fund baiting indefinitely where you should fund sealing works once. Professionals call this proofing, or exclusion-based control. It is the only part of a rodent control programme that you still own once the contract ends.

This article explains how to identify entry points, which materials genuinely hold up, in what order to carry out baiting and sealing, who pays what between the co-ownership, the landlord and the tenant, and which documents to demand so that the works can actually be verified.

Green wheeled bin covered in snow on a pavement, next to a tree and a parked car

Why baiting alone never solves a building-fabric problem

An empty territory gets repopulated

A well-run baiting campaign reduces a population. It does not change the building. And a building that offers three resources — food, water, shelter — plus free access remains attractive to rats and mice from neighbouring properties, the sewer network and nearby construction sites. As soon as the pressure drops, the territory is recolonised.

This is exactly the mechanism described in infestation displacement linked to street works: animals driven out of one spot look for the most permeable building on the street. We covered this in detail in our piece on rats arriving three weeks after nearby works begin. The permeable building is the one whose service penetrations have not been sealed.

The access dimensions worth knowing

Property managers systematically underestimate how easily rodents get through:

A 1 cm gap under a door, an unsealed riser, a torn-out cable grommet: these are open doors. ANSES and professional integrated pest management (IPM) standards in fact place removing access ahead of biocide use in the hierarchy of measures — bait is a catch-up tool, not a building strategy.

Mapping entry points: the 12 zones to inspect

The inspection must be written up, zone by zone, with photos and locations. In a typical Paris-region building, the same defects always show up in the same places.

At ground and basement level

• Service penetrations at the base of walls: water, gas, fibre, telecoms. The duct gets installed; the collar around the duct never does.
• Floor gullies and inspection chambers with no grating or a broken one, in bin stores, laundry rooms and boiler rooms.
• Cellar and plant room doors: gap under the door, gnawed wooden door bottom, warped hinge.
• Low-level ventilation grilles of the crawl space and bike store, in thin aluminium or plastic.
• Car park ramps and up-and-over doors: a missing brush seal over just one metre is enough. The subject is covered in detail in our article on rats in underground car parks.

In vertical circulation areas

• Landing risers and service cupboards: the opening between two floors, left unsealed after pipework replacement. This is defect number one.
• Refuse chutes: hatches that no longer close, cracked ducting.
• Meter rooms: multiple penetrations, often "sealed" with expanding foam. See our analysis of the meter room as the forgotten blind spot.

At roof level and on façades

• Roof verges, ventilation tiles, rafter ends: the entry route for black rats and field mice into the loft space.
• Rainwater downpipes with no leaf guard.
• Suspended ceilings and plenums in ground-floor shops, connecting through to the cellars.
• Expansion joints between adjoining buildings.

Materials that hold up — and those that merely look like work done

This is where most interventions fail. A botched seal is worse than no seal at all: it creates the impression that the point has been dealt with, and it drops off the monitoring list.

To rule out entirely

• Expanding polyurethane foam on its own. A mouse goes through it in one night. You find it in plant rooms every time, and every time it's been breached.
• Silicone, acrylic mastic, adhesive tape, cardboard, rags.
• Bare plaster over a large area with no reinforcement.
• Plastic or very thin aluminium grilles sold off the shelf.

What holds up

• Galvanised steel mesh, 6 mm aperture (or stainless steel in damp areas), mortared in, for ventilation openings and floor openings. The 6 mm aperture is the threshold that also stops mice.
• Stainless steel wool or a steel wool pad, pushed into the gap and then covered with mortar: the most effective combination on service penetrations. The animal gnaws, injures itself and gives up.
• Fast-setting repair mortar or cement mortar to make good duct collars and chases.
• Galvanised sheet metal or a metal profile fitted over wooden door bottoms.
• Metal door-bottom strip with a brush seal, adjustable, for cellar doors, bin store doors and shop stockroom doors.
• Fire collars / metal cable transit systems for duct penetrations, which deal with fire compartmentation at the same time.
• Leaf guards and non-return flaps in cast iron or stainless steel on rainwater downpipes and drains.

For secondary points — a gap under a bike store door, a damaged air brick grille — the manager can have the work done by the caretaker or a general building contractor, provided metal materials are used. A roll of fine stainless steel mesh and a box of stainless steel wool cost a few tens of euros and cover ten entry points. Penetrations in service risers, mortaring work and anything touching fire safety, on the other hand, are a job for a contractor.

Green wheeled bin covered in snow on a pavement, next to a tree and a parked car

The order of operations: never seal before you have treated

This is the most common sequencing mistake, and it creates painful cases.

Why the order matters

If you seal an infested building before reducing the population, you shut the animals inside. The classic consequences: increased pressure on the flats, rodents attacking partition walls from the inside, and above all mortality in inaccessible voids — with the smell of a carcass inside a partition wall then tying up the managing agent for three weeks.

The sequence that works

1. Full diagnosis with mapping of evidence (droppings, runs, gnaw marks, entry points) and an annotated plan.
2. Population reduction phase: baiting in secured stations and/or mechanical trapping, depending on the zone. The placement rules are strict: see our guide to rodenticide baits in apartment buildings.
3. Progressive sealing, from the outside inwards: you first close the perimeter access points feeding the building from the street and the networks, leaving internal routes open towards the baited areas.
4. Activity check: no more bait consumption, no fresh evidence over two consecutive visits.
5. Closing the last internal points, then cleaning and disinfecting the soiled areas. The protocol is set out in our article on disinfection after a rodent infestation.
6. Reduced monitoring: non-lethal detection stations, non-toxic monitoring blocks, quarterly checks.

In practice, for a Paris-region building of 40 to 60 units with a basement, this sequence spreads over six to ten weeks. A contractor who promises "treatment and sealing on the same day" has not looked at the building.

Who decides, who pays: the question that stalls cases

In a co-ownership

Sealing entry points located in the common parts — risers, floor openings, plant rooms, air bricks, cellar doors, façades — is a co-ownership expense. Two regimes to distinguish:

• Small maintenance repairs (a grille to replace, a door-bottom strip to fit, a duct collar to make good): routine maintenance, decided by the managing agent under their powers to preserve the building, charged to the annual budget.
• Structured sealing campaign (making good all the floor openings in one stairwell, overhauling the low-level ventilation, meshing every basement opening): works outside the annual budget, to be voted on at the general meeting. The method for preparing that vote, backed by quotes, is set out in our article on voting through a rodent control contract at the AGM.

A point that is often overlooked: if the sealing concerns a private part (a hole in a flat's partition wall, a gap under a unit's entrance door, a riser passing through a dwelling), the individual owner must do their share. A well-advised managing agent encloses clear instructions to occupants with the campaign letter — otherwise the stairwell stays permeable.

In rented housing

The landlord must provide decent housing, free from infestation by pests — that is Article 2 of Decree no. 2002-120 of 30 January 2002, as amended by the ELAN Act. Sealing the structural entry points of the dwelling (penetrations, risers, door bottoms, ventilation) is the owner's responsibility, not the tenant's. The tenant remains responsible for hygiene, food storage and providing access to the premises. On the question of access, see our article on the tenant who refuses access for treatment.

In commercial or business premises

The commercial lease decides, but the usual dividing lines are:

For a food business, the stakes go beyond cost allocation: Regulation (EC) no. 852/2004 requires premises to prevent access by unwanted animals, and the absence of sealing is recorded as such at inspection. Both readings are developed in our articles on rodents in a ground-floor shop and on a control plan for warehouses.

The documents to demand so that sealing can be verified

Sealing without traceability cannot be checked and cannot be guaranteed. Insist, in the quote as well as in the final report, on:

• a plan or table of numbered entry points, by floor and by stairwell;
• for each point: precise location, nature of the defect, material used, date of works;
• before/after photos for each point dealt with;
• an explicit mention of points not dealt with and why (no access, private part, works to be quoted separately) — this is the most useful line in the report;
• a durability guarantee on the sealing, with remedial work if it is gnawed through within the agreed period;
• a breakdown of what falls to the pest control contractor and what requires a general building contractor.

This level of requirement is written upstream, into the specification: our template specification for a pest control service provides for a "sealing" lot separate from the "treatment" lot, with a schedule of unit prices by type of point. It is the only way to compare bids without being caught out by an all-in "sealing included" price that in reality covers only three points.

The same principle applies to reading monthly reports: a line saying "entry points sealed: several" is worth nothing. We set out the entries that count in our article on checking an intervention report.

What it costs, and why it pays off

Typical orders of magnitude observed in the Paris region, excluding special cases:

• making good a service penetration at the base of a wall: a few tens of euros per point;
• stainless steel mesh mortared over a low-level vent or air brick: in the region of €50 to €150 per opening;
• metal door-bottom strip supplied, fitted and adjusted: €60 to €200 per door;
• making good all the floor openings in one stairwell across 7 floors: a few days' work, to be quoted;
• a sealing campaign for a 50-unit building with a basement: generally comparable to one to two years of a baiting contract, for an effect that lasts ten years.

That is the trade-off residents' committees and property managers should put to the general meeting: renew an identical visit-based contract, or fund once the works that make the contract light. In the cases we follow, the number of occupant reports drops decisively within three months of serious sealing work — because the cause has been removed, not the individuals.

Rodent control without sealing is a rental: you pay every month for a result you lose the day you stop.

Key takeaways

• A mouse gets through 6 mm, a brown rat through 20 to 25 mm: a gap under a door or an unsealed duct collar is enough to feed an entire building.
• Expanding foam, silicone and plastic grilles are not seals. Galvanised steel mesh with a 6 mm aperture, stainless steel wool, mortar and sheet metal are.
• Never seal before reducing the population: you shut the animals in and create odour problems and internal damage.
• In a co-ownership, small repairs are routine maintenance; a structured campaign is voted at the AGM with quotes. In rented housing, closing structural access points is the landlord's responsibility.
• Demand numbered points, before/after photos, the list of points not dealt with and a durability guarantee — otherwise nothing is verifiable.

To have a building accessibility survey carried out and a costing that separates treatment from sealing, our team works across the whole Paris region: see the quote request page, our apartment building and co-ownership offer, or the annual contracts for the monitoring phase.