Fireplaces and wood stoves: why the flue and the log pile let pests in come October

Introduction: the season's first fire reveals six months of occupancy

Between mid-October and All Saints' Day, millions of French households relight their stove, their insert or their open fireplace. It is also the period when pest and rodent control companies record a very particular category of calls: noises in the flue, insects crawling out of the log basket, rodent droppings behind the woodpile.

Occupants rarely make the connection, and yet it is a direct one. A chimney flue left unused from April to October is, from the point of view of a bird, an edible dormouse or a wasp, a dry vertical cavity, sheltered from wind and predators. A stère of wood stacked against a façade wall is, from the point of view of a field mouse or a woodworm beetle, a five-star habitat delivered to the door.

According to ADEME, nearly 7 million households heat with wood in France, a large share of them as a supplementary source. This heating method, encouraged for energy reasons, mechanically creates two entry points that conventional guides on rodents or insects almost never address: the flue and the firewood itself.

This article does not rehash our content on rodents in the attic or late-summer wasp nests. It deals with a precise, seasonal angle: what to inspect, seal and move before lighting the first fire of winter 2026-2027.

Who moves into a chimney flue between April and October?

A masonry or lined flue offers a stable temperature, low humidity and total protection from rain and ground predators. Four categories of occupants come up again and again.

Cavity-nesting birds

The western jackdaw (Corvus monedula) is the most frequent case in mainland France. It builds a bulky nest of twigs, sometimes several tens of centimetres high, which blocks the flue. The consequences are twofold: smoke backdraught and a risk of carbon monoxide poisoning.

An essential legal point: the western jackdaw is a protected species under the order of 29 October 2009 establishing the list of birds protected throughout the territory. Destroying occupied nests and capturing individuals is prohibited without a prefectural derogation. In practice, a nest may only be removed once the young have fledged, which means precisely from the end of summer and into autumn. Hence the importance of the September-October window.

The common swift and the common starling also use chimney stacks, but leave earlier in the season.

Wasps and hornets

A colony of common wasps can quite happily build its nest in a disused flue or in the space between the stack and the liner. The warning sign is regular coming and going around the cowl, visible from outside in July and August. By October, the colony declines and dies off with the first frosts: the empty nest is no longer dangerous, but its materials remain combustible and can feed a chimney fire.

Edible dormice, garden dormice and field mice

The edible dormouse (Glis glis) and the garden dormouse (Eliomys quercinus) are the great specialists of cavities in buildings. A stainless steel flue liner with a peripheral air gap, or a stove casing, offers them a perfect corridor. Their noises are distinctive: scratching and nocturnal movement, sometimes the "rolling" of stored objects.

Be careful: the edible dormouse also enjoys protected species status in France (order of 23 April 2007 on protected mammals). Lethal methods are prohibited; only mechanical exclusion and, under certain conditions, capture-and-release are viable.

Bats

All bat species are fully protected in France. A colony living in a flue means contacting the regional Groupe Chiroptères or the LPO before any intervention. Sealing a flue during the hibernation period (November to March) amounts to killing them and exposes you to criminal penalties.

General rule: before sealing, fumigating or dismantling anything in a flue, identify the occupant. Several of the species involved are protected, and a mistake costs you both legally and in sanitary terms (a decomposing carcass in the flue).

Chimney sweeping: a legal obligation that doubles as a pest diagnosis

Decree no. 2023-641 of 20 July 2023 harmonised chimney sweeping rules in France. Since 1 October 2023, it requires:

The sweep issues a sweeping certificate stating, among other things, that the flue is clear along its entire length. Most insurers require this document in the event of a fire claim.

This annual appointment is the best moment for a pest diagnosis. Explicitly ask the professional:

• whether they found any nesting material (twigs, moss, feathers, chewed paper);
• whether the chimney cowl is present and in good condition;
• whether there are cracks or gaps between the stack and the roof;
• whether the flue is lined, and whether the peripheral air gap is sealed at the top.

A carbon monoxide detector installed in the heated room remains the indispensable safety net: it will flag a backdraught caused by a partial blockage before symptoms appear. Santé publique France points out that carbon monoxide causes around a hundred accidental deaths each year in France, with a peak between October and March.

Sealing without smothering: the cowl and anti-intrusion mesh

The preventive solution is simple, inexpensive and long-lasting: fit the flue outlet with a mesh device.

The specifications that matter

• 10 to 13 mm mesh: fine enough to block birds, dormice and rodents, open enough not to clog too quickly with soot. A mesh smaller than 8 mm will block up within a single season on a wood-burning appliance.
• Stainless or hot-dip galvanised steel: bare steel rusts within two winters in contact with acidic condensates.
• Removable: the sweep must be able to access it. Permanently welded mesh will be torn off at the first intervention.
• Compatible with the appliance: on pellet stoves with a balanced or concentric flue terminal, standard chimney grilles are unsuitable; there are specific anti-rodent grilles for flue terminals.

A stainless steel rain cowl with mesh solves animal intrusion, water ingress and, partly, wind-induced backdraught all at once. Budget for installation by a roofer or chimney sweep: working on the roof is not a weekend DIY job.

Disused flues

An abandoned, unsealed flue is one of the most underestimated entry points in older buildings. It must be sealed at both ends:

• at the top, with a slab or solid cap;
• at the bottom, with a rigid plug (rock wool plus a board, or a permanent inflatable stopper).

But beware: a flue that is completely airtight at both ends will condense. The practice recommended by professionals is to leave a small-section mesh vent at each end, which prevents intrusion while ensuring a sweep of air.

Firewood: the most common Trojan horse

This is the absolute blind spot. You protect the house, you seal the flue — and then every day you carry into the living room a basket of logs taken from a pile sitting straight on the ground against the north wall.

What a badly stored stère contains

Wood-boring insects. Freshly felled or poorly seasoned hardwood logs frequently harbour:

• house longhorn beetles and other longhorns, whose larvae tunnel beneath the bark;
• woodworm beetles (Anobium punctatum, and the deathwatch beetle Xestobium rufovillosum);
• bark beetles, easily spotted by their gallery patterns under the bark;
• powderpost beetles, which attack coarse-pored hardwoods (oak, ash, sweet chestnut).

One reassuring point worth stating clearly: the vast majority of these insects do not attack the dry structural timber of a house. Bark beetles and most longhorns found in firewood need bark and a high moisture content. The real risk, documented by the CTBA and echoed by timber-frame treatment specialists, concerns woodworm and powderpost beetles, which can colonise structural timber if moisture conditions are right.

Arachnids and hymenoptera. A woodpile is a classic overwintering site for spiders, earwigs, Asian ladybirds and fertilised wasp and hornet queens looking for a crevice in which to spend the winter. Hence those "wasps in the middle of January" that wake up in a living room heated to 21 °C.

Rodents. A pile of logs resting on the ground against a wall forms a network of dry tunnels with dozens of entrances. Field mice, voles and house mice move in during October, then explore the adjoining wall in search of a way into the building.

The five storage rules that change everything

1. Never against the house wall. Aim for at least 1 to 2 metres of clearance, or better still a separate shelter at the bottom of the garden. This is the most important rule, and the one least often respected.
2. Never directly on the ground. Storing on pallets, or better on a raised log rack, stops moisture rising and eliminates ground-level tunnels.
3. Cover the top, leave the sides open. A full tarpaulin traps moisture and creates exactly the microclimate wood-borers are looking for. Cover the top only.
4. Dry wood, measured. Below 20% moisture content, wood stops being attractive to most insects and burns properly. A firewood moisture meter costs a few tens of euros and prevents both tar build-up in the flue and the importing of live insects.
5. Bring in no more than one or two days' worth. No week-long reserve in the living room. Going from 5 °C to 20 °C wakes up everything that was dormant in the bark.

Never chemically treat firewood. Insecticides applied to logs intended for burning release toxic compounds when heated. The only valid approach is mechanical and logistical: seasoning, raising off the ground, distance, rotation.

The inspection protocol before the first fire

To be carried out once a year, ideally in October, before lighting up.

Outside

• Inspect the chimney stack with binoculars: is the cowl present? Is the masonry cracked? Are twigs poking out of the flue?
• Check the flashings and weatherings at the base of the stack — a 2 cm gap is enough for a garden dormouse.
• Check the flue terminals of pellet stoves: the original grille is often too coarse and tends to deform.
• Move the woodpile if it is up against the façade, and clean the ground under its former location (nests, burrows, tunnels).

Inside

• Open the damper or sweeping hatch and inspect with a torch: nest debris, feathers, shells, droppings?
• Check the airtightness of the casing around an insert or liner: duct penetrations are motorways for rodents.
• Fit sealing strips or anti-rodent steel wool into the gaps around flues passing through floors. Stainless steel wool is the only material rodents will not chew through in the long run.
• Check the expiry date of the carbon monoxide detector (typical service life: 7 to 10 years).

The signs that call for a professional

Co-ownership, rentals: who does what?

In a rental, chimney sweeping and routine flue maintenance fall to the tenant, under decree no. 87-712 of 26 August 1987 on rental repairs. Conversely, repairing a damaged stack, fitting a missing cowl or installing a liner are the responsibility of the landlord, who must provide decent accommodation in good working order.

In a co-ownership, collective smoke flues and roof stacks are generally common parts: fitting mesh to the outlets therefore requires a decision by the general meeting and funding from the co-owners' association. The steps are similar to those described in our article on pests in co-ownerships and rentals: have the problem formally recorded, report it in writing to the managing agent, and request that it be added to the agenda.

For a full survey of the building and its entry points, our teams work by appointment — see the services page. For preventive equipment (grilles, steel wool, mechanical traps, detectors), browse our selection in the shop.

Key takeaways

• A flue left unused six months a year is an attractive cavity for birds, dormice, wasps and bats.
• Several of the species involved — jackdaws, edible dormice, bats — are protected: identifying them before acting is a legal obligation, not a precaution.
• Sweeping twice a year (decree of 20 July 2023) is simultaneously a regulatory obligation, a fire safety measure and the best pest diagnosis available.
• A removable stainless steel mesh cowl, 10-13 mm mesh, permanently solves intrusion through the flue.
• Badly stored firewood is the most common and most overlooked vector: keep it 1-2 m from the façade, raised off the ground, covered on top only, below 20% moisture, and brought in day by day.
• An up-to-date carbon monoxide detector is non-negotiable as soon as you heat with wood.

October is the only window in which these three jobs — sweeping, meshing and reorganising the woodpile — can be tackled together, after the broods have fledged and before the first frosts. Past November, most roof interventions get postponed, and the flue's occupants settle in for the winter.

Sources: ADEME (domestic wood heating); decree no. 2023-641 of 20 July 2023 on the annual maintenance of heating installations; order of 29 October 2009 (protected birds); order of 23 April 2007 (protected terrestrial mammals); Santé publique France (carbon monoxide poisoning); decree no. 87-712 of 26 August 1987 (rental repairs).