Carpenter Ants and Damp Wood: The Quiet Pest Hollowing Out French Roof Timbers

Introduction: the little pile of sawdust at the foot of the wall

It appeared one morning, on the tiled floor, against the skirting board of the loft bedroom. A millimetre thick, about the size of a two-euro coin, a very fine blond dust flecked with darker fragments. You vacuum it up. Three days later, the pile is back, in exactly the same spot.

This seemingly trivial detail is one of the few outward signals of a problem unfolding inside the wood, out of sight, sometimes for several years already. Wood-boring insects — those that feed on timber or tunnel through it — are the slowest and most expensive pests in the French housing stock. They don't bite, they carry no disease, they cause no itching. They degrade the load-bearing structure of a building.

This site regularly covers "visible" pests: rodents, bed bugs, cockroaches, wasps. This article looks at the exact opposite: a population that remains invisible until it has already consumed a significant share of the material. Carpenter ants, house longhorn beetles, woodworm, powderpost beetles and — in the departments concerned — termites: knowing how to read the clues, tell the species apart and understand what attracts them is what separates a localised repair from a five-figure roof structure rebuild.

The carpenter ant: it doesn't eat the wood, it hollows it out

A common confusion

Contrary to a stubborn misconception, ants of the genus Camponotus — known as carpenter ants — do not feed on wood. They lack the enzymes needed to digest cellulose. What they do is different, and in some respects more problematic: they excavate the wood to build their nest, pushing the shavings out of the galleries.

The result is a network of smooth, clean corridors with almost polished walls, following the softer growth rings of the timber. Seen in cross-section, an infested beam looks like a well-organised sponge. Mechanical strength collapses without the outer surface changing in appearance.

In mainland France, the species most frequently involved in buildings is Camponotus herculeanus and, further south, Camponotus ligniperda. These are large ants, 7 to 15 mm for workers, sometimes more for queens, black or two-toned (a reddish thorax in some cases).

The three signs that should raise the alarm

That last point deserves emphasis. A Camponotus colony only produces winged reproductives once it has reached maturity, which takes several years. Seeing winged ants emerge from a ceiling in April therefore means the problem is long-standing, not that it is just beginning.

The real culprit: moisture

Carpenter ants almost never attack sound, dry timber. They look for wood with a moisture content above 15 to 20%, often already slightly degraded by fungus. In practice, a Camponotus nest is the symptom of a waterproofing defect, not the root cause:

• a failing chimney or roof flashing;
• an overflowing gutter wetting a verge rafter;
• a slow leak under an upstairs shower tray;
• decking joists in permanent contact with the ground;
• structural timber in direct contact with damp masonry without a damp-proof course.

In other words, treating the ants without dealing with the water is like repainting a cracked wall. A pin-type moisture meter for wood, a device costing about as much as a tank of petrol, lets you map the at-risk areas yourself before you even call anyone — above 18%, the area is vulnerable.

The true wood-borers: house longhorn beetle, woodworm, powderpost beetle

Where ants merely excavate, other insects genuinely digest wood. These are beetles whose larvae spend several years inside the material before emerging as adults through a characteristic exit hole.

The house longhorn beetle (Hylotrupes bajulus)

This is the most destructive structural timber insect in France, and it attacks exclusively the sapwood of softwoods (fir, spruce, pine) — that is, the vast majority of roof structures built since the post-war years.

• Exit hole: oval, 5 to 10 mm along the long axis. This is the most reliable identification criterion.
• Frass: very fine, powdery sawdust mixed with cylindrical pellets.
• Larval stage: 3 to 11 years depending on temperature and the nitrogen content of the wood.
• Sound: an audible gnawing in summer, particularly in hot weather, in the loft.

A house longhorn larva can reach 25 mm in diameter at the end of its development. It travels beneath a film of wood half a millimetre thick: a single jab with an awl is enough to break through the surface of a rafter that looks perfectly intact. This is the so-called "screwdriver test", which every surveyor carries out as a matter of course.

The deathwatch beetle (Xestobium rufovillosum)

It prefers old, damp hardwoods: oak in period roof structures, half-timbering beams, floors in unheated buildings. Its exit hole is round, 2 to 4 mm. It is associated with the presence of wood-rotting fungi, which pre-degrade the timber.

Its distinctive feature: in spring, the adult taps the wall of its gallery with its head to attract a mate, producing a regular ticking heard in the silence of a church or an old house. This is the "death watch" of English-speaking folklore.

The common furniture beetle (Anobium punctatum)

A round hole 1 to 2 mm across, sawdust as fine as talc. It attacks furniture, floorboards, staircases and panelling — hardwoods and softwoods alike — as soon as ambient humidity exceeds 60%. This is the insect you find in a flea-market wardrobe or in the treads of an attic staircase. Its structural impact is real but slower than that of the house longhorn beetle.

The powderpost beetle (Lyctus brunneus)

It is interested only in the sapwood of starch-rich hardwoods: oak, chestnut, ash, but also imported tropical timbers. A 1 to 2 mm hole, with extremely fine, free-flowing frass that pours out like sand when you shake the piece. It is found mainly in recent floorboards, panelling and new furniture.

Quick identification table

The special case of termites: a legal obligation

Subterranean termites (Reticulitermes) fall under a separate legal regime. The French construction and housing code requires, in areas defined by prefectoral order, a termite status report to be included in the technical survey file when a property is sold. This document is valid for six months.

Two points owners often overlook:

1. The obligation to declare to the town hall. Anyone who observes the presence of termites in a building or on land must declare it to the town hall. This obligation applies to the owner, the occupant, or the managing agent in a block of flats.
2. Termites leave no exit hole. They work from the inside, scrupulously preserving the surface layer, and advance along the grain. The wood stays smooth, but sounds hollow and flexes. Earth shelter tubes (tubes of compacted soil running along a wall, from a cellar up to a floor) are the most specific sign.

Around forty French departments are covered by such orders, mainly in the South-West, along the Atlantic coast, in the Loire valley, as well as in the Île-de-France region and certain Mediterranean areas. The exact list is available from the relevant town hall or prefecture: it changes over time.

Diagnosis: what you can do yourself

Before any intervention, a careful survey is worth more than blind treatment. Three tools are enough for a first examination.

1. The raking light. A powerful light source held parallel to the wood surface reveals relief invisible head-on: bulges, sagging, tiny holes. A rechargeable torch with an adjustable beam is the only genuinely essential accessory for inspecting a loft.

2. The probe. A simple flat-blade screwdriver or scriber, pressed in without force every 20 cm along a beam, on the faces and above all on the ends built into the masonry — the area most often affected and least often checked. If the point sinks in more than 3 mm in wood that should be dry, make a note of it.

3. Moisture readings. On roof timbers, in the roof space, under floors, in a crawl space. Cross-check with a visual inspection of the roof covering after a spell of rain: the correlation between a ceiling water stain and an insect nest ten years later is the rule rather than the exception.

Record everything on a rough plan, with dated photos. This survey will radically change the quality of the quotes you receive.

Treatment: what works and what doesn't

DIY products

The water-based wood treatment products sold in DIY superstores do have real value — but as a preventive measure, on new, sound, accessible timber: decking joists, cladding, garden shed framing, garage roof timbers still bare. Applied with a brush or sprayer in three coats, they offer effective protection.

On an already infested roof structure, however, their effectiveness is marginal. House longhorn larvae live several centimetres deep; a surface coating never reaches them. Standard curative treatment involves surface stripping (mechanically removing the degraded part down to sound wood), drill-and-inject treatment under pressure into holes bored every 30 to 40 cm in a staggered pattern, then an overall spray application. It is long, messy work, to be carried out with an A2P3 cartridge respirator mask and sealed goggles, whatever products are used.

What a professional must provide you with

Always insist on:

• the company's CTB-A+ certification, the FCBA standard specific to the treatment of in-service timber;
• the marketing authorisation number for the products used, and their safety data sheets;
• an initial condition report showing the location of the treated areas;
• a ten-year warranty on the curative treatment;
• a clear statement of the timber members to be replaced — no product restores a lost section.

Be wary of door-to-door canvassing followed by a "free" survey that concludes there is a widespread infestation and produces a quote signed the same day. The DGCCRF has issued several warnings about such practices in the home treatment sector. Always get two quotes, a few weeks apart. Our indicative intervention prices give useful benchmarks for assessing a proposal.

Prevention, the only cost-effective strategy

• Ventilate the loft: air inlets at the eaves, outlets at the ridge, roof vents kept clear. A ventilated roof space dries out; a sealed one rots.
• Never store firewood indoors for more than a few days: a log can harbour larvae. A small log trolley outside, under a ventilated shelter and not pushed up against the wall, avoids bringing in live populations.
• Break every wood-to-ground contact: deck pedestals, waterproof damp-proof courses, metal shoes under post bases.
• Check flashings and gutters twice a year, in autumn after the leaves have fallen and again in spring.
• Fit an indoor hygrometer in at-risk rooms — cellar, utility room, converted loft: keeping the air below 60% relative humidity makes the whole building unfavourable to woodworm and mould alike.

For owners of period houses, a practical guide to the pathologies of old buildings is a reasonable investment: understanding why a stone wall needs to breathe prevents many a cement render which, by trapping moisture, creates exactly the conditions these insects are looking for.

Key takeaways

Wood-boring insects are not a hygiene problem, unlike cockroaches or rodents. An immaculately kept house can have a roof structure at the end of its life. These are opportunistic organisms that exploit a construction error or a maintenance failure, usually water-related.

Three reflexes are worth more than a long treatise:

1. Any recurring pile of sawdust is a signal, never ordinary dust.
2. A hole in timber should be measured and described: shape, diameter, wood species. That's half the diagnosis.
3. Moisture is the cause, the insect is the consequence. Tackling them in the wrong order means paying twice.

If your inspection reveals questionable areas, or if you find large ants indoors in winter, have an independent second assessment carried out before committing to anything. You can consult our services page to understand how an intervention unfolds and which questions to ask.

Sources and references: FCBA (Institut technologique Forêt Cellulose Bois-construction Ameublement), CTB-A+ certification standard; French construction and housing code, articles relating to termite control and parasitic condition reports; ANSES, database of authorised biocidal products (PT8, wood preservatives); INRS, prevention data sheets on the application of wood treatment products.