Are carpenter ants a sign of water damage?

Usually, yes. Carpenter ants do not damage sound wood. They make their galleries in water-damaged wood that is soft or rotten.^[1] That makes them a useful warning: carpenter ants rarely cause significant structural damage to buildings themselves, but their presence indicates that there is water damage somewhere in the building.^[1] The ants are the symptom. The wet wood is the problem to fix.

Why carpenter ants and damp wood go together

The wood is only a nesting site, not food. The ants push the wood they remove out of the nest in sawdust-like piles, and their galleries are smooth and clean, unlike termite tunnels, which are lined with moist soil.^[1] Soft, decaying wood is simply easier to excavate, so wherever water has been sitting in a piece of framing, trim or insulation for months, it becomes a likely home.

Carpenter ants prefer damp wood over dry wood, so increasing ventilation in attics, basements and crawl spaces helps reduce the moisture they need.^[4]

When the ants do not mean the house is wet

Seeing carpenter ants is not proof of a nest inside. The season is the best clue:

• When carpenter ants are active in the house during late winter or early spring (February and March), the infestation probably comes from a nest inside the house.^[1]
• In late spring, large numbers suddenly seen indoors may be foragers from an outdoor nest and not a sign of a nest inside.^[1] They come in for water and sugary scraps and go home to a stump or woodpile.

A trail that leads out the door and into the yard points outdoors. A trail that disappears into a wall, or ants that keep appearing in the same room in winter, points to damp wood inside. For outdoor nests, note that large numbers of carpenter ants on a tree suggest that the tree has wood decay and is potentially hazardous.^[1]

Where to look for the water

Carpenter ant nests in houses are commonly found in porch pillars, roofs, windowsills and door frames where water has damaged the wood, or in wood with soil contact.^[1] Other places that often have water damage include around eaves, chimneys and skylights, and carpenter ants may also nest in wet foam insulation.^[1] They also turn up in wooden doors, ceiling beams, floor joists, under bathtubs and in attic eaves.^[4] Nests in structural wood are often found in moist areas like basements and crawl spaces.^[2]

In a Virginia house, the usual culprits are:

• Plumbing. A slow drip under a sink or tub, or a cold water pipe that sweats all summer. Insulate pipes if water droplets form on the outside, and repair leaks in the roof, attic, inside the home and in the foundation.^[4]
• Gutters and drainage. Overflowing gutters soak fascia, trim and the sill below. Keep rain gutters clean and drain water away from buildings.^[4] Make sure the ground slopes away from the foundation so water does not enter or collect around it.^[5] See how often gutters should be cleaned.
• Windows and doors. Failed caulk and paint let rain into sills and the bottoms of door frames. How to keep exterior trim from rotting covers the upkeep.
• Crawl spaces and basements. Humid air keeps sills and joists damp. Aim to keep indoor humidity below 60 percent, ideally between 30 and 50 percent,^[5] and how to lower humidity in a crawl space explains the options.
• Wood touching soil. Porch posts, steps and siding at grade wick up water from the ground.

When you do find a fresh leak, act quickly: if wet or damp materials are dried within 24 to 48 hours after a leak or spill, in most cases mold will not grow.^[5]

Fix the water, then the ants

Killing the colony without drying the wood only buys time. Address the causes of the moisture damage and, if need be, remove the damaged wood, or the nest site will remain attractive to other carpenter ants.^[1] Treat, repair and dry in whatever order the job needs, but do all three. Whether you need a professional for carpenter ants explains what you can handle and when to hire.

Make sure they are not termites

Termites also point to moisture, but they eat the wood and are treated differently. Subterranean termites usually show themselves through flattened mud tubes about the width of a pencil, winged swarmers near lights and windows, and damage that can leave the surface of the wood intact.^[3] A wet house can attract both: homes with flat roofs or chronic leaks are sometimes at risk of termites establishing themselves above ground, with no connection to the soil.^[3] Compare with the signs of termites in a house.

Stop and call a pro if

• The damp wood is in the roof, around a chimney or skylight, or anywhere you would need a ladder on the roof to reach. That is work for a roofer.
• Sills, joists, posts or other framing feel soft when you probe them. A contractor should judge whether they need replacing.
• The ants come out of a wall or ceiling and you cannot find the nest. A licensed pest control company can inspect and treat a hidden nest.
• You find mud tubes, discarded wings on windowsills or hollow-sounding wood. Have it inspected for termites.

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