Signs of Carpenter Ant Damage in Your Home

Carpenter ants are Missouri's most structurally damaging ant species. While they cause less damage per unit time than termites — they excavate wood rather than eating it — a large, established colony working in moisture-damaged structural wood can create significant structural compromise over months and years. Catching the signs early limits both the damage and the cost of remediation.

The Most Reliable Sign: Frass

Frass is the debris that carpenter ants push out of their galleries through small kick-out holes. It is the single most diagnostic sign of carpenter ant activity. Carpenter ant frass is distinctive:

• Coarse, sawdust-like texture — but not fine and powdery like actual sawdust
• Mixed content: wood fibers, fragments of insulation, dead ants, ant body parts
• Often found in small piles beneath the kick-out hole — on a windowsill, on a baseboard, on the floor below a wall
• May be found in crawlspaces under structural joists where galleries have been excavated

Finding frass with no recent construction activity is a strong indicator of active carpenter ant infestation. Compare it to termite frass (which is fine, pellet-like, and uniform) — carpenter ant frass is much coarser and mixed.

Hollow Sound When Wood Is Tapped

Tap structural wood — door frames, window frames, baseboards, exposed joists — with a screwdriver handle or knuckle. Wood hollowed by carpenter ant galleries produces a hollow sound rather than the solid sound of intact wood. This test is most useful in areas where frass has been found, to confirm the extent of the damage.

Rustling Sound in Walls

Large carpenter ant colonies in wall voids sometimes produce a faint rustling or crinkling sound — the sound of ants moving through galleries or chewing wood. This is most noticeable at night when household noise is reduced. Place your ear against the wall near suspected activity areas.

Visible Gallery Damage

When damaged wood is opened or broken, carpenter ant galleries are smooth-walled and clean — unlike termite tunnels, which are packed with soil and debris. The galleries run both with and across the wood grain, creating an irregular maze. Wood near the surface may appear intact while the interior has been significantly hollowed.

The Moisture Connection

This is the most important context for carpenter ant damage: carpenter ants strongly prefer to nest in wood that has been softened by moisture damage or decay. Finding carpenter ant activity almost always means there is a moisture problem somewhere in the structure. Common moisture sources associated with carpenter ant damage:

• Window frames or door frames with failed caulking allowing water infiltration
• Roof leaks reaching attic framing or wall top plates
• Plumbing leaks inside walls
• Crawlspace condensation soaking floor joists and sill plates
• Improperly flashed decks, porches, or additions
• Gutters that overflow and saturate the fascia and rafter tails

Treating carpenter ants without finding and fixing the moisture source is a temporary solution — the wet, softened wood that attracted the ants remains, and new colonization is likely.

Seeing Large Black Ants at Night

Carpenter ants are nocturnal foragers. Finding large (6–13mm) black ants inside the home — particularly near windows, crossing floors at night, or on walls near the ceiling — is a behavioral sign of a nearby or in-structure colony. Individual workers may travel over 100 yards from a nest to forage, so seeing one does not definitively mean the nest is inside the house. Repeated sightings of multiple workers inside, particularly at night, raises the likelihood of an indoor nest.

Next Steps

If you find frass or hollow wood, contact a pest control professional for inspection. The goals of the inspection are to confirm the species, locate the nest or satellite nest, assess the extent of damage, identify the moisture source, and develop a treatment plan. See our full guide on carpenter ants in Missouri for more on treatment and control.