Blog · Cambridge
Carpenter ants in a Galt or Hespeler house: what they are telling you about moisture
Large black ants in the kitchen in April are the most common carpenter ant call in Cambridge. The ants are the symptom. Somewhere in the house there is wood that has been wet for a while, and finding it is most of the job.
Last updated September 25, 2026 · Reviewed by the Cambridge Pest Control team
How to recognise carpenter ants
The University of Maryland Extension describes carpenter ant workers as large, dark ants roughly 6 to 13 mm long, with winged reproductives reaching about 19 mm. In Ontario houses the usual one is the black carpenter ant. They are much bigger than the small pavement ants that trail across a driveway or kitchen floor, and they are most active after dark.
Carpenter ants do not eat wood. They excavate it to make nesting galleries, and they push the debris out. What you are likely to notice:
- Frass. Small piles of coarse sawdust that look like pencil shavings, often mixed with insect parts, below a window sill, along a baseboard or in a closet.
- Winged ants indoors. Swarmers usually appear in spring or early summer, and winged ants collecting at windows inside the house suggest a nest in the structure.
- Workers indoors over a long stretch. Colorado State University Extension notes that wingless workers found inside through much of the year, not just on a warm spring day, point to an active nest in the building rather than ants wandering in from outside.
- A faint rustling in a wall or ceiling at night.
Extension services note that carpenter ant galleries are smooth and clean, while termite galleries are packed with mud and debris. If you find wood damage with mud in it, tell the technician, because that is a different conversation.
Why they point to wet wood
The University of Maryland Extension makes the key point directly: a carpenter ant colony inside a structure is usually most important as a sign that there is, or has been, a water problem. They start in damp or decaying wood and can then extend galleries into sound, dry wood next to it.
In Cambridge that wet wood has a few favourite locations, and they follow the age of the housing:
- Galt century homes along the Grand River: sill plates sitting on damp rubble-stone foundations, porch framing, and old window frames where the paint failed long ago.
- Hespeler mill-era frame houses: wood siding near grade, rear additions built on posts, and rot around old chimneys and flashings.
- Preston post-war bungalows: deck ledgers fastened to the house without flashing, fascia behind overflowing gutters, and bathroom subfloors around a leaking toilet or tub.
Parent nests and satellite nests
Colorado State University Extension notes that nests in buildings are often satellites of a larger colony outdoors, in a stump, a dead limb, a landscape timber or a woodpile. That explains the familiar pattern where ants are treated inside, disappear, and come back the next spring. The satellite was dealt with, the parent colony was not, and the wet wood was still there.
Both extension services say the most important and most difficult part of control is finding the nests. That is why a carpenter ant treatment starts with an inspection at the times and places the ants actually move: along foundation lines, under siding, across tree limbs touching the roof, and back to the moisture.
What a lasting fix looks like
- Find and treat the nests. Indoors this can mean baits, or dust placed into a void through a small hole. Outdoors it means treating or removing the parent nest where it can be found.
- Fix the water. Gutters, downspouts, flashing, a leaking plumbing fixture, grading that sends water toward the wall. Without this step the ants are likely to return.
- Replace the damaged wood. Rotted or excavated wood is structural carpentry, done by a carpenter. The pest control plan tells you where the damage is so the right piece gets opened up.
- Take away the bridges. Trim branches touching the roof and shrubs touching the siding, keep firewood away from the house, and do not let mulch or soil build up against wood siding.
- Follow up. A return visit confirms the activity has stopped before the job is closed.
Why spring and fall are when you notice them
The University of Maryland Extension notes that carpenter ant activity picks up as the weather warms in spring and again as it cools in fall. In a Cambridge house that often means the first ants of the year show up in March or April near a sunny south wall or a kitchen window, well before anything is happening in the garden. Ants seen that early, indoors, deserve a closer look than the same ants on a picnic table in July.
What you can do before the inspection
- Note when and where you see ants, especially at night, and take photos of any sawdust piles before you vacuum them.
- If you catch a winged ant, keep it in a sealed bag or jar so it can be identified.
- Check the obvious water sources: gutters, the ground at the downspout outlets, the bathroom floor around the toilet, and the sill beside any exterior door.
- Skip the spray can. Killing the workers you can see does nothing to the nest and can scatter the trail that leads to it.
Planning ranges for nest treatment and follow-up are on the Cambridge pest control cost guide; carpentry is separate. To book an inspection anywhere in Galt, Preston, Hespeler or the townships, use the contact page.