How many carpenter ants is a real problem?

There is no magic number. What matters is when and where you see them: regular sightings indoors, especially in winter, point to a nest inside that is worth finding, often in damp wood.

Ants 4 min read Updated 9 October 2026

The pattern matters more than the count. A carpenter ant that wandered in from the garden in spring is not much to worry about. One a day in different rooms, week after week, and especially in winter, suggests a nest inside the building. That is worth finding, mostly because of what it can point to: damp or damaged wood.

Carpenter ants do not eat wood. They chew out tunnels in it to nest, and they much prefer wood that is already wet or soft. So the ants are often a sign of a leak or rot, and fixing that matters as much as the ants do.

Signs the nest may be inside

Ants active indoors in winter. Outdoor nests go quiet in the cold, so ants moving about in a cold month are likely nesting in a warm part of the house.
Regular sightings in the same few rooms over weeks.
Winged ants coming out indoors. See flying ants in the house.
Small piles of fine wood shavings, like coarse sawdust, sometimes with bits of insect in them, below skirting, window frames or in the loft.
Faint rustling in a wall at night, when they are most active.
Damp spots nearby: a leaking window, bathroom, roof or gutter, or wood touching soil.

The plan

Find and fix the damp. Leaks, blocked gutters, wood touching soil, rotten deck boards or window sills. Move firewood away from the walls and trim branches touching the roof.
Follow the ants. They are busiest at night. Use a torch and see where the trail runs. That tells you where the nest, or the way in, is likely to be.
Bait the trail. Use enclosed bait stations labelled for carpenter ants, on the trail they use. Workers carry the bait back and share it with the colony. Follow the label and check the product is allowed where you live.
Seal the way in once activity stops. See sealing gaps.
Keep watching. Note the date and room of each sighting. A count that falls tells you the plan is working.

Why spraying the whole garden is not the answer

A spray around the outside of the house, or across the garden, treats the places ants walk. It does not reach the nest, and the ants you see are only a small part of the colony. It also hits whatever else lives in the garden, including bees and other pollinators.

If a company offers a whole-garden spray, ask where they think the nest is and how the treatment reaches it. A treatment aimed at a nest they have actually found is a different thing, and that is the kind worth paying for.

Will the colony die off in winter?

Not if it is nesting in a heated part of the house. If you are seeing them in winter, waiting for the cold to finish them off is unlikely to work. Bait the trail and find the damp.

Kids and pets: put bait stations along the trail but behind or under furniture that children and pets cannot get to, and follow the label. If you cannot place them out of reach, leave them out and call a professional.

When a professional inspection makes sense

Get one if you find wood shavings, wood that sounds hollow or feels soft, winged ants coming out indoors, or sightings that carry on after a few weeks of bait. Conflicting advice is normal. Ask each company the same questions:

Where do you think the nest is, and how will you confirm it?
What will you treat, with what, and where?
Will you check for damp and damaged wood?
Is a follow-up visit included?

If you rent, tell your landlord in writing, with dates and photos. Leaks and damp wood are often theirs to fix, though it depends on your agreement and where you live. More on ants in the ant guide.