Carpenter ants in the house: how to find the nest and when to call a pro

Updated November 10, 2026 · KeystoneProGuide editorial team

Are big black ants in the house carpenter ants?

Often. Penn State Extension says the black carpenter ant is a common invader of homes in the northeastern United States. Workers are shiny, dark brown to black, a quarter to half an inch long, and wingless; winged queens and males are up to three quarters of an inch. In nature the ants nest in logs, stumps and hollow trees and help break down dead wood. Homes built in wooded areas, Penn State says, are especially likely to be invaded.

How do you tell them from termites?

By body shape, antennae and wings. Penn State's comparison:

The EPA's termite guide gives the same wing and antenna test. Penn State says the difference matters because each needs a different method of control, and a termite contract will not cover ants: its termite fact sheet says subterranean termite warranties do not extend to carpenter ants.

The damage looks different too. Carpenter ants do not eat wood; they excavate it and push out piles of coarse, stringy shavings mixed with insect parts. Their galleries are smooth and clean. Termite passages are lined with soil.

Do carpenter ants damage the house?

Usually less than people fear. Penn State says the ants seldom tunnel into dry, sound wood. They excavate moist, rotting wood and soft materials such as foam insulation board, and rarely does a nest in a building's timbers cause structural damage. In homes, they mostly build in hollow spaces in walls, ceilings and floors, tunneling through sound timber only to reach a void, which has little effect on its strength.

The more important point is what they reveal. Penn State says carpenter ants usually will not infest sound wood with a moisture content below 15 percent. A nest inside often marks a moisture problem: a leaking roof, faulty flashing, overflowing gutters, sweating pipes, a leaking tub or shower, or poor ventilation.

Where is the nest?

Possibly outside. Penn State says large ants indoors do not prove a nest is in the house; the workers may be foraging from a nest outside. The parent colony is often in a tree, log or waste wood within 100 yards of the house, with satellite colonies inside. Signs of an indoor nest are numerous winged ants at windows inside, and shavings sifting out of cracks, moldings, the basement, the attic or under porches.

Common indoor nest sites, it says, include porch pillars, sills, joists, wall studs, window and door casings, and under insulation in ceilings and walls. Its tips for finding one:

• In summer, look for foraging workers at night with a flashlight in the basement, attic, garage and outside, from May through July between 10 p.m. and 2 a.m., and try to follow them home.
• Listen for rustling in walls and ceilings with a stethoscope or a glass held to the wall.
• Check every damp spot.

Can you get rid of them yourself?

Sometimes, but Penn State sets a high bar: the parent and every satellite colony must be destroyed, or the ants will reappear within days or weeks. It says products of low toxicity to mammals are preferred for homeowners, and that repairing the structural problem and replacing damaged wood should go along with eliminating nests. It also says plainly that it is unlikely a satellite colony can be eliminated without one or more of the materials applied by professional pest management companies.

Baits used by professionals can work, Penn State says, if the technician is diligent about finding trails and refilling baits, though the ants sometimes ignore them.

How do you keep them out?

Penn State's prevention list:

• remove stumps, logs and waste wood within 100 yards of the house;
• keep shrubs and trees, especially evergreens, from touching the house;
• store firewood away from the house and off the ground, and bring it in only when needed;
• fix roofs, flashing, gutters and downspouts, and replace water-damaged wood;
• keep exterior wood painted and sealed;
• seal holes where pipes and wires enter;
• put a plastic moisture barrier over soil in crawl spaces and under porches, and ventilate them.

What should you ask a company?

Ask where it thinks the parent colony is and whether its plan treats both parent and satellites. Ask whether it found the moisture source, since Penn State links infestations to wet wood. The National Pesticide Information Center suggests asking for the product names and where each will be used. A company treating ants in or around your home needs a Pennsylvania pesticide business license and an applicator certified in the household category, category 11.

HomeAdvisor lists ant control at $100 to $500; HomeGuide lists a one-time ant treatment at $200 to $300. Neither gives a separate figure for carpenter ants.

For termites, which need a different approach, see the termite guide. Other signs worth a call are in warning signs.

Sources

1. Carpenter Ants, Penn State Extension
2. Eastern Subterranean Termites, Penn State Extension
3. Termites: How to Identify and Control Them, U.S. Environmental Protection Agency
4. How Much Does Pest Control Cost in 2026?, HomeAdvisor
5. How Much Does Pest Control Cost?, HomeGuide
6. 7 Pa. Code Chapter 128. Pesticides, Commonwealth of Pennsylvania (Pennsylvania Code)
7. Selecting a Pest Control Company, National Pesticide Information Center