Let's begin by clearing up a few common misconceptions about carpenter ants.

First of all, carpenter ants don't "eat" wood. They damage wood by excavating galleries in which to lay their eggs and tend their young. Unlike termites and some wood-destroying beetles, carpenter ants derive no nutritional benefit from the wood itself. (They may, however, raise fungi within the galleries, which they then consume as food.)

Secondly, it's pretty unusual for carpenter ants to attack a piece of sound, dry wood; so news stories about families whose homes were destroyed overnight by carpenter ants are nonsense. Carpenter ants prefer wood that has already been moisture damaged. When assessing the damage caused by carpenter ants to homes and other structures, we have to consider that the wood, in all likelihood, already had a longstanding moisture problem before the carpenter ants arrived.

Finally, carpenter ants don't always damage wood at all. What carpenter ants need is a secluded void with adequate warmth and humidity, and which is well protected from predators. If they come across a place that already meets their needs, then they may not do any excavating at all. In fact, the place they live need not even be made of wood. Carpenter ants often live in non-wooden items such as curtain rods, metal frames of backyard swing sets, metal fence posts, and even garden hoses that have been unused for long periods of time.

Identifying Carpenter Ant Damage

That being said, if left untreated for a long time, carpenter ants can do significant damage above and beyond that caused by moisture itself.

In the photo on the right, we see carpenter ant damage to a house's sill plate and bandboard. This kind of damage can be very expensive to repair because the entire superstructure of the house sits on top of the sill plate, making it very difficult and expensive to replace.
 
Carpenter ant galleries are very smooth inside, with an almost sandpapered appearance. The smooth galleries are one way to distinguish carpenter ant damage from damage caused by termites or wood-devouring beetles. In addition, termite galleries often contain large amounts of mud and soil, which usually are absent in carpenter ant galleries.

Carpenter ant galleries often have "windows" through which the carpenter ants enter and leave the nest when foraging, and through which they eject debris. Sometimes the damage inside a piece of wood will be quite extensive, but the surface of the wood will appear essentially intact, except for the windows (and, of course, the ants traveling in and out of them).