Many basements are naturally damp, particularly if your home is in an area near water or water runoff. Water can seep in during periods of heavy rainfall. Pair that with the fact that basements are underground and rarely see enough sunlight to dry out, and it's easy to understand why basements are notoriously damp. If your basement is flooding or seems to have an excessive amount of water even when it's not raining, you may be wondering what's causing it.
If you have water in your basement, check for internal sources of water leaks. Since most water pipes in the home originate in the basement and wind their way throughout the house, a small leak in the plumbing system could be running down the water or sewer pipes and and pooling in the basement. If the water is localized in an area of your basement, see if you can trace it to a particular source. Your basement could have any number of leaks associated with the home's hot water heater, the washing machine connection, the water main or the plumbing pipes.
Even if it's not currently raining outside, a previous rainfall could still cause water to enter your basement. Rain gutters that have been improperly installed can divert falling rainwater into the backfilled soil around the foundation. Backfill is often looser than compacted soil, and even after the rain has stopped falling, water can be running down through the soil along the foundation. In addition, it can sometimes take several days before rainwater seeps deep enough into the soil to soak through to the basement. Swelling in the underground water supply and runoff from nearby brooks and streams may eventually saturate the soil enough that water begins to penetrate through cracks in your home's foundation.
Condensation can also be the source of water in your basement. Condensation occurs when moisture vapor in the air cools and forms water droplets. Condensation may form on your basement walls if your basement is humid, and the soil surrounding your basement foundation is cool, or vice versa. Condensation can also form on the exterior of appliances in your basement, and the moisture may collect into puddles on the basement floor. If you have any appliances in the basement such as a clothes dryer that blows out a warm-air exhaust, ensure they are properly vented as required by the manufacturer.