The causes of a wet basement generally lie outside it, but even the best landscaping and gutter problem may not solve the problem, and often cracks or holes in basement walls admit water that otherwise would stay outside. Much wetness can be eliminated by fixing interior walls, but work on the exterior may be necessary.
If the basement is damp but not wet you see no patches of water but feel excess humidity or see its effects in mildew the steps are fairly simple. Dampness may arise from water vapor generated by appliances in the house such as dish-and clothes, washers-clothes driers are the worst offenders and their exhausts should be vented outdoors.
Seepage through walls or floor may introduce water as well as humidity, and sometimes humid air generated inside the house may condense into liquid on masonry surfaces, suggesting that the moisture is entering from outdoors. To determine whether the problem is inside or outside and whether ventilation is sufficient solution perform the following test. Tape a 16-inch square of heavy plastic sheeting to the wall below ground level. Remove it after several days: dampness underneath means that water is seeping into the basement between grains of sand and cement in a wall that looks solid. If the plastic-covered area is dry and the wall around is damp, then water is condensing from moist air that is inside the basement.
If seepage is the problem, you may be able to block it with a coat of waterproof, cement paint or, if necessary, layers of patching mortar over masonry interior walls and floor. A dirt floor is a common source of seepage; if you have one, cover it. Polyethylene plastic will do if you need not use the basement; if you must walk on the floor, lay sturdier covering- concrete or, at the least, roll roofing.
Cracks or more serious than seepage or condensation. They can be caused by settling, infiltrating tree roots, water pressure against walls or floor, or even minor earthquake tremors.