Simple ways to determine what's causing moisture in the basement

Basement water — whether leakage during rainfall or general moisture — is not just a nuisance. Over the years, such moisture can weaken your home's foundation. Begin with these tips and determine the source of the problem.

To determine whether moisture on a damp wall comes from inside or outside the house, tape a piece of heavy-gauge plastic, about 40 centimetres (16 inches) square, to the wall below ground level and seal all the edges with tape.

• After several days, remove the plastic.
• Dampness underneath the plastic indicates that water is seeping into the basement from the outside and through the concrete.
• If the plastic-covered area is dry and the wall around it is damp, water is condensing from moist air inside the basement.

Most basement moisture problems can be alleviated or even solved by sending rainwater away from the house.

• Water collecting around the foundation of a home can exert a surprising amount of pressure, which will force water through even tiny cracks.
• Look to your gutters and downspouts first. If they are clean and working, try extending your downspouts further away from the house.
• As an emergency measure, you can attach an extension, such as an extra piece of gutter, to the bottom of each downspout. You can buy splashblocks at any home centre. Or you can buy curled flat plastic splash blocks that unroll only when water goes through them from the drainpipe.
• If any of these devices for diverting rainwater from your foundation cuts down on seepage into the basement, consider having underground drainpipes installed by a landscaper.

• Check the grading around the house; it should slope away from the foundation.
• Examine your house's perimeter after a rainfall. If water collects in a kind of moat, or if there are puddles near the house, that means that water will seep down and exert pressure on your foundation.
• Fill puddles with well-tamped soil, bermed upwards toward the foundation so that the water will run away from the house. If an area slopes towards the house rather than away from it, consult a landscaper to solve the problem.

If the problem is an occasional leak during a hard rain, and if the water all runs into a drain, you might choose to live with it. However, general moisture or persistent leaking should be dealt with. Consider these tips and determine where the moisture is coming from, so you can quickly solve the problem before it leads to permanent damage.