A cool, damp basement, musty smells, and mold or mildew on the walls are all indications of moisture getting into your basement. The main cause is likely a poor drainage system around your home that lets moisture build up in the soil around your foundation, work its way right through your foundation, and seep its way into your home. Fixing drainage problems around your house is actually a relatively inexpensive undertaking, and it often comes down to two major issues: 1) how rainwater is handled and 2) the grading or slope of your property.
Gutters, or eavestroughs as they are called in some parts of the country, are an important part of the drainage system around your house. If yours are nonexistent, in poor condition, or improperly placed on your house, they could be causing problems and putting your home at risk of water damage.
Gutters need to be installed properly so that they catch the water coming off your roof and channel it away from your house and its foundation. Your eavestroughs should have a downward slope of 1/2 inch for every 10 feet of the run to ensure they drain completely. If your house is more than 40 feet wide, the optimal installation would have the gutters sloping down from a high point in the middle to a downspout at each end of the house.
All gutters need to be firmly attached to the center of the downspout aligned under the lip of the roof. Screws are the best option for connecting gutters because they won't back out over time. Gutters can be made up of sections that are usually 10-12 feet long, or they can be installed as one continuous piece up to 37 feet long. If yours are in sections, make sure the seams are well sealed and water doesn't leak through. Gutters that leak are a problem not just because they drip water on your head, but because the water drops land close to the foundation.
Downspouts need to be large enough to handle the large amount of water that can come running off your roof during heavy rainfall. The minimum size for a downspout should be 2x3 inches, but a larger size (like 3x4 inches) is even better. The larger size makes it less likely that anything will be able to block the downspout--even a tennis ball will drop right through, and you know water won't back up into your gutters during heavy rain and possibly overflow.
At ground level, the water from your downspout should be directed at least five feet away from your house. It's best to have the water flowing onto a hard surface (like a driveway) or to have fiberglass or concrete splash guards designed to spread the water and stop it from puddling on the ground. Flexible plastic diverters or hinge attachments for your downspout allow you to swing it up out of the way when cutting your lawn, so you're not constantly tripping over it.
Proper Grading and Slope
Once the water is on the ground, it's important that it doesn't just sit there. This is where grading your property plays a key role. Most building codes require that the ground around a foundation must slope away from the foundation at a minimum of six inches in the first 10 feet and with another foot of slope in the next 100 feet. (This is a minimum--more slope will obviously help get water away from the foundation faster). Don't change the grading of your lot, and if you notice some low spots developing, be sure to fill them.
If you have a problem with water constantly accumulating in a particular area, you might consider installing a "French drain." A French drain is a small trench with a perforated pipe surrounded by sand or gravel that directs the water flow away from the foundation following the grade. Not only will it get the water away from the house, but it will also do it without any possibility of water runoff damage or erosion.
Please refer to part 2: Taking Care of Your Drainage System.