Potential downspout problems
Too often, downspouts are pointed just inches from the house, dumping water that could seep down around the foundation. In older homes where the foundation may not have been waterproofed, the flow of water at the foundation can lead to deterioration, with water seeping through the foundation. Once water has made its way through the foundation, it can cause mildew or mold, a problem that can be aggravated if living in a humid area or not having air conditioning.
Buried downspout drain pipe
You can bury an extension of the downspout to carry the water away from the house to a dry well, which is an underground reservoir that will distribute the water. The top of the dry well is covered with at least 18 inches of soil. You can purchase a dry well kit that comes with a perforated drum; or you can buy a 55-gallon steel oil drum, take the top and bottom off, and make large holes in it (at least 1 1/2 inches in diameter). Some success can be achieved with no liner for the dry well, for a more cost-effective approach.
Before you start digging to create a dry well, have the area examined by your local utility service to make sure there are no buried utilities, like gas pipes or power lines. If you live within city limits, you should also check with your local building code administrator to ensure that the addition of a dry well is allowable.
Digging the trench and dry well
The dry well should be at least 10 feet from the foundation of the building and the top of the dry well should be at least 18 inches below ground. Identify the location for dry well but do not start digging the well yet. Begin digging a trench about 12 inches deep at the location of the downspout. The trench will hold the drain pipe that leads to the dry well. Continue digging the trench toward the proposed dry well location, increasing the depth of the trench by 1/4 to 1/2 inch for every foot of trench to ensure that the water drains to the dry well.
The top of the dry well should be beneath the ending level of the trench or 18 inches under ground level, whichever is greater. Dig the dry well to a diameter and depth to accommodate the kit drum or oil drum; if using a simple, unlined hole, dig the hole 4 feet in diameter and 3 feet deep.