The Best Size Stones for French Drains

Stone Size

• Regardless of the size of the French drain trench, the stones that filter the water should be between 1/2 and 1 1/2 inches in diameter. Larger stones will impede the flow of water through the system. Smaller ones will clog the drain and the water will continue to accumulate.

Stone Material

• Use gravel that has been washed so it doesn’t deteriorate as the water flows over it and cause drainage problems. Most home improvement or hardware stores sell washed gravel and stone. Buy more than you need to facilitate drain maintenance. If washed gravel is not available, buy a filter sock for the drain pipe to prevent sediment buildup. Don’t use crushed limestone in the drain because it disintegrates over time and turns into dense mud that impedes drainage.

Drain Construction

• To build a French drain, dig a sloped trench about 12 inches deep and 6 inches wide. It can be smaller or larger but this size is most common. The angle of the trench needs to be between 0.5 percent and 2 percent to keep the water properly diverted from the foundation. An easy way to figure out the proper slope is to remember that a 1 percent slope is achieved by spreading a 1-foot angle over a distance of 100 feet. If the slope is too steep, you will end up with another oversaturated area at the end of the drain and if the angle is too shallow, the water will drain too slowly to alleviate the original problem. Line the trench with 2 inches of gravel and lay 4 to 6 inches of perforated drainage pipe on top. Cover the pipe with more small stones, followed by a 3- to 4-inch layer of sand for filtering.

Maintenance and Troubleshooting

• Periodically check the French drain and remove any accumulated debris. If the stones appear smaller than when first installed, replace them with fresh ones. French drains occasionally have to be dug out on the sides and bottoms after periods of heavy rainfall.