What Stone to Use in a French Drain?

Gravel and Sand Recommendations

• French drain gravel should be between 1/2 and 1 1/2 inches in diameter. Use prewashed gravel so it doesn’t disintegrate during constant exposure to water and clog the drainage system. Avoid crushed limestone as it slowly turns into mud over time. A layer of sand on top of the gravel acts as a filter for the incoming water. If a drainpipe is used at the bottom of the trench, cover it with an additional layer of gravel and sand.

Filters

• If you cannot find a source for washed rock, use a filter sock to prevent fine sediment from entering the drainpipe. Filter socks, sold at drainage and irrigation supply stores and home improvement centers, are made from synthetic geotextile fabric that comes in rolls and is easily cut into desired sizes and shapes. As a precautionary measure, some contractors use filter socks to enclose all French drain gravel to ensure optimum drainage.

Trench Dimensions

• Although you can build a French drain in any size you want, the typical dimensions are 6 inches wide by 12 inches deep. The slope of the drain is crucial for it to divert the water in the right direction. A 2 percent slope is perfect, and the slope should not be less than 0.5 percent. To calculate the proper slope, remember that a 1 percent slope is equal to a 1 foot decline spread over a distance of 100 feet.

French Drain Structure

• After you dig and slope the trench, lay 2 inches of gravel in the bottom and top it with 4 to 6 inches of perforated drainpipe. Add gravel on top of the pipe, ensuring there is at least an inch of pebbles on all sides of the pipe. Make sure the pipe is long enough for the top end to be accessible from the ground surface to enable easy cleaning and snaking.

Finishing Touches

• When the installation is complete, cover the entire surface of the hole with 3 to 4 inches of coarse sand. You can hide the French drain trench by letting the grass on either side grow over it, plant grass seed on top or place sod over the opening. If you choose to sod it, wash the dirt off the sod roots before laying it to keep the dirt from creeping into the drain.