A French drain is a trench filled with gravel or rock, or both, with or without a perforated pipe that redirects surface water and groundwater away from an area. The perforated pipe is called a weeping tile, drain tile, or perimeter tile. The basic principle of a French drain is to provide a path of least resistance for water to follow instead of building up and flooding the soil. The system was named after Henry French, a judge and farmer who lived in Concord, Massachusetts, who helped to popularize the idea specifically to drain excess water away from crop fields in order to improve the crop yield and prevent flooding.

French drains are primarily used to prevent ground and surface water from penetrating or damaging building foundations and as an alternative to open ditches or storm sewers for streets and highways. They can also be used to distribute water, such as a septic drain field at the outlet of a typical septic tank sewage treatment system. French drains are also used behind retaining walls to relieve ground water pressure.

Here are the basic steps to install a French drain:

1. Plan the location: Figure out where the excess water is pooling and where you want it to go. Choose an outlet for runoff water, look for retention ponds or other bodies of water, or tap into existing drainage. Do not drain onto a neighbors property. Use striping spray paint, stakes, flags, or another method to mark the direction and length of the trench. For proper drainage, the trench and pipe system should always drain from a higher elevation and let out water at a lower elevation. Always direct the water away from your home. Before the French drain installation, make sure your neighborhood has no zoning restrictions and get necessary permissions.
2. Dig a trench: Dig a trench from the place in your yard that needs drainage to your chosen outlet. Check for underground utility lines before digging. The trench should be about 12 inches wide and 18 to 24 inches deep.
3. Add gravel: Add a layer of gravel to the bottom of the trench. The gravel should be about 2 to 3 inches deep.
4. Add the pipe: Place the perforated pipe on top of the gravel, with the holes facing down.
5. Cover the pipe: Cover the pipe with more gravel until the trench is about 3 inches from the surface.
6. Cover with fabric: Cover the gravel with water-permeable fabric to prevent soil from clogging the pipe.
7. Cover with soil: Cover the fabric with soil and sod or other ground cover.

French drains are a useful solution to drainage problems in your yard that can lead to pooling, mold, or water damage in or around your home.