A French drain is basically a ditch lined with rocks or gravel that helps drain water away from an area. It is generally used in gardening or to protect the foundation of a house or other structure from ground and surface water. One may also be used as backup for retaining walls or to drain water away from a septic tank.
This type of drain works on the principle of gravity, being slightly sloped down from the area to be drained to the area where a homeowner wishes to redirect the water. It is typically lined with perforated clay pipe and surrounded with a landscaping textile to prevent dirt or plant roots from clogging the system. Excess ground and surface water percolates into the drain and is directed away.
The French drain is named after Henry French, a judge and farmer, and discussed in his 1859 book on the subject, Farm Drainage. The design predates French by many centuries, however, as a version was used in ancient Rome. The system continues to be one of the most efficient and widely used methods of redirecting water, though it has been improved upon over the years.
Modern drains are often installed well underground and hidden from view, and they may be covered with sod. A French drain can lead to a dry well, a structure that returns excess water to the supply of groundwater, or to a rain garden, an environmentally friendly invention that uses wetland plants to absorb excess water and return it to the atmosphere through transpiration.
Most gardeners or homeowners can build a French drain relatively easily, but many countries, including the United States and the United Kingdom, regulate the use of these drain systems to keep certain contaminants out of the water supply. Anyone considering installing one should consider the relevant city codes and effects on their neighbors' property.