A French drain is a sloping trench that is filled with gravel or rock. The drain often contains a perforated pipe wrapped in filter fabric that is used to carry surface and ground water away from a structure. The gravity created by the drain's slope helps to redirect the water.
Pre-engineered French drains do not use gravel or rock in their construction. Instead, the system uses a lightweight gravel substitute that is wrapped around a perforated corrugated pipe. The gravel substitute is then covered in filter fabric. French drains are also used to distribute water at the outlet of septic drainage fields in addition to being used to relieve the pressure of groundwater behind retaining walls.