A French drain or weeping tile (also blind drain, rubble drain, rock drain,
drain tile, perimeter drain, land drain, French ditch, sub-surface drain, sub-soil drain or agricultural drain) is a trench filled with gravel or rock or
containing a perforated pipe that redirects surface water and groundwater away from an area. A French drain can have perforated hollow pipes along the
bottom (see images) to quickly vent water that seeps down through the upper gravel or rock. Pre-engineered French drain systems that eliminate the need
for gravel and rock have become increasingly popular since their introduction over 40 years ago. The common features of these systems include a
lightweight gravel substitute that is wrapped around perforated corrugated pipe and covered with commonly used filter fabric. French drains are primarily
used to prevent ground and surface water from penetrating or damaging building foundations. Alternatively, French drains may 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.
The earliest forms of French drains were simple ditches, pitched from a
high area to a lower one and filled with gravel. These may have been invented in France but were described and popularized by Henry Flagg French
(1813–1885) of Concord, Massachusetts, a lawyer and Assistant US Treasury Secretary, in his 1859 book Farm Drainage. French's own drains were made of
sections of ordinary roofing tile laid with a 1⁄8 in (0.32 cm) gap left in between the sections to admit water. Later, specialized drain tiles were
designed with perforations.
Video: French Drain tips
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French Drain Construction
The earliest forms of French drains were simple ditches, pitched from a high area to a lower one and filled with gravel. These may have been
invented in Franc but were described and popularized by Henry Flagg French (1813–1885) of Concord, Massachusetts, a lawyer and Assistant US Treasury
Secretary, in his 1859 book Farm Drainage. French's own drains were made of sections of ordinary roofing tile laid with a 1⁄8 in (0.32 cm) gap left
in between the sections to admit water. Later, specialized drain tiles were designed with perforations. To prevent clogging, the gravel size varied from
coarse at the center to fine at the outside and was designed based on the gradation of the soil surrounding the drain. The particle sizing was critical
to keep the surrounding soil from washing into the voids in the gravel and clogging the drain. The development of geotextiles greatly simplified this
Ditches may be dug by hand or with a trencher. An inclination of 1 in 100 to 1 in 200 is typical. Lining the bottom of the ditch with clay or
plastic pipe increases the volume of water that can flow through the drain. Modern French drain systems can be made with perforated pipe (weeping tile)
surrounded by sand or gravel and geotextile or landscaping textile. Landscaping textiles are used to prevent migration of the drainage material as well
as preventing dirt and roots from entering and clogging the drainage pipe. The perforated pipe provides a minor underground storage volume but the prime
purpose is for the perforations to drain the area along the full length of the pipe and to discharge any surplus water at its end. The direction of
percolation will depend on the relative conditions inside and outside the pipe.
Subsurface drainage systems have been in common use for centuries. They take many forms, but are all similar in design and function to the
traditional French drain. French drains are excavated trenches filled with aggregate surrounding a slotted or perforated pipe that conveys excess
surface and groundwater to a discharge point away from the drainage area.