French drain

A French drain, drain tile, or land drain is a ditch filled with gravel or rock that redirects surface and ground water away from an area. French drains are common drainage systems, primarily used to prevent ground and surface water from penetrating or damaging building foundations. Alternatively, the French drain technique may be used to distribute water, such as that which flows from 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 filled with gravel. Later, French drains were made of sections of tile, invented by Henry French of Concord*, that were laid with a 1/8" gap left in between the sections to admit water. Later, the drain was designed with perforations. Ditches may be dug by hand or with a trencher. An inclination of 1 in 100 to 1 in 200 is typical. [cite web | title = Land Drainage for fields and gardens | work = | author = McCormick AJ | url = | accessdate = 2007-05-03] Lining the bottom of 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.

Variations on the French drain model include:

* filter drain is to drain ground water
* collector drain (or inteceptor drain) combines groundwater drainage with the interception of surface water or run-off
* dispersal drain distributes the waste water from a septic tank
* fin drain uses a perforated pipe with a thin vertical section (the fin) of drainage composite above. The advantage is that the fin drain is narrower (200mm or 7 inches) than a traditional French drain (450mm or 17 inches and up), and cheaper to build.

French drains can lead to dry wells or environmentally-friendly rain gardens where the extra water is held and absorbed by plants, when city water systems, or other waste water areas can not be used.


French drains are often installed around a home foundation in two different ways:

* Buried around the foundation wall on the external side of the foundation
* Installed underneath the basement floor on the inside perimeter of the basement

In most homes, an external French drain or drain tile is installed around the foundation walls before the foundation soil is backfilled. It's laid on the bottom of the excavated area, and a layer of stone is laid on top. In many cases, a filter fabric is then laid on top of the stone to keep fine sediments and particles from entering. Once the drain is installed, the area is backfilled and the system is left alone unless it clogs.

Disadvantages: While an external French drain can operate for ten years or more without the need for maintenance, it's prone to clogging without any warning and can eventually lead to a flooded basement. When there is no filter fiber, sediments can make their way through the stone as years pass and clog the drain, and when the filter fabric is present, that can instead clog with sediments. Also, a French drain that is not installed with a sump pump counts on gravity alone to drain foundation water, and if the house is not located on a hill or near a steep incline, finding this slope can be problematic.* Additionally, maintenance on an external French drain involves expensive exterior excavation, which includes removal of walkways, shrubberies, porches, gardens, and anything else along the perimeter.

Installing a French drain around the inside perimeter is most commonly done after the house has been built. Most commonly, this is done in response to a wet basement or right before performing a basement finishing. To install this kind of drain, the perimeter of the basement floor is jackhammered down to the footing and the cement is removed. A layer of stone is laid down, and the drain is laid on top of it. Water is collected from the basement wall floor joint as it enters, and a sump pump is installed to pump the water out of the house and away from the foundation.

Once completed, the area, save for a 2" gap around the edge, is cemented over. This gap exists in order to allow water in from the basement walls. This can be installed very quickly- 1-2 days by an experienced crew. The system is easy to maintain once installed, and the sump pump will need annual maintenance to perform properly. An interior French drain is much less likely to clog than an exterior, partially due to the fact that it is not sitting underneath several feet of soil.

Disadvantages: Interior French drain installation is an effective way to waterproof a basement but requires the use of a sump pump. Many contractors will install plastic sump pumps that can quickly break down or neglect to install a battery backup sump pump, making the basement vulnerable to flooding during power outages. Sump pumps should be installed with a battery backup system in a proper sump liner of 20 gallon size or larger to prevent the sump from having too little water and turning on and off continuously.

French drain has evolved significantly from its origins- starting off as a hand-dug ditch, moving on to ceramic tile, PVC pipe, and eventually to the new French drain innovations on the market like WaterGuard. Each new system is able to address weaknesses of the old as the French drain continues to improve and evolve.

Legal issues

In the U.S., municipalities may require permits for building drainage systems as federal law requires water sent to storm drains to be free of certain contaminants and sediment.

In the UK, local authorities may have specific requirements for the outfall of a French drain into a ditch or watercourse.


See also

*Dry well
*Infiltration basin
*Percolation trench


External links

* [ Graphical descriptions of French drain installations]
* [ Residential French drains and etymology]
* [ What's so French about French Drains?]
* Non-residential French drains are [ regulated in the U.S.] - US EPA
* [ How to Install French Drains]
* [ Disadvantages to external French drains around foundations]
* [ The origins of the French Drain]

Wikimedia Foundation. 2010.

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