A French drain, blind drain, rubble drain, rock drain, drain tile, perimeter drain, land drain or French ditch is a trench

Trench

A trench is a type of excavation or depression in the ground. Trenches are generally defined by being deeper than they are wide , and by being narrow compared to their length ....

 covered with gravel

Gravel

Gravel is composed of unconsolidated rock fragments that have a general particle size range and include size classes from granule- to boulder-sized fragments. Gravel can be sub-categorized into granule and cobble...

 or rock

Rock (geology)

In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...

 that redirects surface

Surface water

Surface water is water collecting on the ground or in a stream, river, lake, wetland, or ocean; it is related to water collecting as groundwater or atmospheric water....

 and groundwater

Groundwater

Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of rock formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock...

 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. French drains are common drainage

Drainage

Drainage is the natural or artificial removal of surface and sub-surface water from an area. Many agricultural soils need drainage to improve production or to manage water supplies.-Early history:...

 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 a septic drain field

Septic drain field

Septic drain fields, also called leach fields or leach drains are used to remove contaminants and impurities from the liquid that emerges from the septic tank. A septic tank, the septic drain field, and the associated piping compose a complete septic system. The septic drain field is effective for...

 at the outlet of a typical septic tank

Septic tank

A septic tank is a key component of the septic system, a small-scale sewage treatment system common in areas with no connection to main sewage pipes provided by local governments or private corporations...

 sewage treatment system. French drains are also used behind retaining wall

Retaining wall

Retaining walls are built in order to hold back earth which would otherwise move downwards. Their purpose is to stabilize slopes and provide useful areas at different elevations, e.g...

s to relieve ground water pressure.

History and construction

The earliest forms of French drains were simple ditches, pitched from a high area to a lower one and filled with gravel. These were described and popularised by Henry French

Henry French

Henry French may refer to:* Henry F. French, an American judge, assistant US Treasury secretary, father of Daniel Chester French, and inventor of the French drain* Henry French, steamboat builder: see Henry French House...

 (1813-1885) a lawyer and Assistant US Treasury Secretary

United States Assistant Secretary of the Treasury

The United States Assistant Secretary of the Treasury is one of several positions in the United States Department of the Treasury, serving under the United States Secretary of the Treasury....

 from Concord, Massachusettshttp://www.concordma.com/magazine/janfeb00/frenchdrains.html in his book Farm drainage. French's own drains were made of sections of ordinary roofing tile laid with a 1/8 in gap left in between the sections to admit water. Later, specialised 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 geotextile

Geotextile

Geotextiles are permeable fabrics which, when used in association with soil, have the ability to separate, filter, reinforce, protect, or drain...

s greatly simplified this procedure.

Ditches may be dug by hand or with a trencher

Trencher (machine)

A trencher is a piece of construction equipment used to dig trenches, typically for laying pipes or cables, or for drainage. Trenchers may range in size from walk-behind models, to attachments for a skid loader or tractor, to very heavy tracked heavy equipment.-Types:Trenchers come in different...

. 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

Weeping tile

A weeping tile is a porous pipe used for underground drainage. The pipe is typically plastic with small slits cut lengthwise into it. It is buried and surrounded by aggregate larger than the slits. The aggregate rocks prevent excessive soil from falling through the slits into the 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 in structure

Variations on the French drain model include:

Name	Description
filter drain	drains the ground water
collector drain	combines groundwater drainage with the interception of surface water or run-off and can connect directly into the underground pipes (to rapidly divert surface water); however, it should have a cleanable filter to avoid sending surface debris, to the underground area, to clog the buried pipes.
interceptor drain
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 length is 200 mm (7.9 in) and is cheaper to build than a traditional French drain.

French drains can lead to a downhill slope or to dry well

Dry well

A dry well is an underground structure that disposes of unwanted water, most commonly stormwater runoff, by dissipating it into the ground, where it merges with the local groundwater. Often called a soakaway in the UK....

s or rain garden

Rain garden

A rain garden is a planted depression that allows rainwater runoff from impervious urban areas like roofs, driveways, walkways, parking lots, and compacted lawn areas the opportunity to be absorbed...

s where the extra water is held and absorbed by plants. This is useful when city water systems or other wastewater areas cannot be used.

Sizing considerations

Depending on the expected level and volume of rain water or runoff, French drains can be widened or also founded on 2 or 3 underground drain pipes. Multiple pipes also provide for redundancy, in case one pipe becomes overfilled or clogged by a rupture or defect in the piping. A pipe might become overfilled if it is on a side of the drain which receives a much larger volume of water, such as one pipe being closer to an uphill slope, or closer to a roofline that drips near the French drain. When a pipe becomes overfilled, water can seep, sideways, into a parallel pipe, as a form of load-balancing, so that neither pipe becomes slowed by air bubbles, as might happen in a full-pipe with no upper air space.

Foundation

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.

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. It may be wise to provide cleanouts, much as is done with sanitary sewers, to provide access for inspection with a camera snake. Also, a French drain that is not installed with a sump pump

Sump pump

A sump pump is a pump used to remove water that has accumulated in a water collecting sump basin, commonly found in the basement of homes. The water may enter via the perimeter drains of a basement waterproofing system, funneling into the basin or because of rain or natural ground water, if the...

 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 in (5.1 cm) 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.

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 US gal (75.7 l) 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 and Grate Channel. Each new system is able to address weaknesses of the old as the French drain continues to improve and evolve. For example, whereas Henry French used chippings of tree bark to provide anti-microbial properties, an anti-microbial additive can now be included in the material of the plastic channel.

Legal issues

In the U.S., municipalities may require permits for building drainage systems as federal law

Federal law

Federal law is the body of law created by the federal government of a country. A federal government is formed when a group of political units, such as states or provinces join together in a federation, surrendering their individual sovereignty and many powers to the central government while...

 requires water sent to storm drain

Storm drain

A storm drain, storm sewer , stormwater drain or drainage well system or simply a drain or drain system is designed to drain excess rain and ground water from paved streets, parking lots, sidewalks, and roofs. Storm drains vary in design from small residential dry wells to large municipal systems...

s 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.

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
• How To Fix French Drain Problems
• Disadvantages to external French drains around foundations
• The origins of the French Drain