A French drain,[1] blind drain,[1] rubble drain,[1] rock drain,[1] drain tile, perimeter drain, land drain or French ditch is a trench covered with gravel or rock that redirects surface 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. 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 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.
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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 (1813-1885) a lawyer and Assistant US Treasury Secretary from Concord, Massachusetts[2] in his book Farm drainage.[3] 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, 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 geotextiles greatly simplified this procedure.
Ditches may be dug by hand or with a trencher. An inclination of 1 in 100 to 1 in 200 is typical.[4] 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.
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 wells or rain gardens where the extra water is held and absorbed by plants. This is useful when city water systems or other wastewater areas cannot be used.
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.
French drains are often installed around a home foundation in two different ways:
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 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 a perforated drain pipe 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 (76 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.
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.