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French Drain: All About French Drains

Learn How to Install a French Drain

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French drains on both sides of wall
toddneville/Flickr/CC BY 2.0

French Drain: What is a French Drain?

A French drain is basically a trench filled with gravel, sometimes with a perforated pipe at the bottom of the trench. A geotextile fabric wraps the pipe and gravel so it can act as a sieve where fine sediments will not pass to the area where the gravel is located. A French drain, when properly designed, will reduce hydrostatic pressure exerted upon retaining walls and subgrade walls. This drainage system is an indispensable component of any subgrade waterproofing project.

French Drain Applications

A French drain, carries moisture out to positive drainage areas, reducing moisture conditions and flooding. French drains could be used in:

  • Basement Walls
  • Parking garages
  • Behind retaining walls
  • Split level buildings
  • Low lying patios
  • Areas with high water table

French Drain Recommendations

A French drain systems should not be more than 12 inches wide, between 6 and 8 inches will be ideal, because it will probably last longer and water will be collected and transported better with less obstructions. As a good practice, you can also have at least 2” of gravel around the pipe. The geotextile fabric must overlap and should be installed so the pipe is completely covered with it. The pipe installed in the trench must also be installed with the holes on the bottom.

A French drain must be installed properly; otherwise cost will add up and reduce your profit. The drainage system should be installed deep enough, so water will eventually run into the pipe, instead of moving near or into the structure. The usual recommended slope of the perforated pipe for a French drain system is one inch drop per ten feet horizontal run.

How deep does the pipe or French drain must be? Well, that depends on the application of the drain. If a French drain is used to protect a living space, the invert of the bottom shall be placed at least 2 inches below the level of the finish floor. However, for the correct elevation of a French drain, a professional shall be consulted as it involves a deeper analysis on how much water is seeping into the structure.

French Drain Pipe

Not all type of piping is allowed on a French drain. Pipes specified to be used on French drains are normally 6 inches PVC SDR-35 perforated. Sometimes schedule 40 pipes can be found, but are not normally used. If the job or area to be worked is small, you might want to try a four inch pipe. Some experts recommend using smooth wall pipes, while others tend to use corrugated pipes. A perforated pipe is manufactured with two rows of round holes, normally installed facing the bottom of the trench.

French Drain Gravel

French drain gravel should be washed three quarter inch minimum and as large as 1 ½ ” crushed stone. The upper 12 inches over the pipe shall be filled with native soil, to avoid having crushed stone over the perforated pipe that could damage the pipe. It is important to place the gravel around the pipe because they will filter any sediment that the geotextile was not able to remove.

French Drain Geotextile Filter

An important piece of a French drain is the geotextile that cover the pipe. All crushed stone shall be protected and wrapped by a geotextile filter, a long-lasting manufactured material that will protect the pipe. The introduction of a drainage fabric around the aggregate can prevent in-situ soils from being washed into and clogging the drain. These geotextile filter fabric will prevent soil fines from entering the drainage system, extending drain life and increasing performance.

French Drain Alternatives

There is prefabricated drainage systems consist of two parts: a geotextile fabric and a molded plastic core. Geotextile fabrics are attached to the core, providing excellent drainage while preventing the backfill from entering the core channel. These prefab drainage systems can reduce installation costs because they install 50% faster than traditional French drains.

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