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What is a French Drain and Why are they Necessary?

A French drain or (sub-surface drain, or sub-soil drain) is a trench of varying dimensions (1' to 3' wide, and 2' to 5')
deep) that is filled with clean gravel or rock (usually 3/4" class A), and contains a perforated drainage pipe (4', or 6", or 8") that directs groundwater away from a problem area.  

For areas of heavy clay concentration, a sump-pump station is often required for removal of water.  Additionally, if the drainage area can't naturally gravity flow the water away, then a sump-pump station will also be required to lift and pump the water up the hill.

To prevent damage to foundations, use of a French drains is primarily used to prevent ground and surface water from penetrating or accumulating near the foundation.  Also, French drains are routinely built to distribute water behind a retaining wall that needs relief of hydrostatic water pressure. Ditches are dug by hand or with a trencher on the larger and deeper trenches.  

In most homes, an external French drain is installed around the foundation walls before the foundation soil is backfilled or replaced after trenching.  It is laid on the bottom of the excavated area, and a layer of rock is laid on top.  Then a thick filter fabric is laid on top of the rock to keep fine sediments and particles from clogging the pipe.  Once the drainage system is installed, the area is backfilled, gutter downspouts are hooked up, and the pipe(s) are routed to the sump-pump station, if one was required for installation.

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