What it is

A French drain is a trench covered in gravel and rock that diverts surface and groundwater away from a specific area. French drains are a commonly used drainage system, primarily applied as a preventative measure against surface and ground water penetrating and/or damaging 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 or strategically placed in flat areas to drain standing water from the yard.

How it Works

In order to prevent clogging, gravel sizes in the typical French Drain vary from course at the center to fine at the outside, and are designed with the gradation of the soil around the drain in mind. To keep the soil at bay, it is particularly important to get the particle sizing just right. If this is done improperly, the chances of soil washing into voids in the gravel and clogging the drain drastically increases. Thankfully, the development of geotextiles has greatly simplified this procedure.

Ditches are usually crafted by hand or with tools called a Trencher or track hoe, for the best results the larger the ditch the more water it collects and are commonly pitched at 1 in per every 10 ft. Using clay or a plastic pipe to line the bottom of the ditch is a common technique used to increase the volume of water moved through the drain. Modern French drains are made with perforated pipe that has sand/gravel and then geotextile/landscaping tile layered on top of it. The landscaping tiles serve to prevent the movement of the drainage material, and keeps roots and soil from clogging up the drain pipe. The main purpose of the perforated pipe is to pull surplus water along its length to be deposited safely at its end, with the direction of percolation depending on the relative conditions inside and outside the pipe.

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

1. Buried around the foundation wall on the external side of the foundation

In most homes that have external French drains, the drain itself and/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. Often times filter fabric is then laid in all earth surrounding areas along with top of the stone to keep sediment and other particles from entering the drain. Once the installation is complete, the area can be backfilled with topsoil seed and straw or a larger stone.

External French drains have been known to operate for ten years or more without needing maintenance. However, it's prone to clogging without any warning and can eventually lead to basement flooding. When there is no filter fiber, sediment can make its way through the stone and clog the drain, and when the filter fabric is present, that can instead clog with sediments, and it may be wise to provide clean-outs. Also, a French drain that is not installed with a sump pump counts on gravity alone to drain the foundation water, and if the house is not located on a hill or near a steep incline, finding this slope can be problematic.

2. Installed underneath the basement floor on the inside perimeter of the basement

Installing a French drain around the inside perimeter is most commonly done after the house has been built. To install this kind of drain, the perimeter of the basement floor is worn down to the footing and the cement is removed. A filtration system should separate the soil from the drain tile that is laid at the bottom of the trench and 57 river wash gravel is placed on top of the tile. Water is collected from the basement wall floor joint as it enters, with a sump pump installed to pump the water out of the house and away from the foundation.

The system is easy to maintain once installed, though the sump pump will need annual maintenance to perform properly. An interior French drain is much

Why it is Important

Drying Out Land

French drains are often used to gather sub-surface water and funnel it away, helping to dry the land and keep it from becoming overly saturated with ground water. To install this form of French drain, the highest point of the land must be found. The drain trench is then dug across the property at this high point, and paths to a lower area of land. This method works by capturing the water in the drain and using gravity to pull the water away to an area where it will do no harm.

Keeping Building Foundations Dry

Another reason for using French drains is to keep water from seeping into foundations. This is done by forming a moat around the foundation. The installation method for this use of French drain is to dig a trench four to six feet away from the foundation in a 'U'' Shape. This method catches and pools the water in the trench and drains it away, stopping it from seeping into the foundation.