What is a French Drain System?

A French drain, which may also be called a curtain drain, perimeter drain, weeping tile, or agricultural drain, is a gravel-filled trench that includes a perforated or slotted pipe. These drains are used to direct surface water or groundwater away from a specific area, such as a home's foundation.

French drains direct surface level water toward the lowest point and allow it to seep through the surface level gravel into the drain. This gravel also blocks the passage of excess debris. The water is then collected in the perforated pipe, running at the base of the drain, and directed away from the home and toward a more suitable area for daylighting or infiltration.

French drains differ from typical surface drains because they collect water over the entire length of the drain instead of one particular spot. French drains can also prevent water from collecting and pooling in specific areas, saturating the ground below, which may lead to water problems at the surface or below. Instead, this water is directed to a more desirable location such as a dry well or an area of your choosing.Subsurface drainage systems have been in use for centuries, helping with everything from controlling agricultural runoff to providing yard drainage.

They take many forms, but are all similar in design and function to the traditional French drain system.


How Does a French Drain Work?

Remember that liquid always seeks out the lowest point it can reach along the easiest path, readily moving into empty pockets in loose soil. That's the secret to a French drain: It provides a reliably easy path, creating a sunken channel that encourages water to percolate out of the surrounding soil and flow along a smooth course. Gravity is essential for a French drain to function properly, as it first forces water down from the surface and out of saturated soil, then pulls it along the downward-sloping pipe to the desired discharge point.

French Drain Cost

As with any project, estimating the cost of French drain installation should be done before starting work. French drain costs are driven primarily by the materials used, including the piping, aggregate, fabric, couplers, and end caps. There may be other associated permits, fees, or equipment rentals as well. Many professionals figure that a traditional French drain costs about $30 to $40 per foot, but home-built French drain costs may also involve equipment rentals such as a trencher or compactor.


French Drain Pipe Characteristics

One of the most common pipe materials used to create a French drain is plastic because of its durability. There are two frequently chosen pipe variations: corrugated and PVC. Both are lightweight, but corrugated pipe can be flexible, while PVC is stronger and more rigid.

Regardless of what the pipe is made of, it must be slotted or perforated at regular intervals to allow water to enter and flow through the system.

Another option for areas that require French drain pipe installation is the EZflow system, which is a pipe that comes already surrounded by an aggregate and fabric mesh. The drain pipe itself is made of high-density polyethylene, and is corrugated and perforated.

French drain pipe alternatives such as this one are flexible and come in pre-set lengths, typically of 5 or 10 feet each. Considering other dimensions is also integral to installing French drain piping, as the pipe's diameter will impact how much water can flow through. A 4-inch diameter perforated French drain pipe is likely adequate for most residential applications, but areas expected to experience a good amount of water collection and heavy soil saturation may require a larger-diameter pipe. Commercial or agricultural uses may also require a larger-diameter perforated pipe for French drain systems.