French Drain Installation

Those new to the concept of landscape drainage will find the following paragraphs helpful, as we will describe the most commonly used landscape drainage system, especially in the Houston area. We will describe primarily the basic steps of a French Drain installation while also providing interesting and useful facts.

French Drain History:

Although rustic models of the French drain systems have been used for centuries, Henry F. French invented the official French Drain in the mid 1800s. Like many aristocrats of his time, he was an educated gentleman and landowner with his own agricultural business. Even though he was a lawyer and also served for a time as the Assistant Secretary of Treasury in Washington, he still found time to develop innovative drainage ideas to solve the problem of the heavy rainy seasons of spring and summer in Massachusetts. Wishing to impart the wealth of knowledge he gained to the farmers of America, he published Farm Drainage: The Principles, Processes, Effects of Draining Land with Stones, Wood, Plows, and Open Ditches, and Especially with Tiles.  The work of this brilliant man gave us invaluable knowledge that is considered essential to landscape drainage in modern times.

The Modern French Drain Introduction:

The difference between the French drain invented by Henry F. French and the modern French drains is the use of PVC pipes. Nowadays, there are two types of French drain systems, slightly differing due to their placement. The first type of French drain is used for structures, normally closer to the edifice itself, whether it’s a residential home or a commercial building. Due to its construction and proximity to the building, it will be covered only with gravel. This type will be used if the problem area that is receiving excess water is close to the building, posing a threat to the foundation. The second type of French drain system can be used under any turf and for any problem area. This type can be installed under a lawn or a landscape bed, which is the major difference between the two types. The first type cannot be covered with dirt or grass. The following paragraphs will outline the process of the French drain installation for the two types.

Overview of How a French Drain works:

When it rains, water will drain to the bottom of the trench. We put gravel in so that water can go straight to the bottom where the pipe is located. Gravel is used to fill up the trench instead of dirt because dirt prevents the water from flowing to the bottom and also clog the pipes. When the water goes to the bottom, it will start to fill in, rise within the trench, and enter the pipe through the perforations. Then, the perforated PVC pipe will easily divert water into the solid pipe that will take it to its final destination. Since we have sloped the trench, the water will follow the slope and be diverted.

The Materials:

Before beginning the French Drain installation process, the installer must have the proper materials. For a French drain for a structure, perforated 4-in PVC pipes, gravel, solid 4-inch PVC pipes, and geotextile materials are needed besides the shovels needed to dig the trench. The best PVC pipe for a french drain is a the SDR-35. It is little more expensive than some pipes used in this kind of drainage, but it is the most durable product. It protects against root systems and movements in the soil.The most common gravel used is River Rock and Bull Rock.

French Drain Installation: Type 1

There are roughly four parts to a French drain installation:

  1. digging the trench, laying the geotextile
  2. laying the perforated PVC pipe correctly
  3. covering the pipe with gravel on all sides
  4. inserting the edging

The first part, digging the trench, is the most labor-intensive part of the process. The trenches have to be manually dug because often times there are utility lines that the installer must be mindful of. The beginning of the trench will be approximately six inches deep. Then, the trench will have to be dug at a slope, one inch deeper for every ten feet of piping, in order to force the water to its final destination. Sloping the trench in this way accounts for friction loss and flow velocities caused by the moving water.

The second step is to fully cover the trench with a landscaping geotextile fabric to keep the perforated PVC pipe clean from debris that could potentially clog the system.  After that, lay down the PVC pipe with the holes facing down. Then, cover the pipe with gravel on all sides. Once the job is completed, you will see only the gravel at the same level as the ground.

The final step in the French drain installation is the edging that will be the barrier between the gravel and the grass or the gravel and the landscaping bed. Otherwise, the grass will try to grow over the French drain, which does not look good. The French drain will only be installed in the problem area. Eventually the French drain will be connected to a pipe that is completely solid all around with no perforations that will take the water to its deposit area. The solid pipes with no holes will be covered with dirt, and grass will be laid on top of it since these pipes cut straight through the grass area of the lawn.

French Drain Installation: Type 2

As previously stated, the difference between type one and type two of the French drain is that type two can be installed under any turf. It will use all the same materials with a few differences in the installation process. 

The six steps for the French Drain installation under any turf

  1. dig the trench
  2. lay the geotextile
  3. connect the perforated pipe with the solid pipes,
  4. put in the gravel
  5. wrap both the gravel and the perforated PVC pipes completely with the geotextile material
  6. cover it with dirt and the preferred turf

Since it is usually installed under a grass turf, the trenches will have to be dug deeper. To have grass grow on top of it, the trench needs to be about 1 foot deep at the start. The gravel and the pipes will each take up 4 inches. The grass will need the remaining 4 inches of soil in order to grow properly. Then, the trench will be sloped 1 inch for every 10 feet of piping.  This one will be more expensive because of the labor involved in digging deeper trenches. It will also be more expensive because there will be a lot more dirt that needs to be hauled out and stored somewhere until the trenches are completely dug and the piping is installed.

Next, the geotextile material will be laid on the trench, making sure to put enough down to be able to completely cover the gravel and the pipes. Then, the perforated pipes will be connected with the solid pipes that will take the excess water to the exit area since the French drain is only used in the problem area. Since soil and sod will be put on top of the French drain, the gravel along with the perforated PVC pipe must be completely wrapped around with the geotextile material to protect the drainage system from debris. Lastly, the soil and the grass will be laid on top.

Other Considerations for French Drain Installation:

French drains cannot be connected directly to any other type of landscape drainage system; there needs to be at least 5 feet of solid piping separating it from another landscape drainage system, such as a catch basin. Putting one any closer would effect the opposite of what the French drain is meant to do. A French drain cannot be connected to another landscape drainage system on the higher side of the slope because a French drain is not meant to have more water flow into it from another system, it is only meant to deal with a problem area. Having another system higher on the slope of the trench will cause the French drain to overflow or even to be clogged.

French Drain Installation Cost:

A French drain costs anywhere from $1600 to $4500. The French drain installation cost will vary depending on the following factors: pool equipment, utility lines, sprinkler systems, trees and shrubbery, and fences. They are factors that impact prices because they will be time-consuming obstacles. The most expensive factor of a French drain installation is labor. The more time consuming the labor is, then the higher the price of the installation. When our landscape drainage specialist provides an estimate for a customer, he has to inspect the property for things that will be obstacles during the installation, which will be included in the proposed price for the French drain installation.  The size of the property will also be a factor because a bigger property will require more piping; therefore, more labor will be needed to dig longer trenches.

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