External French drain system types. How to build an external drainage system?


French drainage is a fairly effective solution applicable to remove excess moisture from the foundation basement. Also, such a system is often equipped behind retaining walls, allowing you to significantly reduce the pressure of sewage. This design is most often used as an alternative to conventional storm sewers and open ditches.

Experts distinguish several types of French drainages, among which are external and internal systems. In the first case, we are talking about a buried pipe system (around the foundation of the building). In the second – about the system, which is mounted directly under the basement floor along the inner perimeter of the building’s basement (usually equipped at the stage of building a house or other construction).

External Drainage features and benefits

In the vast majority of buildings, a French drain is arranged around the foundation walls until it is backfilled. In this case, the system is laid out at the bottom of the prepared area, and a layer of stone is placed on its top. Special filter material is fixed on top of this layer to prevent small particles and debris from entering the system. After arranging the drainage, the entire area is filled up and used according to the basic recommendations.

A well-designed and installed French drain system can serve homeowners for up to 10 years or more without regular inspection and maintenance. However, it is worth remembering that it can periodically become clogged, which ultimately leads to the basement flooding.

In the absence of filter material, any fine deposits can accidentally penetrate through the stone layer and gradually contribute to the clogging of the system. It is also important to remember that the maintenance of an external type of French drain involves a number of external excavations (liquidation of paths, and digging up shrubs and trees around the perimeter of the area). The advantages of an external French drain include:

  • High efficiency, subject to the correct and competent drafting of the project.
  • Design reliability.
  • Stable moisture removal.
  • Long service life.
  • The ability to clean the system in case of clogging

How to install an external French drain?

First of all, before arranging the moisture removal system, you should check the drawings and projects of the house so as not to accidentally damage communications (wires or pipes). Set up a trench (minimum width 20 cm, depth 45 cm, length 150 cm) under the outlet of the gutter system. Next, it is important to make a slope in the trench so that moisture can enter strictly for its intended purpose. On average, the slope should be 1 cm for each of the meters in length. Geotextiles should be placed in the finished trench, which will protect the pipe from falling into the soil, which can interfere with effective drainage. As a result, a stable transfer of moisture from the outside of the structure will be ensured.

Further, keeping the designed angle of inclination, fill the ditch with gravel. Now it comes through the arrangement of the system with a slope, for which it will be necessary to lay a branch connected to the rest of the system by part of the pipe. Make holes with a diameter of 6-7 mm in three rows in this piece of pipe (the distance between the holes is 10 cm). For the final fixation of the entire structure, glue the cover to the perforated pipe at the end of the drain (as a result, water will flow freely through the holes).

Types of external French drainage systems

Planning to equip the external French drain, first familiarize yourself with its varieties. When choosing one or another solution, be guided by the dominant type of soil on the site, individual wishes, and features of the building’s basement. Among the most common solutions that are offered to all the houses and other buildings’ owners, are the following:

  • French channel-type drainage.
  • Drainage ditch.
  • Bio drainage.

The most frequently applicable, as practice shows, is the French channel-type drainage. This is a closed-type trench drain that performs the function of a collector. With its help, the interception and subsequent removal of the excess volume of surface water, which is formed in the arable soil layer and is not able to seep through the subsoil on its own, is ensured. In order to avoid large volumes of earthworks, such a system is made rather narrow. On average, its width along the top is from 50 to 60 cm, and along the bottom 25-20 cm.

External French drain system can also be equipped in the format of a drainage ditch. This is the most primitive and budget option. Trenches should be built along the entire perimeter (according to a pre-agreed project). This solution is optimal for areas with no natural slope for water runoff, that is, mainly in flat territories or lowlands. This kind of system can take the form of a classic ditch, in which the accumulated moisture flows into a pre-equipped water collector, or gradually evaporates naturally. What can be used as a water collector? It can be a natural slope with a drain below, a reservoir (river), and a sewage well (or any other artificial reservoir). The trenches of such a system need regular maintenance, as they can periodically become clogged and overgrown with weeds. Be sure to consider this nuance when choosing the appropriate type of French drainage structure.

Another type of system for removing excess moisture is called bio-drainage. In this case, the method of arranging ditches is also used, but they are covered with soil or gravel. The material must necessarily pass moisture through itself well. Plants that are easy to care for can be planted over the ditches. They will help to better absorb moisture, subsequently simply evaporating it into the air. The advantage of this drainage method is that you keep the design of the site and can emphasize its originality.


All types of external French drainage are characterized by ease of arrangement and high efficiency. When choosing a certain type of system, pay attention to the features of its arrangement, the requirements for the soil, and the site. With proper organization, the drainage system will harmoniously fit into the landscape design of your site and make it more thoughtful and functional.


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